Re: [Qemu-devel] [PATCH] memory: fetch pmem size in get_file_size()

[Eduardo Habkost]

On Fri, Aug 30, 2019 at 10:30:56AM +0100, Stefan Hajnoczi wrote:
> Neither stat(2) nor lseek(2) report the size of Linux devdax pmem
> character device nodes.  Commit 314aec4a6e06844937f1677f6cba21981005f389
> ("hostmem-file: reject invalid pmem file sizes") added code to
> hostmem-file.c to fetch the size from sysfs and compare against the
> user-provided size=NUM parameter:
> 
>   if (backend->size > size) {
>       error_setg(errp, "size property %" PRIu64 " is larger than "
>                  "pmem file \"%s\" size %" PRIu64, backend->size,
>                  fb->mem_path, size);
>       return;
>   }
> 
> It turns out that exec.c:qemu_ram_alloc_from_fd() already has an
> equivalent size check but it skips devdax pmem character devices because
> lseek(2) returns 0:
> 
>   if (file_size > 0 && file_size < size) {
>       error_setg(errp, "backing store %s size 0x%" PRIx64
>                  " does not match 'size' option 0x" RAM_ADDR_FMT,
>                  mem_path, file_size, size);
>       return NULL;
>   }
> 
> This patch moves the devdax pmem file size code into get_file_size() so
> that we check the memory size in a single place:
> qemu_ram_alloc_from_fd().  This simplifies the code and makes it more
> general.
> 
> This also fixes the problem that hostmem-file only checks the devdax
> pmem file size when the pmem=on parameter is given.  An unchecked
> size=NUM parameter can lead to SIGBUS in QEMU so we must always fetch
> the file size for Linux devdax pmem character device nodes.
> 
> Signed-off-by: Stefan Hajnoczi <address@hidden>

Reviewed-by: Eduardo Habkost <address@hidden>

Paolo, do you want to queue this, or should it go through my
memory backend queue?

-- 
Eduardo