accel/qaic: Add AIC200 support

[drm/drm-misc.git] / fs / xfs / scrub / nlinks.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Copyright (c) 2021-2024 Oracle.  All Rights Reserved.
 * Author: Darrick J. Wong <djwong@kernel.org>
 */
#include "xfs.h"
#include "xfs_fs.h"
#include "xfs_shared.h"
#include "xfs_format.h"
#include "xfs_trans_resv.h"
#include "xfs_mount.h"
#include "xfs_log_format.h"
#include "xfs_trans.h"
#include "xfs_inode.h"
#include "xfs_icache.h"
#include "xfs_iwalk.h"
#include "xfs_ialloc.h"
#include "xfs_dir2.h"
#include "xfs_dir2_priv.h"
#include "xfs_ag.h"
#include "xfs_parent.h"
#include "scrub/scrub.h"
#include "scrub/common.h"
#include "scrub/repair.h"
#include "scrub/xfile.h"
#include "scrub/xfarray.h"
#include "scrub/iscan.h"
#include "scrub/orphanage.h"
#include "scrub/nlinks.h"
#include "scrub/trace.h"
#include "scrub/readdir.h"
#include "scrub/tempfile.h"
#include "scrub/listxattr.h"

/*
 * Live Inode Link Count Checking
 * ==============================
 *
 * Inode link counts are "summary" metadata, in the sense that they are
 * computed as the number of directory entries referencing each file on the
 * filesystem.  Therefore, we compute the correct link counts by creating a
 * shadow link count structure and walking every inode.
 */

/* Set us up to scrub inode link counts. */
int
xchk_setup_nlinks(
        struct xfs_scrub        *sc)
{
        struct xchk_nlink_ctrs  *xnc;
        int                     error;

        xchk_fsgates_enable(sc, XCHK_FSGATES_DIRENTS);

        if (xchk_could_repair(sc)) {
                error = xrep_setup_nlinks(sc);
                if (error)
                        return error;
        }

        xnc = kvzalloc(sizeof(struct xchk_nlink_ctrs), XCHK_GFP_FLAGS);
        if (!xnc)
                return -ENOMEM;
        xnc->xname.name = xnc->namebuf;
        xnc->sc = sc;
        sc->buf = xnc;

        return xchk_setup_fs(sc);
}

/*
 * Part 1: Collecting file link counts.  For each file, we create a shadow link
 * counting structure, then walk the entire directory tree, incrementing parent
 * and child link counts for each directory entry seen.
 *
 * To avoid false corruption reports in part 2, any failure in this part must
 * set the INCOMPLETE flag even when a negative errno is returned.  This care
 * must be taken with certain errno values (i.e. EFSBADCRC, EFSCORRUPTED,
 * ECANCELED) that are absorbed into a scrub state flag update by
 * xchk_*_process_error.  Scrub and repair share the same incore data
 * structures, so the INCOMPLETE flag is critical to prevent a repair based on
 * insufficient information.
 *
 * Because we are scanning a live filesystem, it's possible that another thread
 * will try to update the link counts for an inode that we've already scanned.
 * This will cause our counts to be incorrect.  Therefore, we hook all
 * directory entry updates because that is when link count updates occur.  By
 * shadowing transaction updates in this manner, live nlink check can ensure by
 * locking the inode and the shadow structure that its own copies are not out
 * of date.  Because the hook code runs in a different process context from the
 * scrub code and the scrub state flags are not accessed atomically, failures
 * in the hook code must abort the iscan and the scrubber must notice the
 * aborted scan and set the incomplete flag.
 *
 * Note that we use jump labels and srcu notifier hooks to minimize the
 * overhead when live nlinks is /not/ running.  Locking order for nlink
 * observations is inode ILOCK -> iscan_lock/xchk_nlink_ctrs lock.
 */

/*
 * Add a delta to an nlink counter, clamping the value to U32_MAX.  Because
 * XFS_MAXLINK < U32_MAX, the checking code will produce the correct results
 * even if we lose some precision.
 */
static inline void
careful_add(
        xfs_nlink_t     *nlinkp,
        int             delta)
{
        uint64_t        new_value = (uint64_t)(*nlinkp) + delta;

        BUILD_BUG_ON(XFS_MAXLINK > U32_MAX);
        *nlinkp = min_t(uint64_t, new_value, U32_MAX);
}

/* Update incore link count information.  Caller must hold the nlinks lock. */
STATIC int
xchk_nlinks_update_incore(
        struct xchk_nlink_ctrs  *xnc,
        xfs_ino_t               ino,
        int                     parents_delta,
        int                     backrefs_delta,
        int                     children_delta)
{
        struct xchk_nlink       nl;
        int                     error;

        if (!xnc->nlinks)
                return 0;

        error = xfarray_load_sparse(xnc->nlinks, ino, &nl);
        if (error)
                return error;

        trace_xchk_nlinks_update_incore(xnc->sc->mp, ino, &nl, parents_delta,
                        backrefs_delta, children_delta);

        careful_add(&nl.parents, parents_delta);
        careful_add(&nl.backrefs, backrefs_delta);
        careful_add(&nl.children, children_delta);

        nl.flags |= XCHK_NLINK_WRITTEN;
        error = xfarray_store(xnc->nlinks, ino, &nl);
        if (error == -EFBIG) {
                /*
                 * EFBIG means we tried to store data at too high a byte offset
                 * in the sparse array.  IOWs, we cannot complete the check and
                 * must notify userspace that the check was incomplete.
                 */
                error = -ECANCELED;
        }
        return error;
}

/*
 * Apply a link count change from the regular filesystem into our shadow link
 * count structure based on a directory update in progress.
 */
STATIC int
xchk_nlinks_live_update(
        struct notifier_block           *nb,
        unsigned long                   action,
        void                            *data)
{
        struct xfs_dir_update_params    *p = data;
        struct xchk_nlink_ctrs          *xnc;
        int                             error;

        xnc = container_of(nb, struct xchk_nlink_ctrs, dhook.dirent_hook.nb);

        /*
         * Ignore temporary directories being used to stage dir repairs, since
         * we don't bump the link counts of the children.
         */
        if (xrep_is_tempfile(p->dp))
                return NOTIFY_DONE;

        trace_xchk_nlinks_live_update(xnc->sc->mp, p->dp, action, p->ip->i_ino,
                        p->delta, p->name->name, p->name->len);

        /*
         * If we've already scanned @dp, update the number of parents that link
         * to @ip.  If @ip is a subdirectory, update the number of child links
         * going out of @dp.
         */
        if (xchk_iscan_want_live_update(&xnc->collect_iscan, p->dp->i_ino)) {
                mutex_lock(&xnc->lock);
                error = xchk_nlinks_update_incore(xnc, p->ip->i_ino, p->delta,
                                0, 0);
                if (!error && S_ISDIR(VFS_IC(p->ip)->i_mode))
                        error = xchk_nlinks_update_incore(xnc, p->dp->i_ino, 0,
                                        0, p->delta);
                mutex_unlock(&xnc->lock);
                if (error)
                        goto out_abort;
        }

        /*
         * If @ip is a subdirectory and we've already scanned it, update the
         * number of backrefs pointing to @dp.
         */
        if (S_ISDIR(VFS_IC(p->ip)->i_mode) &&
            xchk_iscan_want_live_update(&xnc->collect_iscan, p->ip->i_ino)) {
                mutex_lock(&xnc->lock);
                error = xchk_nlinks_update_incore(xnc, p->dp->i_ino, 0,
                                p->delta, 0);
                mutex_unlock(&xnc->lock);
                if (error)
                        goto out_abort;
        }

        return NOTIFY_DONE;

out_abort:
        xchk_iscan_abort(&xnc->collect_iscan);
        return NOTIFY_DONE;
}

/* Bump the observed link count for the inode referenced by this entry. */
STATIC int
xchk_nlinks_collect_dirent(
        struct xfs_scrub        *sc,
        struct xfs_inode        *dp,
        xfs_dir2_dataptr_t      dapos,
        const struct xfs_name   *name,
        xfs_ino_t               ino,
        void                    *priv)
{
        struct xchk_nlink_ctrs  *xnc = priv;
        bool                    dot = false, dotdot = false;
        int                     error;

        /* Does this name make sense? */
        if (name->len == 0 || !xfs_dir2_namecheck(name->name, name->len)) {
                error = -ECANCELED;
                goto out_abort;
        }

        if (name->len == 1 && name->name[0] == '.')
                dot = true;
        else if (name->len == 2 && name->name[0] == '.' &&
                                   name->name[1] == '.')
                dotdot = true;

        /* Don't accept a '.' entry that points somewhere else. */
        if (dot && ino != dp->i_ino) {
                error = -ECANCELED;
                goto out_abort;
        }

        /* Don't accept an invalid inode number. */
        if (!xfs_verify_dir_ino(sc->mp, ino)) {
                error = -ECANCELED;
                goto out_abort;
        }

        /* Update the shadow link counts if we haven't already failed. */

        if (xchk_iscan_aborted(&xnc->collect_iscan)) {
                error = -ECANCELED;
                goto out_incomplete;
        }

        trace_xchk_nlinks_collect_dirent(sc->mp, dp, ino, name);

        mutex_lock(&xnc->lock);

        /*
         * If this is a dotdot entry, it is a back link from dp to ino.  How
         * we handle this depends on whether or not dp is the root directory.
         *
         * The root directory is its own parent, so we pretend the dotdot entry
         * establishes the "parent" of the root directory.  Increment the
         * number of parents of the root directory.
         *
         * Otherwise, increment the number of backrefs pointing back to ino.
         *
         * If the filesystem has parent pointers, we walk the pptrs to
         * determine the backref count.
         */
        if (dotdot) {
                if (xchk_inode_is_dirtree_root(dp))
                        error = xchk_nlinks_update_incore(xnc, ino, 1, 0, 0);
                else if (!xfs_has_parent(sc->mp))
                        error = xchk_nlinks_update_incore(xnc, ino, 0, 1, 0);
                else
                        error = 0;
                if (error)
                        goto out_unlock;
        }

        /*
         * If this dirent is a forward link from dp to ino, increment the
         * number of parents linking into ino.
         */
        if (!dot && !dotdot) {
                error = xchk_nlinks_update_incore(xnc, ino, 1, 0, 0);
                if (error)
                        goto out_unlock;
        }

        /*
         * If this dirent is a forward link to a subdirectory, increment the
         * number of child links of dp.
         */
        if (!dot && !dotdot && name->type == XFS_DIR3_FT_DIR) {
                error = xchk_nlinks_update_incore(xnc, dp->i_ino, 0, 0, 1);
                if (error)
                        goto out_unlock;
        }

        mutex_unlock(&xnc->lock);
        return 0;

out_unlock:
        mutex_unlock(&xnc->lock);
out_abort:
        xchk_iscan_abort(&xnc->collect_iscan);
out_incomplete:
        xchk_set_incomplete(sc);
        return error;
}

/* Bump the backref count for the inode referenced by this parent pointer. */
STATIC int
xchk_nlinks_collect_pptr(
        struct xfs_scrub                *sc,
        struct xfs_inode                *ip,
        unsigned int                    attr_flags,
        const unsigned char             *name,
        unsigned int                    namelen,
        const void                      *value,
        unsigned int                    valuelen,
        void                            *priv)
{
        struct xfs_name                 xname = {
                .name                   = name,
                .len                    = namelen,
        };
        struct xchk_nlink_ctrs          *xnc = priv;
        const struct xfs_parent_rec     *pptr_rec = value;
        xfs_ino_t                       parent_ino;
        int                             error;

        /* Update the shadow link counts if we haven't already failed. */

        if (xchk_iscan_aborted(&xnc->collect_iscan)) {
                error = -ECANCELED;
                goto out_incomplete;
        }

        if (!(attr_flags & XFS_ATTR_PARENT))
                return 0;

        error = xfs_parent_from_attr(sc->mp, attr_flags, name, namelen, value,
                        valuelen, &parent_ino, NULL);
        if (error)
                return error;

        trace_xchk_nlinks_collect_pptr(sc->mp, ip, &xname, pptr_rec);

        mutex_lock(&xnc->lock);

        error = xchk_nlinks_update_incore(xnc, parent_ino, 0, 1, 0);
        if (error)
                goto out_unlock;

        mutex_unlock(&xnc->lock);
        return 0;

out_unlock:
        mutex_unlock(&xnc->lock);
        xchk_iscan_abort(&xnc->collect_iscan);
out_incomplete:
        xchk_set_incomplete(sc);
        return error;
}

/* Walk a directory to bump the observed link counts of the children. */
STATIC int
xchk_nlinks_collect_dir(
        struct xchk_nlink_ctrs  *xnc,
        struct xfs_inode        *dp)
{
        struct xfs_scrub        *sc = xnc->sc;
        unsigned int            lock_mode;
        int                     error = 0;

        /*
         * Ignore temporary directories being used to stage dir repairs, since
         * we don't bump the link counts of the children.
         */
        if (xrep_is_tempfile(dp))
                return 0;

        /* Prevent anyone from changing this directory while we walk it. */
        xfs_ilock(dp, XFS_IOLOCK_SHARED);
        lock_mode = xfs_ilock_data_map_shared(dp);

        /*
         * The dotdot entry of an unlinked directory still points to the last
         * parent, but the parent no longer links to this directory.  Skip the
         * directory to avoid overcounting.
         */
        if (VFS_I(dp)->i_nlink == 0)
                goto out_unlock;

        /*
         * We cannot count file links if the directory looks as though it has
         * been zapped by the inode record repair code.
         */
        if (xchk_dir_looks_zapped(dp)) {
                error = -EBUSY;
                goto out_abort;
        }

        error = xchk_dir_walk(sc, dp, xchk_nlinks_collect_dirent, xnc);
        if (error == -ECANCELED) {
                error = 0;
                goto out_unlock;
        }
        if (error)
                goto out_abort;

        /* Walk the parent pointers to get real backref counts. */
        if (xfs_has_parent(sc->mp)) {
                /*
                 * If the extended attributes look as though they has been
                 * zapped by the inode record repair code, we cannot scan for
                 * parent pointers.
                 */
                if (xchk_pptr_looks_zapped(dp)) {
                        error = -EBUSY;
                        goto out_unlock;
                }

                error = xchk_xattr_walk(sc, dp, xchk_nlinks_collect_pptr, NULL,
                                xnc);
                if (error == -ECANCELED) {
                        error = 0;
                        goto out_unlock;
                }
                if (error)
                        goto out_abort;
        }

        xchk_iscan_mark_visited(&xnc->collect_iscan, dp);
        goto out_unlock;

out_abort:
        xchk_set_incomplete(sc);
        xchk_iscan_abort(&xnc->collect_iscan);
out_unlock:
        xfs_iunlock(dp, lock_mode);
        xfs_iunlock(dp, XFS_IOLOCK_SHARED);
        return error;
}

/* If this looks like a valid pointer, count it. */
static inline int
xchk_nlinks_collect_metafile(
        struct xchk_nlink_ctrs  *xnc,
        xfs_ino_t               ino)
{
        if (!xfs_verify_ino(xnc->sc->mp, ino))
                return 0;

        trace_xchk_nlinks_collect_metafile(xnc->sc->mp, ino);
        return xchk_nlinks_update_incore(xnc, ino, 1, 0, 0);
}

/* Bump the link counts of metadata files rooted in the superblock. */
STATIC int
xchk_nlinks_collect_metafiles(
        struct xchk_nlink_ctrs  *xnc)
{
        struct xfs_mount        *mp = xnc->sc->mp;
        int                     error = -ECANCELED;


        if (xchk_iscan_aborted(&xnc->collect_iscan))
                goto out_incomplete;

        mutex_lock(&xnc->lock);
        error = xchk_nlinks_collect_metafile(xnc, mp->m_sb.sb_rbmino);
        if (error)
                goto out_abort;

        error = xchk_nlinks_collect_metafile(xnc, mp->m_sb.sb_rsumino);
        if (error)
                goto out_abort;

        error = xchk_nlinks_collect_metafile(xnc, mp->m_sb.sb_uquotino);
        if (error)
                goto out_abort;

        error = xchk_nlinks_collect_metafile(xnc, mp->m_sb.sb_gquotino);
        if (error)
                goto out_abort;

        error = xchk_nlinks_collect_metafile(xnc, mp->m_sb.sb_pquotino);
        if (error)
                goto out_abort;
        mutex_unlock(&xnc->lock);

        return 0;

out_abort:
        mutex_unlock(&xnc->lock);
        xchk_iscan_abort(&xnc->collect_iscan);
out_incomplete:
        xchk_set_incomplete(xnc->sc);
        return error;
}

/* Advance the collection scan cursor for this non-directory file. */
static inline int
xchk_nlinks_collect_file(
        struct xchk_nlink_ctrs  *xnc,
        struct xfs_inode        *ip)
{
        xfs_ilock(ip, XFS_IOLOCK_SHARED);
        xchk_iscan_mark_visited(&xnc->collect_iscan, ip);
        xfs_iunlock(ip, XFS_IOLOCK_SHARED);
        return 0;
}

/* Walk all directories and count inode links. */
STATIC int
xchk_nlinks_collect(
        struct xchk_nlink_ctrs  *xnc)
{
        struct xfs_scrub        *sc = xnc->sc;
        struct xfs_inode        *ip;
        int                     error;

        /* Count the rt and quota files that are rooted in the superblock. */
        error = xchk_nlinks_collect_metafiles(xnc);
        if (error)
                return error;

        /*
         * Set up for a potentially lengthy filesystem scan by reducing our
         * transaction resource usage for the duration.  Specifically:
         *
         * Cancel the transaction to release the log grant space while we scan
         * the filesystem.
         *
         * Create a new empty transaction to eliminate the possibility of the
         * inode scan deadlocking on cyclical metadata.
         *
         * We pass the empty transaction to the file scanning function to avoid
         * repeatedly cycling empty transactions.  This can be done even though
         * we take the IOLOCK to quiesce the file because empty transactions
         * do not take sb_internal.
         */
        xchk_trans_cancel(sc);
        error = xchk_trans_alloc_empty(sc);
        if (error)
                return error;

        while ((error = xchk_iscan_iter(&xnc->collect_iscan, &ip)) == 1) {
                if (S_ISDIR(VFS_I(ip)->i_mode))
                        error = xchk_nlinks_collect_dir(xnc, ip);
                else
                        error = xchk_nlinks_collect_file(xnc, ip);
                xchk_irele(sc, ip);
                if (error)
                        break;

                if (xchk_should_terminate(sc, &error))
                        break;
        }
        xchk_iscan_iter_finish(&xnc->collect_iscan);
        if (error) {
                xchk_set_incomplete(sc);
                /*
                 * If we couldn't grab an inode that was busy with a state
                 * change, change the error code so that we exit to userspace
                 * as quickly as possible.
                 */
                if (error == -EBUSY)
                        return -ECANCELED;
                return error;
        }

        /*
         * Switch out for a real transaction in preparation for building a new
         * tree.
         */
        xchk_trans_cancel(sc);
        return xchk_setup_fs(sc);
}

/*
 * Part 2: Comparing file link counters.  Walk each inode and compare the link
 * counts against our shadow information; and then walk each shadow link count
 * structure (that wasn't covered in the first part), comparing it against the
 * file.
 */

/* Read the observed link count for comparison with the actual inode. */
STATIC int
xchk_nlinks_comparison_read(
        struct xchk_nlink_ctrs  *xnc,
        xfs_ino_t               ino,
        struct xchk_nlink       *obs)
{
        struct xchk_nlink       nl;
        int                     error;

        error = xfarray_load_sparse(xnc->nlinks, ino, &nl);
        if (error)
                return error;

        nl.flags |= (XCHK_NLINK_COMPARE_SCANNED | XCHK_NLINK_WRITTEN);

        error = xfarray_store(xnc->nlinks, ino, &nl);
        if (error == -EFBIG) {
                /*
                 * EFBIG means we tried to store data at too high a byte offset
                 * in the sparse array.  IOWs, we cannot complete the check and
                 * must notify userspace that the check was incomplete.  This
                 * shouldn't really happen outside of the collection phase.
                 */
                xchk_set_incomplete(xnc->sc);
                return -ECANCELED;
        }
        if (error)
                return error;

        /* Copy the counters, but do not expose the internal state. */
        obs->parents = nl.parents;
        obs->backrefs = nl.backrefs;
        obs->children = nl.children;
        obs->flags = 0;
        return 0;
}

/* Check our link count against an inode. */
STATIC int
xchk_nlinks_compare_inode(
        struct xchk_nlink_ctrs  *xnc,
        struct xfs_inode        *ip)
{
        struct xchk_nlink       obs;
        struct xfs_scrub        *sc = xnc->sc;
        uint64_t                total_links;
        unsigned int            actual_nlink;
        int                     error;

        /*
         * Ignore temporary files being used to stage repairs, since we assume
         * they're correct for non-directories, and the directory repair code
         * doesn't bump the link counts for the children.
         */
        if (xrep_is_tempfile(ip))
                return 0;

        xfs_ilock(ip, XFS_ILOCK_SHARED);
        mutex_lock(&xnc->lock);

        if (xchk_iscan_aborted(&xnc->collect_iscan)) {
                xchk_set_incomplete(xnc->sc);
                error = -ECANCELED;
                goto out_scanlock;
        }

        error = xchk_nlinks_comparison_read(xnc, ip->i_ino, &obs);
        if (error)
                goto out_scanlock;

        /*
         * If we don't have ftype to get an accurate count of the subdirectory
         * entries in this directory, take advantage of the fact that on a
         * consistent ftype=0 filesystem, the number of subdirectory
         * backreferences (dotdot entries) pointing towards this directory
         * should be equal to the number of subdirectory entries in the
         * directory.
         */
        if (!xfs_has_ftype(sc->mp) && S_ISDIR(VFS_I(ip)->i_mode))
                obs.children = obs.backrefs;

        total_links = xchk_nlink_total(ip, &obs);
        actual_nlink = VFS_I(ip)->i_nlink;

        trace_xchk_nlinks_compare_inode(sc->mp, ip, &obs);

        /*
         * If we found so many parents that we'd overflow i_nlink, we must flag
         * this as a corruption.  The VFS won't let users increase the link
         * count, but it will let them decrease it.
         */
        if (total_links > XFS_NLINK_PINNED) {
                xchk_ino_set_corrupt(sc, ip->i_ino);
                goto out_corrupt;
        } else if (total_links > XFS_MAXLINK) {
                xchk_ino_set_warning(sc, ip->i_ino);
        }

        /* Link counts should match. */
        if (total_links != actual_nlink) {
                xchk_ino_set_corrupt(sc, ip->i_ino);
                goto out_corrupt;
        }

        if (S_ISDIR(VFS_I(ip)->i_mode) && actual_nlink > 0) {
                /*
                 * The collection phase ignores directories with zero link
                 * count, so we ignore them here too.
                 *
                 * The number of subdirectory backreferences (dotdot entries)
                 * pointing towards this directory should be equal to the
                 * number of subdirectory entries in the directory.
                 */
                if (obs.children != obs.backrefs)
                        xchk_ino_xref_set_corrupt(sc, ip->i_ino);
        } else {
                /*
                 * Non-directories and unlinked directories should not have
                 * back references.
                 */
                if (obs.backrefs != 0) {
                        xchk_ino_set_corrupt(sc, ip->i_ino);
                        goto out_corrupt;
                }

                /*
                 * Non-directories and unlinked directories should not have
                 * children.
                 */
                if (obs.children != 0) {
                        xchk_ino_set_corrupt(sc, ip->i_ino);
                        goto out_corrupt;
                }
        }

        if (xchk_inode_is_dirtree_root(ip)) {
                /*
                 * For the root of a directory tree, both the '.' and '..'
                 * entries should point to the root directory.  The dotdot
                 * entry is counted as a parent of the root /and/ a backref of
                 * the root directory.
                 */
                if (obs.parents != 1) {
                        xchk_ino_set_corrupt(sc, ip->i_ino);
                        goto out_corrupt;
                }
        } else if (actual_nlink > 0) {
                /*
                 * Linked files that are not the root directory should have at
                 * least one parent.
                 */
                if (obs.parents == 0) {
                        xchk_ino_set_corrupt(sc, ip->i_ino);
                        goto out_corrupt;
                }
        }

out_corrupt:
        if (sc->sm->sm_flags & XFS_SCRUB_OFLAG_CORRUPT)
                error = -ECANCELED;
out_scanlock:
        mutex_unlock(&xnc->lock);
        xfs_iunlock(ip, XFS_ILOCK_SHARED);
        return error;
}

/*
 * Check our link count against an inode that wasn't checked previously.  This
 * is intended to catch directories with dangling links, though we could be
 * racing with inode allocation in other threads.
 */
STATIC int
xchk_nlinks_compare_inum(
        struct xchk_nlink_ctrs  *xnc,
        xfs_ino_t               ino)
{
        struct xchk_nlink       obs;
        struct xfs_mount        *mp = xnc->sc->mp;
        struct xfs_trans        *tp = xnc->sc->tp;
        struct xfs_buf          *agi_bp;
        struct xfs_inode        *ip;
        int                     error;

        /*
         * The first iget failed, so try again with the variant that returns
         * either an incore inode or the AGI buffer.  If the function returns
         * EINVAL/ENOENT, it should have passed us the AGI buffer so that we
         * can guarantee that the inode won't be allocated while we check for
         * a zero link count in the observed link count data.
         */
        error = xchk_iget_agi(xnc->sc, ino, &agi_bp, &ip);
        if (!error) {
                /* Actually got an inode, so use the inode compare. */
                error = xchk_nlinks_compare_inode(xnc, ip);
                xchk_irele(xnc->sc, ip);
                return error;
        }
        if (error == -ENOENT || error == -EINVAL) {
                /* No inode was found.  Check for zero link count below. */
                error = 0;
        }
        if (error)
                goto out_agi;

        /* Ensure that we have protected against inode allocation/freeing. */
        if (agi_bp == NULL) {
                ASSERT(agi_bp != NULL);
                xchk_set_incomplete(xnc->sc);
                return -ECANCELED;
        }

        if (xchk_iscan_aborted(&xnc->collect_iscan)) {
                xchk_set_incomplete(xnc->sc);
                error = -ECANCELED;
                goto out_agi;
        }

        mutex_lock(&xnc->lock);
        error = xchk_nlinks_comparison_read(xnc, ino, &obs);
        if (error)
                goto out_scanlock;

        trace_xchk_nlinks_check_zero(mp, ino, &obs);

        /*
         * If we can't grab the inode, the link count had better be zero.  We
         * still hold the AGI to prevent inode allocation/freeing.
         */
        if (xchk_nlink_total(NULL, &obs) != 0) {
                xchk_ino_set_corrupt(xnc->sc, ino);
                error = -ECANCELED;
        }

out_scanlock:
        mutex_unlock(&xnc->lock);
out_agi:
        if (agi_bp)
                xfs_trans_brelse(tp, agi_bp);
        return error;
}

/*
 * Try to visit every inode in the filesystem to compare the link count.  Move
 * on if we can't grab an inode, since we'll revisit unchecked nlink records in
 * the second part.
 */
static int
xchk_nlinks_compare_iter(
        struct xchk_nlink_ctrs  *xnc,
        struct xfs_inode        **ipp)
{
        int                     error;

        do {
                error = xchk_iscan_iter(&xnc->compare_iscan, ipp);
        } while (error == -EBUSY);

        return error;
}

/* Compare the link counts we observed against the live information. */
STATIC int
xchk_nlinks_compare(
        struct xchk_nlink_ctrs  *xnc)
{
        struct xchk_nlink       nl;
        struct xfs_scrub        *sc = xnc->sc;
        struct xfs_inode        *ip;
        xfarray_idx_t           cur = XFARRAY_CURSOR_INIT;
        int                     error;

        if (sc->sm->sm_flags & XFS_SCRUB_OFLAG_CORRUPT)
                return 0;

        /*
         * Create a new empty transaction so that we can advance the iscan
         * cursor without deadlocking if the inobt has a cycle and push on the
         * inactivation workqueue.
         */
        xchk_trans_cancel(sc);
        error = xchk_trans_alloc_empty(sc);
        if (error)
                return error;

        /*
         * Use the inobt to walk all allocated inodes to compare the link
         * counts.  Inodes skipped by _compare_iter will be tried again in the
         * next phase of the scan.
         */
        xchk_iscan_start(sc, 0, 0, &xnc->compare_iscan);
        while ((error = xchk_nlinks_compare_iter(xnc, &ip)) == 1) {
                error = xchk_nlinks_compare_inode(xnc, ip);
                xchk_iscan_mark_visited(&xnc->compare_iscan, ip);
                xchk_irele(sc, ip);
                if (error)
                        break;

                if (xchk_should_terminate(sc, &error))
                        break;
        }
        xchk_iscan_iter_finish(&xnc->compare_iscan);
        xchk_iscan_teardown(&xnc->compare_iscan);
        if (error)
                return error;

        if (sc->sm->sm_flags & XFS_SCRUB_OFLAG_CORRUPT)
                return 0;

        /*
         * Walk all the non-null nlink observations that weren't checked in the
         * previous step.
         */
        mutex_lock(&xnc->lock);
        while ((error = xfarray_iter(xnc->nlinks, &cur, &nl)) == 1) {
                xfs_ino_t       ino = cur - 1;

                if (nl.flags & XCHK_NLINK_COMPARE_SCANNED)
                        continue;

                mutex_unlock(&xnc->lock);

                error = xchk_nlinks_compare_inum(xnc, ino);
                if (error)
                        return error;

                if (xchk_should_terminate(xnc->sc, &error))
                        return error;

                mutex_lock(&xnc->lock);
        }
        mutex_unlock(&xnc->lock);

        return error;
}

/* Tear down everything associated with a nlinks check. */
static void
xchk_nlinks_teardown_scan(
        void                    *priv)
{
        struct xchk_nlink_ctrs  *xnc = priv;

        /* Discourage any hook functions that might be running. */
        xchk_iscan_abort(&xnc->collect_iscan);

        xfs_dir_hook_del(xnc->sc->mp, &xnc->dhook);

        xfarray_destroy(xnc->nlinks);
        xnc->nlinks = NULL;

        xchk_iscan_teardown(&xnc->collect_iscan);
        mutex_destroy(&xnc->lock);
        xnc->sc = NULL;
}

/*
 * Scan all inodes in the entire filesystem to generate link count data.  If
 * the scan is successful, the counts will be left alive for a repair.  If any
 * error occurs, we'll tear everything down.
 */
STATIC int
xchk_nlinks_setup_scan(
        struct xfs_scrub        *sc,
        struct xchk_nlink_ctrs  *xnc)
{
        struct xfs_mount        *mp = sc->mp;
        char                    *descr;
        unsigned long long      max_inos;
        xfs_agnumber_t          last_agno = mp->m_sb.sb_agcount - 1;
        xfs_agino_t             first_agino, last_agino;
        int                     error;

        mutex_init(&xnc->lock);

        /* Retry iget every tenth of a second for up to 30 seconds. */
        xchk_iscan_start(sc, 30000, 100, &xnc->collect_iscan);

        /*
         * Set up enough space to store an nlink record for the highest
         * possible inode number in this system.
         */
        xfs_agino_range(mp, last_agno, &first_agino, &last_agino);
        max_inos = XFS_AGINO_TO_INO(mp, last_agno, last_agino) + 1;
        descr = xchk_xfile_descr(sc, "file link counts");
        error = xfarray_create(descr, min(XFS_MAXINUMBER + 1, max_inos),
                        sizeof(struct xchk_nlink), &xnc->nlinks);
        kfree(descr);
        if (error)
                goto out_teardown;

        /*
         * Hook into the directory entry code so that we can capture updates to
         * file link counts.  The hook only triggers for inodes that were
         * already scanned, and the scanner thread takes each inode's ILOCK,
         * which means that any in-progress inode updates will finish before we
         * can scan the inode.
         */
        ASSERT(sc->flags & XCHK_FSGATES_DIRENTS);
        xfs_dir_hook_setup(&xnc->dhook, xchk_nlinks_live_update);
        error = xfs_dir_hook_add(mp, &xnc->dhook);
        if (error)
                goto out_teardown;

        /* Use deferred cleanup to pass the inode link count data to repair. */
        sc->buf_cleanup = xchk_nlinks_teardown_scan;
        return 0;

out_teardown:
        xchk_nlinks_teardown_scan(xnc);
        return error;
}

/* Scrub the link count of all inodes on the filesystem. */
int
xchk_nlinks(
        struct xfs_scrub        *sc)
{
        struct xchk_nlink_ctrs  *xnc = sc->buf;
        int                     error = 0;

        /* Set ourselves up to check link counts on the live filesystem. */
        error = xchk_nlinks_setup_scan(sc, xnc);
        if (error)
                return error;

        /* Walk all inodes, picking up link count information. */
        error = xchk_nlinks_collect(xnc);
        if (!xchk_xref_process_error(sc, 0, 0, &error))
                return error;

        /* Fail fast if we're not playing with a full dataset. */
        if (xchk_iscan_aborted(&xnc->collect_iscan))
                xchk_set_incomplete(sc);
        if (sc->sm->sm_flags & XFS_SCRUB_OFLAG_INCOMPLETE)
                return 0;

        /* Compare link counts. */
        error = xchk_nlinks_compare(xnc);
        if (!xchk_xref_process_error(sc, 0, 0, &error))
                return error;

        /* Check one last time for an incomplete dataset. */
        if (xchk_iscan_aborted(&xnc->collect_iscan))
                xchk_set_incomplete(sc);

        return 0;
}