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qemu/util/mmap-alloc.c

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/*
* 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;
}
size_t qemu_mempath_getpagesize(const char *mem_path)
{
#ifdef CONFIG_LINUX
struct statfs fs;
int ret;
if (mem_path) {
do {
ret = statfs(mem_path, &fs);
} while (ret != 0 && errno == EINTR);
if (ret != 0) {
fprintf(stderr, "Couldn't statfs() memory path: %s\n",
strerror(errno));
exit(1);
}
if (fs.f_type == HUGETLBFS_MAGIC) {
/* It's hugepage, return the huge page size */
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);
}
/*
* 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));
}
}
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