Re: [PATCH V5 02/23] physmem: qemu_ram_alloc_from_fd extensions

[Peter Xu <peterx@redhat.com>]

On Tue, Dec 24, 2024 at 08:16:47AM -0800, Steve Sistare wrote:
> Extend qemu_ram_alloc_from_fd to support resizable ram, and define
> qemu_ram_resize_cb to clean up the API.
> 
> Add a grow parameter to extend the file if necessary.  However, if
> grow is false, a zero-sized file is always extended.
> 
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> ---
>  include/exec/ram_addr.h | 13 +++++++++----
>  system/memory.c         |  4 ++--
>  system/physmem.c        | 35 ++++++++++++++++++++---------------
>  3 files changed, 31 insertions(+), 21 deletions(-)
> 
> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> index ff157c1..94bb3cc 100644
> --- a/include/exec/ram_addr.h
> +++ b/include/exec/ram_addr.h
> @@ -111,23 +111,30 @@ long qemu_maxrampagesize(void);
>   *
>   * Parameters:
>   *  @size: the size in bytes of the ram block
> + *  @max_size: the maximum size of the block after resizing
>   *  @mr: the memory region where the ram block is
> + *  @resized: callback after calls to qemu_ram_resize
>   *  @ram_flags: RamBlock flags. Supported flags: RAM_SHARED, RAM_PMEM,
>   *              RAM_NORESERVE, RAM_PROTECTED, RAM_NAMED_FILE, RAM_READONLY,
>   *              RAM_READONLY_FD, RAM_GUEST_MEMFD
>   *  @mem_path or @fd: specify the backing file or device
>   *  @offset: Offset into target file
> + *  @grow: extend file if necessary (but an empty file is always extended).
>   *  @errp: pointer to Error*, to store an error if it happens
>   *
>   * Return:
>   *  On success, return a pointer to the ram block.
>   *  On failure, return NULL.
>   */
> +typedef void (*qemu_ram_resize_cb)(const char *, uint64_t length, void *host);
> +
>  RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
>                                     uint32_t ram_flags, const char *mem_path,
>                                     off_t offset, Error **errp);
> -RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
> +RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, ram_addr_t max_size,
> +                                 qemu_ram_resize_cb resized, MemoryRegion *mr,
>                                   uint32_t ram_flags, int fd, off_t offset,
> +                                 bool grow,
>                                   Error **errp);
>  
>  RAMBlock *qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
> @@ -135,9 +142,7 @@ RAMBlock *qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
>  RAMBlock *qemu_ram_alloc(ram_addr_t size, uint32_t ram_flags, MemoryRegion *mr,
>                           Error **errp);
>  RAMBlock *qemu_ram_alloc_resizeable(ram_addr_t size, ram_addr_t max_size,
> -                                    void (*resized)(const char*,
> -                                                    uint64_t length,
> -                                                    void *host),
> +                                    qemu_ram_resize_cb resized,
>                                      MemoryRegion *mr, Error **errp);
>  void qemu_ram_free(RAMBlock *block);
>  
> diff --git a/system/memory.c b/system/memory.c
> index 78e17e0..290c522 100644
> --- a/system/memory.c
> +++ b/system/memory.c
> @@ -1680,8 +1680,8 @@ bool memory_region_init_ram_from_fd(MemoryRegion *mr,
>      mr->readonly = !!(ram_flags & RAM_READONLY);
>      mr->terminates = true;
>      mr->destructor = memory_region_destructor_ram;
> -    mr->ram_block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, offset,
> -                                           &err);
> +    mr->ram_block = qemu_ram_alloc_from_fd(size, size, NULL, mr, ram_flags, fd,
> +                                           offset, false, &err);
>      if (err) {
>          mr->size = int128_zero();
>          object_unparent(OBJECT(mr));
> diff --git a/system/physmem.c b/system/physmem.c
> index c76503a..48c544f 100644
> --- a/system/physmem.c
> +++ b/system/physmem.c
> @@ -1942,8 +1942,10 @@ out_free:
>  }
>  
>  #ifdef CONFIG_POSIX
> -RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
> +RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, ram_addr_t max_size,
> +                                 qemu_ram_resize_cb resized, MemoryRegion *mr,
>                                   uint32_t ram_flags, int fd, off_t offset,
> +                                 bool grow,
>                                   Error **errp)
>  {
>      RAMBlock *new_block;
> @@ -1953,7 +1955,9 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>      /* Just support these ram flags by now. */
>      assert((ram_flags & ~(RAM_SHARED | RAM_PMEM | RAM_NORESERVE |
>                            RAM_PROTECTED | RAM_NAMED_FILE | RAM_READONLY |
> -                          RAM_READONLY_FD | RAM_GUEST_MEMFD)) == 0);
> +                          RAM_READONLY_FD | RAM_GUEST_MEMFD |
> +                          RAM_RESIZEABLE)) == 0);
> +    assert(max_size >= size);
>  
>      if (xen_enabled()) {
>          error_setg(errp, "-mem-path not supported with Xen");
> @@ -1968,12 +1972,14 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>  
>      size = TARGET_PAGE_ALIGN(size);
>      size = REAL_HOST_PAGE_ALIGN(size);
> +    max_size = TARGET_PAGE_ALIGN(max_size);
> +    max_size = REAL_HOST_PAGE_ALIGN(max_size);
>  
>      file_size = get_file_size(fd);
> -    if (file_size > offset && file_size < (offset + size)) {
> +    if (file_size && file_size < offset + max_size && !grow) {

Is this a bugfix for the case offset < fsize?  If so, better make it a
small patch and copy stable..

$ touch ramfile
$ truncate -s 64M ramfile
$ ./qemu-system-x86_64 -object memory-backend-file,mem-path=./ramfile,offset=128M,size=128M,id=mem1,prealloc=on
qemu-system-x86_64: qemu_prealloc_mem: preallocating memory failed: Bad address

So yes, it's a bug..

>          error_setg(errp, "backing store size 0x%" PRIx64
>                     " does not match 'size' option 0x" RAM_ADDR_FMT,
> -                   file_size, size);
> +                   file_size, max_size);
>          return NULL;
>      }
>  
> @@ -1988,11 +1994,13 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>      new_block = g_malloc0(sizeof(*new_block));
>      new_block->mr = mr;
>      new_block->used_length = size;
> -    new_block->max_length = size;
> +    new_block->max_length = max_size;
> +    new_block->resized = resized;
>      new_block->flags = ram_flags;
>      new_block->guest_memfd = -1;
> -    new_block->host = file_ram_alloc(new_block, size, fd, !file_size, offset,
> -                                     errp);
> +    new_block->host = file_ram_alloc(new_block, max_size, fd,
> +                                     file_size < offset + max_size,

Same here, looks like relevant to above.

In general, we want to either fail properly detecting wrong-sized files, or
making sure qemu can boot correctly with the fd.  I confess pretty corner
cases.. but still.

So the whole patch looks correct, but maybe need a split.

> +                                     offset, errp);
>      if (!new_block->host) {
>          g_free(new_block);
>          return NULL;
> @@ -2044,7 +2052,8 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
>          return NULL;
>      }
>  
> -    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, offset, errp);
> +    block = qemu_ram_alloc_from_fd(size, size, NULL, mr, ram_flags, fd, offset,
> +                                   false, errp);
>      if (!block) {
>          if (created) {
>              unlink(mem_path);
> @@ -2059,9 +2068,7 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
>  
>  static
>  RAMBlock *qemu_ram_alloc_internal(ram_addr_t size, ram_addr_t max_size,
> -                                  void (*resized)(const char*,
> -                                                  uint64_t length,
> -                                                  void *host),
> +                                  qemu_ram_resize_cb resized,
>                                    void *host, uint32_t ram_flags,
>                                    MemoryRegion *mr, Error **errp)
>  {
> @@ -2113,10 +2120,8 @@ RAMBlock *qemu_ram_alloc(ram_addr_t size, uint32_t ram_flags,
>  }
>  
>  RAMBlock *qemu_ram_alloc_resizeable(ram_addr_t size, ram_addr_t maxsz,
> -                                     void (*resized)(const char*,
> -                                                     uint64_t length,
> -                                                     void *host),
> -                                     MemoryRegion *mr, Error **errp)
> +                                    qemu_ram_resize_cb resized,
> +                                    MemoryRegion *mr, Error **errp)
>  {
>      return qemu_ram_alloc_internal(size, maxsz, resized, NULL,
>                                     RAM_RESIZEABLE, mr, errp);
> -- 
> 1.8.3.1
> 

-- 
Peter Xu