util/mmap-alloc.c

/*
 * Support for RAM backed by mmaped host memory.
 *
 * Copyright (c) 2015 Red Hat, Inc.
 *
 * Authors:
 *  Michael S. Tsirkin <mst@redhat.com>
 *
 * This work is licensed under the terms of the GNU GPL, version 2 or
 * later.  See the COPYING file in the top-level directory.
 */

#ifdef CONFIG_LINUX
#include <linux/mman.h>
#else  /* !CONFIG_LINUX */
#define MAP_SYNC              0x0
#define MAP_SHARED_VALIDATE   0x0
#endif /* CONFIG_LINUX */

#include "qemu/osdep.h"
#include "qemu/mmap-alloc.h"
#include "qemu/host-utils.h"
#include "qemu/cutils.h"
#include "qemu/error-report.h"

#define HUGETLBFS_MAGIC       0x958458f6

#ifdef CONFIG_LINUX
#include <sys/vfs.h>
#endif

size_t qemu_fd_getpagesize(int fd)
{
#ifdef CONFIG_LINUX
    struct statfs fs;
    int ret;

    if (fd != -1) {
        do {
            ret = fstatfs(fd, &fs);
        } while (ret != 0 && errno == EINTR);

        if (ret == 0 && fs.f_type == HUGETLBFS_MAGIC) {
            return fs.f_bsize;
        }
    }
#ifdef __sparc__
    /* SPARC Linux needs greater alignment than the pagesize */
    return QEMU_VMALLOC_ALIGN;
#endif
#endif

    return qemu_real_host_page_size();
}

#define OVERCOMMIT_MEMORY_PATH "/proc/sys/vm/overcommit_memory"
static bool map_noreserve_effective(int fd, uint32_t qemu_map_flags)
{
#if defined(__linux__)
    const bool readonly = qemu_map_flags & QEMU_MAP_READONLY;
    const bool shared = qemu_map_flags & QEMU_MAP_SHARED;
    gchar *content = NULL;
    const char *endptr;
    unsigned int tmp;

    /*
     * hugeltb accounting is different than ordinary swap reservation:
     * a) Hugetlb pages from the pool are reserved for both private and
     *    shared mappings. For shared mappings, all mappers have to specify
     *    MAP_NORESERVE.
     * b) MAP_NORESERVE is not affected by /proc/sys/vm/overcommit_memory.
     */
    if (qemu_fd_getpagesize(fd) != qemu_real_host_page_size()) {
        return true;
    }

    /*
     * Accountable mappings in the kernel that can be affected by MAP_NORESEVE
     * are private writable mappings (see mm/mmap.c:accountable_mapping() in
     * Linux). For all shared or readonly mappings, MAP_NORESERVE is always
     * implicitly active -- no reservation; this includes shmem. The only
     * exception is shared anonymous memory, it is accounted like private
     * anonymous memory.
     */
    if (readonly || (shared && fd >= 0)) {
        return true;
    }

    /*
     * MAP_NORESERVE is globally ignored for applicable !hugetlb mappings when
     * memory overcommit is set to "never". Sparse memory regions aren't really
     * possible in this system configuration.
     *
     * Bail out now instead of silently committing way more memory than
     * currently desired by the user.
     */
    if (g_file_get_contents(OVERCOMMIT_MEMORY_PATH, &content, NULL, NULL) &&
        !qemu_strtoui(content, &endptr, 0, &tmp) &&
        (!endptr || *endptr == '\n')) {
        if (tmp == 2) {
            error_report("Skipping reservation of swap space is not supported:"
                         " \"" OVERCOMMIT_MEMORY_PATH "\" is \"2\"");
            return false;
        }
        return true;
    }
    /* this interface has been around since Linux 2.6 */
    error_report("Skipping reservation of swap space is not supported:"
                 " Could not read: \"" OVERCOMMIT_MEMORY_PATH "\"");
    return false;
#endif
    /*
     * E.g., FreeBSD used to define MAP_NORESERVE, never implemented it,
     * and removed it a while ago.
     */
    error_report("Skipping reservation of swap space is not supported");
    return false;
}

/*
 * Reserve a new memory region of the requested size to be used for mapping
 * from the given fd (if any).
 */
static void *mmap_reserve(size_t size, int fd)
{
    int flags = MAP_PRIVATE;

#if defined(__powerpc64__) && defined(__linux__)
    /*
     * On ppc64 mappings in the same segment (aka slice) must share the same
     * page size. Since we will be re-allocating part of this segment
     * from the supplied fd, we should make sure to use the same page size, to
     * this end we mmap the supplied fd.  In this case, set MAP_NORESERVE to
     * avoid allocating backing store memory.
     * We do this unless we are using the system page size, in which case
     * anonymous memory is OK.
     */
    if (fd == -1 || qemu_fd_getpagesize(fd) == qemu_real_host_page_size()) {
        fd = -1;
        flags |= MAP_ANONYMOUS;
    } else {
        flags |= MAP_NORESERVE;
    }
#else
    fd = -1;
    flags |= MAP_ANONYMOUS;
#endif

    return mmap(0, size, PROT_NONE, flags, fd, 0);
}

/*
 * Activate memory in a reserved region from the given fd (if any), to make
 * it accessible.
 */
static void *mmap_activate(void *ptr, size_t size, int fd,
                           uint32_t qemu_map_flags, off_t map_offset)
{
    const bool noreserve = qemu_map_flags & QEMU_MAP_NORESERVE;
    const bool readonly = qemu_map_flags & QEMU_MAP_READONLY;
    const bool shared = qemu_map_flags & QEMU_MAP_SHARED;
    const bool sync = qemu_map_flags & QEMU_MAP_SYNC;
    const int prot = PROT_READ | (readonly ? 0 : PROT_WRITE);
    int map_sync_flags = 0;
    int flags = MAP_FIXED;
    void *activated_ptr;

    if (noreserve && !map_noreserve_effective(fd, qemu_map_flags)) {
        return MAP_FAILED;
    }

    flags |= fd == -1 ? MAP_ANONYMOUS : 0;
    flags |= shared ? MAP_SHARED : MAP_PRIVATE;
    flags |= noreserve ? MAP_NORESERVE : 0;
    if (shared && sync) {
        map_sync_flags = MAP_SYNC | MAP_SHARED_VALIDATE;
    }

    activated_ptr = mmap(ptr, size, prot, flags | map_sync_flags, fd,
                         map_offset);
    if (activated_ptr == MAP_FAILED && map_sync_flags) {
        if (errno == ENOTSUP) {
            char *proc_link = g_strdup_printf("/proc/self/fd/%d", fd);
            char *file_name = g_malloc0(PATH_MAX);
            int len = readlink(proc_link, file_name, PATH_MAX - 1);

            if (len < 0) {
                len = 0;
            }
            file_name[len] = '\0';
            fprintf(stderr, "Warning: requesting persistence across crashes "
                    "for backend file %s failed. Proceeding without "
                    "persistence, data might become corrupted in case of host "
                    "crash.\n", file_name);
            g_free(proc_link);
            g_free(file_name);
            warn_report("Using non DAX backing file with 'pmem=on' option"
                        " is deprecated");
        }
        /*
         * If mmap failed with MAP_SHARED_VALIDATE | MAP_SYNC, we will try
         * again without these flags to handle backwards compatibility.
         */
        activated_ptr = mmap(ptr, size, prot, flags, fd, map_offset);
    }
    return activated_ptr;
}

static inline size_t mmap_guard_pagesize(int fd)
{
#if defined(__powerpc64__) && defined(__linux__)
    /* Mappings in the same segment must share the same page size */
    return qemu_fd_getpagesize(fd);
#else
    return qemu_real_host_page_size();
#endif
}

void *qemu_ram_mmap(int fd,
                    size_t size,
                    size_t align,
                    uint32_t qemu_map_flags,
                    off_t map_offset)
{
    const size_t guard_pagesize = mmap_guard_pagesize(fd);
    size_t offset, total;
    void *ptr, *guardptr;

    /*
     * Note: this always allocates at least one extra page of virtual address
     * space, even if size is already aligned.
     */
    total = size + align;

    guardptr = mmap_reserve(total, fd);
    if (guardptr == MAP_FAILED) {
        return MAP_FAILED;
    }

    assert(is_power_of_2(align));
    /* Always align to host page size */
    assert(align >= guard_pagesize);

    offset = QEMU_ALIGN_UP((uintptr_t)guardptr, align) - (uintptr_t)guardptr;

    ptr = mmap_activate(guardptr + offset, size, fd, qemu_map_flags,
                        map_offset);
    if (ptr == MAP_FAILED) {
        munmap(guardptr, total);
        return MAP_FAILED;
    }

    if (offset > 0) {
        munmap(guardptr, offset);
    }

    /*
     * Leave a single PROT_NONE page allocated after the RAM block, to serve as
     * a guard page guarding against potential buffer overflows.
     */
    total -= offset;
    if (total > size + guard_pagesize) {
        munmap(ptr + size + guard_pagesize, total - size - guard_pagesize);
    }

    return ptr;
}

void qemu_ram_munmap(int fd, void *ptr, size_t size)
{
    if (ptr) {
        /* Unmap both the RAM block and the guard page */
        munmap(ptr, size + mmap_guard_pagesize(fd));
    }
}