util/mmap-alloc.c

   1 /*
   2  * Support for RAM backed by mmaped host memory.
   3  *
   4  * Copyright (c) 2015 Red Hat, Inc.
   5  *
   6  * Authors:
   7  *  Michael S. Tsirkin <mst@redhat.com>
   8  *
   9  * This work is licensed under the terms of the GNU GPL, version 2 or
  10  * later.  See the COPYING file in the top-level directory.
  11  */
  12
  13 #ifdef CONFIG_LINUX
  14 #include <linux/mman.h>
  15 #else  /* !CONFIG_LINUX */
  16 #define MAP_SYNC              0x0
  17 #define MAP_SHARED_VALIDATE   0x0
  18 #endif /* CONFIG_LINUX */
  19
  20 #include "qemu/osdep.h"
  21 #include "qemu/mmap-alloc.h"
  22 #include "qemu/host-utils.h"
  23 #include "qemu/cutils.h"
  24 #include "qemu/error-report.h"
  25
  26 #define HUGETLBFS_MAGIC       0x958458f6
  27
  28 #ifdef CONFIG_LINUX
  29 #include <sys/vfs.h>
  30 #include <linux/magic.h>
  31 #endif
  32
  33 QemuFsType qemu_fd_getfs(int fd)
  34 {
  35 #ifdef CONFIG_LINUX
  36     struct statfs fs;
  37     int ret;
  38
  39     if (fd < 0) {
  40         return QEMU_FS_TYPE_UNKNOWN;
  41     }
  42
  43     do {
  44         ret = fstatfs(fd, &fs);
  45     } while (ret != 0 && errno == EINTR);
  46
  47     switch (fs.f_type) {
  48     case TMPFS_MAGIC:
  49         return QEMU_FS_TYPE_TMPFS;
  50     case HUGETLBFS_MAGIC:
  51         return QEMU_FS_TYPE_HUGETLBFS;
  52     default:
  53         return QEMU_FS_TYPE_UNKNOWN;
  54     }
  55 #else
  56     return QEMU_FS_TYPE_UNKNOWN;
  57 #endif
  58 }
  59
  60 size_t qemu_fd_getpagesize(int fd)
  61 {
  62 #ifdef CONFIG_LINUX
  63     struct statfs fs;
  64     int ret;
  65
  66     if (fd != -1) {
  67         do {
  68             ret = fstatfs(fd, &fs);
  69         } while (ret != 0 && errno == EINTR);
  70
  71         if (ret == 0 && fs.f_type == HUGETLBFS_MAGIC) {
  72             return fs.f_bsize;
  73         }
  74     }
  75 #ifdef __sparc__
  76     /* SPARC Linux needs greater alignment than the pagesize */
  77     return QEMU_VMALLOC_ALIGN;
  78 #endif
  79 #endif
  80
  81     return qemu_real_host_page_size();
  82 }
  83
  84 #define OVERCOMMIT_MEMORY_PATH "/proc/sys/vm/overcommit_memory"
  85 static bool map_noreserve_effective(int fd, uint32_t qemu_map_flags)
  86 {
  87 #if defined(__linux__)
  88     const bool readonly = qemu_map_flags & QEMU_MAP_READONLY;
  89     const bool shared = qemu_map_flags & QEMU_MAP_SHARED;
  90     gchar *content = NULL;
  91     const char *endptr;
  92     unsigned int tmp;
  93
  94     /*
  95      * hugeltb accounting is different than ordinary swap reservation:
  96      * a) Hugetlb pages from the pool are reserved for both private and
  97      *    shared mappings. For shared mappings, all mappers have to specify
  98      *    MAP_NORESERVE.
  99      * b) MAP_NORESERVE is not affected by /proc/sys/vm/overcommit_memory.
 100      */
 101     if (qemu_fd_getpagesize(fd) != qemu_real_host_page_size()) {
 102         return true;
 103     }
 104
 105     /*
 106      * Accountable mappings in the kernel that can be affected by MAP_NORESEVE
 107      * are private writable mappings (see mm/mmap.c:accountable_mapping() in
 108      * Linux). For all shared or readonly mappings, MAP_NORESERVE is always
 109      * implicitly active -- no reservation; this includes shmem. The only
 110      * exception is shared anonymous memory, it is accounted like private
 111      * anonymous memory.
 112      */
 113     if (readonly || (shared && fd >= 0)) {
 114         return true;
 115     }
 116
 117     /*
 118      * MAP_NORESERVE is globally ignored for applicable !hugetlb mappings when
 119      * memory overcommit is set to "never". Sparse memory regions aren't really
 120      * possible in this system configuration.
 121      *
 122      * Bail out now instead of silently committing way more memory than
 123      * currently desired by the user.
 124      */
 125     if (g_file_get_contents(OVERCOMMIT_MEMORY_PATH, &content, NULL, NULL) &&
 126         !qemu_strtoui(content, &endptr, 0, &tmp) &&
 127         (!endptr || *endptr == '\n')) {
 128         if (tmp == 2) {
 129             error_report("Skipping reservation of swap space is not supported:"
 130                          " \"" OVERCOMMIT_MEMORY_PATH "\" is \"2\"");
 131             return false;
 132         }
 133         return true;
 134     }
 135     /* this interface has been around since Linux 2.6 */
 136     error_report("Skipping reservation of swap space is not supported:"
 137                  " Could not read: \"" OVERCOMMIT_MEMORY_PATH "\"");
 138     return false;
 139 #endif
 140     /*
 141      * E.g., FreeBSD used to define MAP_NORESERVE, never implemented it,
 142      * and removed it a while ago.
 143      */
 144     error_report("Skipping reservation of swap space is not supported");
 145     return false;
 146 }
 147
 148 /*
 149  * Reserve a new memory region of the requested size to be used for mapping
 150  * from the given fd (if any).
 151  */
 152 static void *mmap_reserve(size_t size, int fd)
 153 {
 154     int flags = MAP_PRIVATE;
 155
 156 #if defined(__powerpc64__) && defined(__linux__)
 157     /*
 158      * On ppc64 mappings in the same segment (aka slice) must share the same
 159      * page size. Since we will be re-allocating part of this segment
 160      * from the supplied fd, we should make sure to use the same page size, to
 161      * this end we mmap the supplied fd.  In this case, set MAP_NORESERVE to
 162      * avoid allocating backing store memory.
 163      * We do this unless we are using the system page size, in which case
 164      * anonymous memory is OK.
 165      */
 166     if (fd == -1 || qemu_fd_getpagesize(fd) == qemu_real_host_page_size()) {
 167         fd = -1;
 168         flags |= MAP_ANONYMOUS;
 169     } else {
 170         flags |= MAP_NORESERVE;
 171     }
 172 #else
 173     fd = -1;
 174     flags |= MAP_ANONYMOUS;
 175 #endif
 176
 177     return mmap(0, size, PROT_NONE, flags, fd, 0);
 178 }
 179
 180 /*
 181  * Activate memory in a reserved region from the given fd (if any), to make
 182  * it accessible.
 183  */
 184 static void *mmap_activate(void *ptr, size_t size, int fd,
 185                            uint32_t qemu_map_flags, off_t map_offset)
 186 {
 187     const bool noreserve = qemu_map_flags & QEMU_MAP_NORESERVE;
 188     const bool readonly = qemu_map_flags & QEMU_MAP_READONLY;
 189     const bool shared = qemu_map_flags & QEMU_MAP_SHARED;
 190     const bool sync = qemu_map_flags & QEMU_MAP_SYNC;
 191     const int prot = PROT_READ | (readonly ? 0 : PROT_WRITE);
 192     int map_sync_flags = 0;
 193     int flags = MAP_FIXED;
 194     void *activated_ptr;
 195
 196     if (noreserve && !map_noreserve_effective(fd, qemu_map_flags)) {
 197         return MAP_FAILED;
 198     }
 199
 200     flags |= fd == -1 ? MAP_ANONYMOUS : 0;
 201     flags |= shared ? MAP_SHARED : MAP_PRIVATE;
 202     flags |= noreserve ? MAP_NORESERVE : 0;
 203     if (shared && sync) {
 204         map_sync_flags = MAP_SYNC | MAP_SHARED_VALIDATE;
 205     }
 206
 207     activated_ptr = mmap(ptr, size, prot, flags | map_sync_flags, fd,
 208                          map_offset);
 209     if (activated_ptr == MAP_FAILED && map_sync_flags) {
 210         if (errno == ENOTSUP) {
 211             char *proc_link = g_strdup_printf("/proc/self/fd/%d", fd);
 212             char *file_name = g_malloc0(PATH_MAX);
 213             int len = readlink(proc_link, file_name, PATH_MAX - 1);
 214
 215             if (len < 0) {
 216                 len = 0;
 217             }
 218             file_name[len] = '\0';
 219             fprintf(stderr, "Warning: requesting persistence across crashes "
 220                     "for backend file %s failed. Proceeding without "
 221                     "persistence, data might become corrupted in case of host "
 222                     "crash.\n", file_name);
 223             g_free(proc_link);
 224             g_free(file_name);
 225             warn_report("Using non DAX backing file with 'pmem=on' option"
 226                         " is deprecated");
 227         }
 228         /*
 229          * If mmap failed with MAP_SHARED_VALIDATE | MAP_SYNC, we will try
 230          * again without these flags to handle backwards compatibility.
 231          */
 232         activated_ptr = mmap(ptr, size, prot, flags, fd, map_offset);
 233     }
 234     return activated_ptr;
 235 }
 236
 237 static inline size_t mmap_guard_pagesize(int fd)
 238 {
 239 #if defined(__powerpc64__) && defined(__linux__)
 240     /* Mappings in the same segment must share the same page size */
 241     return qemu_fd_getpagesize(fd);
 242 #else
 243     return qemu_real_host_page_size();
 244 #endif
 245 }
 246
 247 void *qemu_ram_mmap(int fd,
 248                     size_t size,
 249                     size_t align,
 250                     uint32_t qemu_map_flags,
 251                     off_t map_offset)
 252 {
 253     const size_t guard_pagesize = mmap_guard_pagesize(fd);
 254     size_t offset, total;
 255     void *ptr, *guardptr;
 256
 257     /*
 258      * Note: this always allocates at least one extra page of virtual address
 259      * space, even if size is already aligned.
 260      */
 261     total = size + align;
 262
 263     guardptr = mmap_reserve(total, fd);
 264     if (guardptr == MAP_FAILED) {
 265         return MAP_FAILED;
 266     }
 267
 268     assert(is_power_of_2(align));
 269     /* Always align to host page size */
 270     assert(align >= guard_pagesize);
 271
 272     offset = QEMU_ALIGN_UP((uintptr_t)guardptr, align) - (uintptr_t)guardptr;
 273
 274     ptr = mmap_activate(guardptr + offset, size, fd, qemu_map_flags,
 275                         map_offset);
 276     if (ptr == MAP_FAILED) {
 277         munmap(guardptr, total);
 278         return MAP_FAILED;
 279     }
 280
 281     if (offset > 0) {
 282         munmap(guardptr, offset);
 283     }
 284
 285     /*
 286      * Leave a single PROT_NONE page allocated after the RAM block, to serve as
 287      * a guard page guarding against potential buffer overflows.
 288      */
 289     total -= offset;
 290     if (total > size + guard_pagesize) {
 291         munmap(ptr + size + guard_pagesize, total - size - guard_pagesize);
 292     }
 293
 294     return ptr;
 295 }
 296
 297 void qemu_ram_munmap(int fd, void *ptr, size_t size)
 298 {
 299     if (ptr) {
 300         /* Unmap both the RAM block and the guard page */
 301         munmap(ptr, size + mmap_guard_pagesize(fd));
 302     }
 303 }