Re: [Qemu-devel] [PULL 2/7] exec: factor out duplicate mmap code

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/10/2015 10:49, Michael S. Tsirkin wrote:
> Anonymous and file-backed RAM allocation are now almost exactly the same.
> 
> Reduce code duplication by moving RAM mmap code out of oslib-posix.c and
> exec.c.
> 
> Reported-by: Marc-André Lureau <mlureau@redhat.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> Acked-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  include/qemu/mmap-alloc.h | 10 +++++++
>  exec.c                    | 47 +++++++------------------------
>  util/mmap-alloc.c         | 71 +++++++++++++++++++++++++++++++++++++++++++++++
>  util/oslib-posix.c        | 28 +++----------------
>  util/Makefile.objs        |  1 +
>  5 files changed, 96 insertions(+), 61 deletions(-)
>  create mode 100644 include/qemu/mmap-alloc.h
>  create mode 100644 util/mmap-alloc.c
> 
> diff --git a/include/qemu/mmap-alloc.h b/include/qemu/mmap-alloc.h
> new file mode 100644
> index 0000000..56388e6
> --- /dev/null
> +++ b/include/qemu/mmap-alloc.h
> @@ -0,0 +1,10 @@
> +#ifndef QEMU_MMAP_ALLOC
> +#define QEMU_MMAP_ALLOC
> +
> +#include "qemu-common.h"
> +
> +void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared);
> +
> +void qemu_ram_munmap(void *ptr, size_t size);
> +
> +#endif
> diff --git a/exec.c b/exec.c
> index 7d90a52..4505dc7 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -55,6 +55,9 @@
>  #include "exec/ram_addr.h"
>  
>  #include "qemu/range.h"
> +#ifndef _WIN32
> +#include "qemu/mmap-alloc.h"
> +#endif
>  
>  //#define DEBUG_SUBPAGE
>  
> @@ -84,9 +87,9 @@ static MemoryRegion io_mem_unassigned;
>   */
>  #define RAM_RESIZEABLE (1 << 2)
>  
> -/* An extra page is mapped on top of this RAM.
> +/* RAM is backed by an mmapped file.
>   */
> -#define RAM_EXTRA (1 << 3)
> +#define RAM_FILE (1 << 3)
>  #endif
>  
>  struct CPUTailQ cpus = QTAILQ_HEAD_INITIALIZER(cpus);
> @@ -1188,13 +1191,10 @@ static void *file_ram_alloc(RAMBlock *block,
>      char *filename;
>      char *sanitized_name;
>      char *c;
> -    void *ptr;
> -    void *area = NULL;
> +    void *area;
>      int fd;
>      uint64_t hpagesize;
> -    uint64_t total;
>      Error *local_err = NULL;
> -    size_t offset;
>  
>      hpagesize = gethugepagesize(path, &local_err);
>      if (local_err) {
> @@ -1238,7 +1238,6 @@ static void *file_ram_alloc(RAMBlock *block,
>      g_free(filename);
>  
>      memory = ROUND_UP(memory, hpagesize);
> -    total = memory + hpagesize;
>  
>      /*
>       * ftruncate is not supported by hugetlbfs in older
> @@ -1250,40 +1249,14 @@ static void *file_ram_alloc(RAMBlock *block,
>          perror("ftruncate");
>      }
>  
> -    ptr = mmap(0, total, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS,
> -                -1, 0);
> -    if (ptr == MAP_FAILED) {
> -        error_setg_errno(errp, errno,
> -                         "unable to allocate memory range for hugepages");
> -        close(fd);
> -        goto error;
> -    }
> -
> -    offset = QEMU_ALIGN_UP((uintptr_t)ptr, hpagesize) - (uintptr_t)ptr;
> -
> -    area = mmap(ptr + offset, memory, PROT_READ | PROT_WRITE,
> -                (block->flags & RAM_SHARED ? MAP_SHARED : MAP_PRIVATE) |
> -                MAP_FIXED,
> -                fd, 0);
> +    area = qemu_ram_mmap(fd, memory, hpagesize, block->flags & RAM_SHARED);
>      if (area == MAP_FAILED) {
>          error_setg_errno(errp, errno,
>                           "unable to map backing store for hugepages");
> -        munmap(ptr, total);
>          close(fd);
>          goto error;
>      }
>  
> -    if (offset > 0) {
> -        munmap(ptr, offset);
> -    }
> -    ptr += offset;
> -    total -= offset;
> -
> -    if (total > memory + getpagesize()) {
> -        munmap(ptr + memory + getpagesize(),
> -               total - memory - getpagesize());
> -    }
> -
>      if (mem_prealloc) {
>          os_mem_prealloc(fd, area, memory);
>      }
> @@ -1601,7 +1574,7 @@ ram_addr_t qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
>      new_block->used_length = size;
>      new_block->max_length = size;
>      new_block->flags = share ? RAM_SHARED : 0;
> -    new_block->flags |= RAM_EXTRA;
> +    new_block->flags |= RAM_FILE;
>      new_block->host = file_ram_alloc(new_block, size,
>                                       mem_path, errp);
>      if (!new_block->host) {
> @@ -1703,8 +1676,8 @@ static void reclaim_ramblock(RAMBlock *block)
>          xen_invalidate_map_cache_entry(block->host);
>  #ifndef _WIN32
>      } else if (block->fd >= 0) {
> -        if (block->flags & RAM_EXTRA) {
> -            munmap(block->host, block->max_length + getpagesize());
> +        if (block->flags & RAM_FILE) {
> +            qemu_ram_munmap(block->host, block->max_length);
>          } else {
>              munmap(block->host, block->max_length);
>          }
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> new file mode 100644
> index 0000000..1394269
> --- /dev/null
> +++ b/util/mmap-alloc.c
> @@ -0,0 +1,71 @@
> +/*
> + * 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.
> + */
> +#include <qemu/mmap-alloc.h>
> +#include <sys/types.h>
> +#include <sys/mman.h>
> +#include <assert.h>
> +
> +void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared)
> +{
> +    /*
> +     * Note: this always allocates at least one extra page of virtual address
> +     * space, even if size is already aligned.
> +     */
> +    size_t total = size + align;
> +    void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> +    size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
> +    void *ptr1;
> +
> +    if (ptr == MAP_FAILED) {
> +        return NULL;

Existing code is checking for MAP_FAILED (and NULL is a valid return
value of mmap), so you should return ptr.

> +    }
> +
> +    /* Make sure align is a power of 2 */
> +    assert(!(align & (align - 1)));
> +    /* Always align to host page size */
> +    assert(align >= getpagesize());
> +
> +    ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE,
> +                MAP_FIXED |
> +                (fd == -1 ? MAP_ANONYMOUS : 0) |
> +                (shared ? MAP_SHARED : MAP_PRIVATE),
> +                fd, 0);
> +    if (ptr1 == MAP_FAILED) {
> +        munmap(ptr, total);
> +        return NULL;

Same here---return ptr1 instead of NULL.

Paolo

> +    }
> +
> +    ptr += offset;
> +    total -= offset;
> +
> +    if (offset > 0) {
> +        munmap(ptr - offset, offset);
> +    }
> +
> +    /*
> +     * Leave a single PROT_NONE page allocated after the RAM block, to serve as
> +     * a guard page guarding against potential buffer overflows.
> +     */
> +    if (total > size + getpagesize()) {
> +        munmap(ptr + size + getpagesize(), total - size - getpagesize());
> +    }
> +
> +    return ptr;
> +}
> +
> +void qemu_ram_munmap(void *ptr, size_t size)
> +{
> +    if (ptr) {
> +        /* Unmap both the RAM block and the guard page */
> +        munmap(ptr, size + getpagesize());
> +    }
> +}
> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> index a0fcdc2..892d2d8 100644
> --- a/util/oslib-posix.c
> +++ b/util/oslib-posix.c
> @@ -72,6 +72,8 @@ extern int daemon(int, int);
>  #include <sys/sysctl.h>
>  #endif
>  
> +#include <qemu/mmap-alloc.h>
> +
>  int qemu_get_thread_id(void)
>  {
>  #if defined(__linux__)
> @@ -128,10 +130,7 @@ void *qemu_memalign(size_t alignment, size_t size)
>  void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment)
>  {
>      size_t align = QEMU_VMALLOC_ALIGN;
> -    size_t total = size + align;
> -    void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> -    size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
> -    void *ptr1;
> +    void *ptr = qemu_ram_mmap(-1, size, align, false);
>  
>      if (ptr == MAP_FAILED) {
>          return NULL;
> @@ -141,23 +140,6 @@ void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment)
>          *alignment = align;
>      }
>  
> -    ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE,
> -                MAP_FIXED | MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> -    if (ptr1 == MAP_FAILED) {
> -        munmap(ptr, total);
> -        return NULL;
> -    }
> -
> -    ptr += offset;
> -    total -= offset;
> -
> -    if (offset > 0) {
> -        munmap(ptr - offset, offset);
> -    }
> -    if (total > size + getpagesize()) {
> -        munmap(ptr + size + getpagesize(), total - size - getpagesize());
> -    }
> -
>      trace_qemu_anon_ram_alloc(size, ptr);
>      return ptr;
>  }
> @@ -171,9 +153,7 @@ void qemu_vfree(void *ptr)
>  void qemu_anon_ram_free(void *ptr, size_t size)
>  {
>      trace_qemu_anon_ram_free(ptr, size);
> -    if (ptr) {
> -        munmap(ptr, size + getpagesize());
> -    }
> +    qemu_ram_munmap(ptr, size);
>  }
>  
>  void qemu_set_block(int fd)
> diff --git a/util/Makefile.objs b/util/Makefile.objs
> index 114d657..f011b81 100644
> --- a/util/Makefile.objs
> +++ b/util/Makefile.objs
> @@ -1,6 +1,7 @@
>  util-obj-y = osdep.o cutils.o unicode.o qemu-timer-common.o
>  util-obj-$(CONFIG_WIN32) += oslib-win32.o qemu-thread-win32.o event_notifier-win32.o
>  util-obj-$(CONFIG_POSIX) += oslib-posix.o qemu-thread-posix.o event_notifier-posix.o qemu-openpty.o
> +util-obj-$(CONFIG_POSIX) += mmap-alloc.o
>  util-obj-y += envlist.o path.o module.o
>  util-obj-$(call lnot,$(CONFIG_INT128)) += host-utils.o
>  util-obj-y += bitmap.o bitops.o hbitmap.o
>