Merge tag 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost

[cris-mirror.git] / drivers / md / dm-verity-fec.c

/*
 * Copyright (C) 2015 Google, Inc.
 *
 * Author: Sami Tolvanen <samitolvanen@google.com>
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by the Free
 * Software Foundation; either version 2 of the License, or (at your option)
 * any later version.
 */

#include "dm-verity-fec.h"
#include <linux/math64.h>

#define DM_MSG_PREFIX   "verity-fec"

/*
 * If error correction has been configured, returns true.
 */
bool verity_fec_is_enabled(struct dm_verity *v)
{
        return v->fec && v->fec->dev;
}

/*
 * Return a pointer to dm_verity_fec_io after dm_verity_io and its variable
 * length fields.
 */
static inline struct dm_verity_fec_io *fec_io(struct dm_verity_io *io)
{
        return (struct dm_verity_fec_io *) verity_io_digest_end(io->v, io);
}

/*
 * Return an interleaved offset for a byte in RS block.
 */
static inline u64 fec_interleave(struct dm_verity *v, u64 offset)
{
        u32 mod;

        mod = do_div(offset, v->fec->rsn);
        return offset + mod * (v->fec->rounds << v->data_dev_block_bits);
}

/*
 * Decode an RS block using Reed-Solomon.
 */
static int fec_decode_rs8(struct dm_verity *v, struct dm_verity_fec_io *fio,
                          u8 *data, u8 *fec, int neras)
{
        int i;
        uint16_t par[DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN];

        for (i = 0; i < v->fec->roots; i++)
                par[i] = fec[i];

        return decode_rs8(fio->rs, data, par, v->fec->rsn, NULL, neras,
                          fio->erasures, 0, NULL);
}

/*
 * Read error-correcting codes for the requested RS block. Returns a pointer
 * to the data block. Caller is responsible for releasing buf.
 */
static u8 *fec_read_parity(struct dm_verity *v, u64 rsb, int index,
                           unsigned *offset, struct dm_buffer **buf)
{
        u64 position, block;
        u8 *res;

        position = (index + rsb) * v->fec->roots;
        block = position >> v->data_dev_block_bits;
        *offset = (unsigned)(position - (block << v->data_dev_block_bits));

        res = dm_bufio_read(v->fec->bufio, v->fec->start + block, buf);
        if (unlikely(IS_ERR(res))) {
                DMERR("%s: FEC %llu: parity read failed (block %llu): %ld",
                      v->data_dev->name, (unsigned long long)rsb,
                      (unsigned long long)(v->fec->start + block),
                      PTR_ERR(res));
                *buf = NULL;
        }

        return res;
}

/* Loop over each preallocated buffer slot. */
#define fec_for_each_prealloc_buffer(__i) \
        for (__i = 0; __i < DM_VERITY_FEC_BUF_PREALLOC; __i++)

/* Loop over each extra buffer slot. */
#define fec_for_each_extra_buffer(io, __i) \
        for (__i = DM_VERITY_FEC_BUF_PREALLOC; __i < DM_VERITY_FEC_BUF_MAX; __i++)

/* Loop over each allocated buffer. */
#define fec_for_each_buffer(io, __i) \
        for (__i = 0; __i < (io)->nbufs; __i++)

/* Loop over each RS block in each allocated buffer. */
#define fec_for_each_buffer_rs_block(io, __i, __j) \
        fec_for_each_buffer(io, __i) \
                for (__j = 0; __j < 1 << DM_VERITY_FEC_BUF_RS_BITS; __j++)

/*
 * Return a pointer to the current RS block when called inside
 * fec_for_each_buffer_rs_block.
 */
static inline u8 *fec_buffer_rs_block(struct dm_verity *v,
                                      struct dm_verity_fec_io *fio,
                                      unsigned i, unsigned j)
{
        return &fio->bufs[i][j * v->fec->rsn];
}

/*
 * Return an index to the current RS block when called inside
 * fec_for_each_buffer_rs_block.
 */
static inline unsigned fec_buffer_rs_index(unsigned i, unsigned j)
{
        return (i << DM_VERITY_FEC_BUF_RS_BITS) + j;
}

/*
 * Decode all RS blocks from buffers and copy corrected bytes into fio->output
 * starting from block_offset.
 */
static int fec_decode_bufs(struct dm_verity *v, struct dm_verity_fec_io *fio,
                           u64 rsb, int byte_index, unsigned block_offset,
                           int neras)
{
        int r, corrected = 0, res;
        struct dm_buffer *buf;
        unsigned n, i, offset;
        u8 *par, *block;

        par = fec_read_parity(v, rsb, block_offset, &offset, &buf);
        if (IS_ERR(par))
                return PTR_ERR(par);

        /*
         * Decode the RS blocks we have in bufs. Each RS block results in
         * one corrected target byte and consumes fec->roots parity bytes.
         */
        fec_for_each_buffer_rs_block(fio, n, i) {
                block = fec_buffer_rs_block(v, fio, n, i);
                res = fec_decode_rs8(v, fio, block, &par[offset], neras);
                if (res < 0) {
                        r = res;
                        goto error;
                }

                corrected += res;
                fio->output[block_offset] = block[byte_index];

                block_offset++;
                if (block_offset >= 1 << v->data_dev_block_bits)
                        goto done;

                /* read the next block when we run out of parity bytes */
                offset += v->fec->roots;
                if (offset >= 1 << v->data_dev_block_bits) {
                        dm_bufio_release(buf);

                        par = fec_read_parity(v, rsb, block_offset, &offset, &buf);
                        if (unlikely(IS_ERR(par)))
                                return PTR_ERR(par);
                }
        }
done:
        r = corrected;
error:
        dm_bufio_release(buf);

        if (r < 0 && neras)
                DMERR_LIMIT("%s: FEC %llu: failed to correct: %d",
                            v->data_dev->name, (unsigned long long)rsb, r);
        else if (r > 0)
                DMWARN_LIMIT("%s: FEC %llu: corrected %d errors",
                             v->data_dev->name, (unsigned long long)rsb, r);

        return r;
}

/*
 * Locate data block erasures using verity hashes.
 */
static int fec_is_erasure(struct dm_verity *v, struct dm_verity_io *io,
                          u8 *want_digest, u8 *data)
{
        if (unlikely(verity_hash(v, verity_io_hash_req(v, io),
                                 data, 1 << v->data_dev_block_bits,
                                 verity_io_real_digest(v, io))))
                return 0;

        return memcmp(verity_io_real_digest(v, io), want_digest,
                      v->digest_size) != 0;
}

/*
 * Read data blocks that are part of the RS block and deinterleave as much as
 * fits into buffers. Check for erasure locations if @neras is non-NULL.
 */
static int fec_read_bufs(struct dm_verity *v, struct dm_verity_io *io,
                         u64 rsb, u64 target, unsigned block_offset,
                         int *neras)
{
        bool is_zero;
        int i, j, target_index = -1;
        struct dm_buffer *buf;
        struct dm_bufio_client *bufio;
        struct dm_verity_fec_io *fio = fec_io(io);
        u64 block, ileaved;
        u8 *bbuf, *rs_block;
        u8 want_digest[v->digest_size];
        unsigned n, k;

        if (neras)
                *neras = 0;

        /*
         * read each of the rsn data blocks that are part of the RS block, and
         * interleave contents to available bufs
         */
        for (i = 0; i < v->fec->rsn; i++) {
                ileaved = fec_interleave(v, rsb * v->fec->rsn + i);

                /*
                 * target is the data block we want to correct, target_index is
                 * the index of this block within the rsn RS blocks
                 */
                if (ileaved == target)
                        target_index = i;

                block = ileaved >> v->data_dev_block_bits;
                bufio = v->fec->data_bufio;

                if (block >= v->data_blocks) {
                        block -= v->data_blocks;

                        /*
                         * blocks outside the area were assumed to contain
                         * zeros when encoding data was generated
                         */
                        if (unlikely(block >= v->fec->hash_blocks))
                                continue;

                        block += v->hash_start;
                        bufio = v->bufio;
                }

                bbuf = dm_bufio_read(bufio, block, &buf);
                if (unlikely(IS_ERR(bbuf))) {
                        DMWARN_LIMIT("%s: FEC %llu: read failed (%llu): %ld",
                                     v->data_dev->name,
                                     (unsigned long long)rsb,
                                     (unsigned long long)block, PTR_ERR(bbuf));

                        /* assume the block is corrupted */
                        if (neras && *neras <= v->fec->roots)
                                fio->erasures[(*neras)++] = i;

                        continue;
                }

                /* locate erasures if the block is on the data device */
                if (bufio == v->fec->data_bufio &&
                    verity_hash_for_block(v, io, block, want_digest,
                                          &is_zero) == 0) {
                        /* skip known zero blocks entirely */
                        if (is_zero)
                                goto done;

                        /*
                         * skip if we have already found the theoretical
                         * maximum number (i.e. fec->roots) of erasures
                         */
                        if (neras && *neras <= v->fec->roots &&
                            fec_is_erasure(v, io, want_digest, bbuf))
                                fio->erasures[(*neras)++] = i;
                }

                /*
                 * deinterleave and copy the bytes that fit into bufs,
                 * starting from block_offset
                 */
                fec_for_each_buffer_rs_block(fio, n, j) {
                        k = fec_buffer_rs_index(n, j) + block_offset;

                        if (k >= 1 << v->data_dev_block_bits)
                                goto done;

                        rs_block = fec_buffer_rs_block(v, fio, n, j);
                        rs_block[i] = bbuf[k];
                }
done:
                dm_bufio_release(buf);
        }

        return target_index;
}

/*
 * Allocate RS control structure and FEC buffers from preallocated mempools,
 * and attempt to allocate as many extra buffers as available.
 */
static int fec_alloc_bufs(struct dm_verity *v, struct dm_verity_fec_io *fio)
{
        unsigned n;

        if (!fio->rs)
                fio->rs = mempool_alloc(v->fec->rs_pool, GFP_NOIO);

        fec_for_each_prealloc_buffer(n) {
                if (fio->bufs[n])
                        continue;

                fio->bufs[n] = mempool_alloc(v->fec->prealloc_pool, GFP_NOWAIT);
                if (unlikely(!fio->bufs[n])) {
                        DMERR("failed to allocate FEC buffer");
                        return -ENOMEM;
                }
        }

        /* try to allocate the maximum number of buffers */
        fec_for_each_extra_buffer(fio, n) {
                if (fio->bufs[n])
                        continue;

                fio->bufs[n] = mempool_alloc(v->fec->extra_pool, GFP_NOWAIT);
                /* we can manage with even one buffer if necessary */
                if (unlikely(!fio->bufs[n]))
                        break;
        }
        fio->nbufs = n;

        if (!fio->output)
                fio->output = mempool_alloc(v->fec->output_pool, GFP_NOIO);

        return 0;
}

/*
 * Initialize buffers and clear erasures. fec_read_bufs() assumes buffers are
 * zeroed before deinterleaving.
 */
static void fec_init_bufs(struct dm_verity *v, struct dm_verity_fec_io *fio)
{
        unsigned n;

        fec_for_each_buffer(fio, n)
                memset(fio->bufs[n], 0, v->fec->rsn << DM_VERITY_FEC_BUF_RS_BITS);

        memset(fio->erasures, 0, sizeof(fio->erasures));
}

/*
 * Decode all RS blocks in a single data block and return the target block
 * (indicated by @offset) in fio->output. If @use_erasures is non-zero, uses
 * hashes to locate erasures.
 */
static int fec_decode_rsb(struct dm_verity *v, struct dm_verity_io *io,
                          struct dm_verity_fec_io *fio, u64 rsb, u64 offset,
                          bool use_erasures)
{
        int r, neras = 0;
        unsigned pos;

        r = fec_alloc_bufs(v, fio);
        if (unlikely(r < 0))
                return r;

        for (pos = 0; pos < 1 << v->data_dev_block_bits; ) {
                fec_init_bufs(v, fio);

                r = fec_read_bufs(v, io, rsb, offset, pos,
                                  use_erasures ? &neras : NULL);
                if (unlikely(r < 0))
                        return r;

                r = fec_decode_bufs(v, fio, rsb, r, pos, neras);
                if (r < 0)
                        return r;

                pos += fio->nbufs << DM_VERITY_FEC_BUF_RS_BITS;
        }

        /* Always re-validate the corrected block against the expected hash */
        r = verity_hash(v, verity_io_hash_req(v, io), fio->output,
                        1 << v->data_dev_block_bits,
                        verity_io_real_digest(v, io));
        if (unlikely(r < 0))
                return r;

        if (memcmp(verity_io_real_digest(v, io), verity_io_want_digest(v, io),
                   v->digest_size)) {
                DMERR_LIMIT("%s: FEC %llu: failed to correct (%d erasures)",
                            v->data_dev->name, (unsigned long long)rsb, neras);
                return -EILSEQ;
        }

        return 0;
}

static int fec_bv_copy(struct dm_verity *v, struct dm_verity_io *io, u8 *data,
                       size_t len)
{
        struct dm_verity_fec_io *fio = fec_io(io);

        memcpy(data, &fio->output[fio->output_pos], len);
        fio->output_pos += len;

        return 0;
}

/*
 * Correct errors in a block. Copies corrected block to dest if non-NULL,
 * otherwise to a bio_vec starting from iter.
 */
int verity_fec_decode(struct dm_verity *v, struct dm_verity_io *io,
                      enum verity_block_type type, sector_t block, u8 *dest,
                      struct bvec_iter *iter)
{
        int r;
        struct dm_verity_fec_io *fio = fec_io(io);
        u64 offset, res, rsb;

        if (!verity_fec_is_enabled(v))
                return -EOPNOTSUPP;

        if (fio->level >= DM_VERITY_FEC_MAX_RECURSION) {
                DMWARN_LIMIT("%s: FEC: recursion too deep", v->data_dev->name);
                return -EIO;
        }

        fio->level++;

        if (type == DM_VERITY_BLOCK_TYPE_METADATA)
                block += v->data_blocks;

        /*
         * For RS(M, N), the continuous FEC data is divided into blocks of N
         * bytes. Since block size may not be divisible by N, the last block
         * is zero padded when decoding.
         *
         * Each byte of the block is covered by a different RS(M, N) code,
         * and each code is interleaved over N blocks to make it less likely
         * that bursty corruption will leave us in unrecoverable state.
         */

        offset = block << v->data_dev_block_bits;
        res = div64_u64(offset, v->fec->rounds << v->data_dev_block_bits);

        /*
         * The base RS block we can feed to the interleaver to find out all
         * blocks required for decoding.
         */
        rsb = offset - res * (v->fec->rounds << v->data_dev_block_bits);

        /*
         * Locating erasures is slow, so attempt to recover the block without
         * them first. Do a second attempt with erasures if the corruption is
         * bad enough.
         */
        r = fec_decode_rsb(v, io, fio, rsb, offset, false);
        if (r < 0) {
                r = fec_decode_rsb(v, io, fio, rsb, offset, true);
                if (r < 0)
                        goto done;
        }

        if (dest)
                memcpy(dest, fio->output, 1 << v->data_dev_block_bits);
        else if (iter) {
                fio->output_pos = 0;
                r = verity_for_bv_block(v, io, iter, fec_bv_copy);
        }

done:
        fio->level--;
        return r;
}

/*
 * Clean up per-bio data.
 */
void verity_fec_finish_io(struct dm_verity_io *io)
{
        unsigned n;
        struct dm_verity_fec *f = io->v->fec;
        struct dm_verity_fec_io *fio = fec_io(io);

        if (!verity_fec_is_enabled(io->v))
                return;

        mempool_free(fio->rs, f->rs_pool);

        fec_for_each_prealloc_buffer(n)
                mempool_free(fio->bufs[n], f->prealloc_pool);

        fec_for_each_extra_buffer(fio, n)
                mempool_free(fio->bufs[n], f->extra_pool);

        mempool_free(fio->output, f->output_pool);
}

/*
 * Initialize per-bio data.
 */
void verity_fec_init_io(struct dm_verity_io *io)
{
        struct dm_verity_fec_io *fio = fec_io(io);

        if (!verity_fec_is_enabled(io->v))
                return;

        fio->rs = NULL;
        memset(fio->bufs, 0, sizeof(fio->bufs));
        fio->nbufs = 0;
        fio->output = NULL;
        fio->level = 0;
}

/*
 * Append feature arguments and values to the status table.
 */
unsigned verity_fec_status_table(struct dm_verity *v, unsigned sz,
                                 char *result, unsigned maxlen)
{
        if (!verity_fec_is_enabled(v))
                return sz;

        DMEMIT(" " DM_VERITY_OPT_FEC_DEV " %s "
               DM_VERITY_OPT_FEC_BLOCKS " %llu "
               DM_VERITY_OPT_FEC_START " %llu "
               DM_VERITY_OPT_FEC_ROOTS " %d",
               v->fec->dev->name,
               (unsigned long long)v->fec->blocks,
               (unsigned long long)v->fec->start,
               v->fec->roots);

        return sz;
}

void verity_fec_dtr(struct dm_verity *v)
{
        struct dm_verity_fec *f = v->fec;

        if (!verity_fec_is_enabled(v))
                goto out;

        mempool_destroy(f->rs_pool);
        mempool_destroy(f->prealloc_pool);
        mempool_destroy(f->extra_pool);
        kmem_cache_destroy(f->cache);

        if (f->data_bufio)
                dm_bufio_client_destroy(f->data_bufio);
        if (f->bufio)
                dm_bufio_client_destroy(f->bufio);

        if (f->dev)
                dm_put_device(v->ti, f->dev);
out:
        kfree(f);
        v->fec = NULL;
}

static void *fec_rs_alloc(gfp_t gfp_mask, void *pool_data)
{
        struct dm_verity *v = (struct dm_verity *)pool_data;

        return init_rs(8, 0x11d, 0, 1, v->fec->roots);
}

static void fec_rs_free(void *element, void *pool_data)
{
        struct rs_control *rs = (struct rs_control *)element;

        if (rs)
                free_rs(rs);
}

bool verity_is_fec_opt_arg(const char *arg_name)
{
        return (!strcasecmp(arg_name, DM_VERITY_OPT_FEC_DEV) ||
                !strcasecmp(arg_name, DM_VERITY_OPT_FEC_BLOCKS) ||
                !strcasecmp(arg_name, DM_VERITY_OPT_FEC_START) ||
                !strcasecmp(arg_name, DM_VERITY_OPT_FEC_ROOTS));
}

int verity_fec_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v,
                              unsigned *argc, const char *arg_name)
{
        int r;
        struct dm_target *ti = v->ti;
        const char *arg_value;
        unsigned long long num_ll;
        unsigned char num_c;
        char dummy;

        if (!*argc) {
                ti->error = "FEC feature arguments require a value";
                return -EINVAL;
        }

        arg_value = dm_shift_arg(as);
        (*argc)--;

        if (!strcasecmp(arg_name, DM_VERITY_OPT_FEC_DEV)) {
                r = dm_get_device(ti, arg_value, FMODE_READ, &v->fec->dev);
                if (r) {
                        ti->error = "FEC device lookup failed";
                        return r;
                }

        } else if (!strcasecmp(arg_name, DM_VERITY_OPT_FEC_BLOCKS)) {
                if (sscanf(arg_value, "%llu%c", &num_ll, &dummy) != 1 ||
                    ((sector_t)(num_ll << (v->data_dev_block_bits - SECTOR_SHIFT))
                     >> (v->data_dev_block_bits - SECTOR_SHIFT) != num_ll)) {
                        ti->error = "Invalid " DM_VERITY_OPT_FEC_BLOCKS;
                        return -EINVAL;
                }
                v->fec->blocks = num_ll;

        } else if (!strcasecmp(arg_name, DM_VERITY_OPT_FEC_START)) {
                if (sscanf(arg_value, "%llu%c", &num_ll, &dummy) != 1 ||
                    ((sector_t)(num_ll << (v->data_dev_block_bits - SECTOR_SHIFT)) >>
                     (v->data_dev_block_bits - SECTOR_SHIFT) != num_ll)) {
                        ti->error = "Invalid " DM_VERITY_OPT_FEC_START;
                        return -EINVAL;
                }
                v->fec->start = num_ll;

        } else if (!strcasecmp(arg_name, DM_VERITY_OPT_FEC_ROOTS)) {
                if (sscanf(arg_value, "%hhu%c", &num_c, &dummy) != 1 || !num_c ||
                    num_c < (DM_VERITY_FEC_RSM - DM_VERITY_FEC_MAX_RSN) ||
                    num_c > (DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN)) {
                        ti->error = "Invalid " DM_VERITY_OPT_FEC_ROOTS;
                        return -EINVAL;
                }
                v->fec->roots = num_c;

        } else {
                ti->error = "Unrecognized verity FEC feature request";
                return -EINVAL;
        }

        return 0;
}

/*
 * Allocate dm_verity_fec for v->fec. Must be called before verity_fec_ctr.
 */
int verity_fec_ctr_alloc(struct dm_verity *v)
{
        struct dm_verity_fec *f;

        f = kzalloc(sizeof(struct dm_verity_fec), GFP_KERNEL);
        if (!f) {
                v->ti->error = "Cannot allocate FEC structure";
                return -ENOMEM;
        }
        v->fec = f;

        return 0;
}

/*
 * Validate arguments and preallocate memory. Must be called after arguments
 * have been parsed using verity_fec_parse_opt_args.
 */
int verity_fec_ctr(struct dm_verity *v)
{
        struct dm_verity_fec *f = v->fec;
        struct dm_target *ti = v->ti;
        u64 hash_blocks;

        if (!verity_fec_is_enabled(v)) {
                verity_fec_dtr(v);
                return 0;
        }

        /*
         * FEC is computed over data blocks, possible metadata, and
         * hash blocks. In other words, FEC covers total of fec_blocks
         * blocks consisting of the following:
         *
         *  data blocks | hash blocks | metadata (optional)
         *
         * We allow metadata after hash blocks to support a use case
         * where all data is stored on the same device and FEC covers
         * the entire area.
         *
         * If metadata is included, we require it to be available on the
         * hash device after the hash blocks.
         */

        hash_blocks = v->hash_blocks - v->hash_start;

        /*
         * Require matching block sizes for data and hash devices for
         * simplicity.
         */
        if (v->data_dev_block_bits != v->hash_dev_block_bits) {
                ti->error = "Block sizes must match to use FEC";
                return -EINVAL;
        }

        if (!f->roots) {
                ti->error = "Missing " DM_VERITY_OPT_FEC_ROOTS;
                return -EINVAL;
        }
        f->rsn = DM_VERITY_FEC_RSM - f->roots;

        if (!f->blocks) {
                ti->error = "Missing " DM_VERITY_OPT_FEC_BLOCKS;
                return -EINVAL;
        }

        f->rounds = f->blocks;
        if (sector_div(f->rounds, f->rsn))
                f->rounds++;

        /*
         * Due to optional metadata, f->blocks can be larger than
         * data_blocks and hash_blocks combined.
         */
        if (f->blocks < v->data_blocks + hash_blocks || !f->rounds) {
                ti->error = "Invalid " DM_VERITY_OPT_FEC_BLOCKS;
                return -EINVAL;
        }

        /*
         * Metadata is accessed through the hash device, so we require
         * it to be large enough.
         */
        f->hash_blocks = f->blocks - v->data_blocks;
        if (dm_bufio_get_device_size(v->bufio) < f->hash_blocks) {
                ti->error = "Hash device is too small for "
                        DM_VERITY_OPT_FEC_BLOCKS;
                return -E2BIG;
        }

        f->bufio = dm_bufio_client_create(f->dev->bdev,
                                          1 << v->data_dev_block_bits,
                                          1, 0, NULL, NULL);
        if (IS_ERR(f->bufio)) {
                ti->error = "Cannot initialize FEC bufio client";
                return PTR_ERR(f->bufio);
        }

        if (dm_bufio_get_device_size(f->bufio) <
            ((f->start + f->rounds * f->roots) >> v->data_dev_block_bits)) {
                ti->error = "FEC device is too small";
                return -E2BIG;
        }

        f->data_bufio = dm_bufio_client_create(v->data_dev->bdev,
                                               1 << v->data_dev_block_bits,
                                               1, 0, NULL, NULL);
        if (IS_ERR(f->data_bufio)) {
                ti->error = "Cannot initialize FEC data bufio client";
                return PTR_ERR(f->data_bufio);
        }

        if (dm_bufio_get_device_size(f->data_bufio) < v->data_blocks) {
                ti->error = "Data device is too small";
                return -E2BIG;
        }

        /* Preallocate an rs_control structure for each worker thread */
        f->rs_pool = mempool_create(num_online_cpus(), fec_rs_alloc,
                                    fec_rs_free, (void *) v);
        if (!f->rs_pool) {
                ti->error = "Cannot allocate RS pool";
                return -ENOMEM;
        }

        f->cache = kmem_cache_create("dm_verity_fec_buffers",
                                     f->rsn << DM_VERITY_FEC_BUF_RS_BITS,
                                     0, 0, NULL);
        if (!f->cache) {
                ti->error = "Cannot create FEC buffer cache";
                return -ENOMEM;
        }

        /* Preallocate DM_VERITY_FEC_BUF_PREALLOC buffers for each thread */
        f->prealloc_pool = mempool_create_slab_pool(num_online_cpus() *
                                                    DM_VERITY_FEC_BUF_PREALLOC,
                                                    f->cache);
        if (!f->prealloc_pool) {
                ti->error = "Cannot allocate FEC buffer prealloc pool";
                return -ENOMEM;
        }

        f->extra_pool = mempool_create_slab_pool(0, f->cache);
        if (!f->extra_pool) {
                ti->error = "Cannot allocate FEC buffer extra pool";
                return -ENOMEM;
        }

        /* Preallocate an output buffer for each thread */
        f->output_pool = mempool_create_kmalloc_pool(num_online_cpus(),
                                                     1 << v->data_dev_block_bits);
        if (!f->output_pool) {
                ti->error = "Cannot allocate FEC output pool";
                return -ENOMEM;
        }

        /* Reserve space for our per-bio data */
        ti->per_io_data_size += sizeof(struct dm_verity_fec_io);

        return 0;
}