Re: [Virtio-fs] [PATCH v8 7/7] Documentation/filesystem/dax: DAX on virtiofs

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Nov 25, 2021 at 03:05:30PM +0800, Jeffle Xu wrote:
> Record DAX on virtiofs and the semantic difference with that on ext4
> and xfs.
> 
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>

Reviewed-by: Vivek Goyal <vgoyal@redhat.com>

Vivek

> ---
>  Documentation/filesystems/dax.rst | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/filesystems/dax.rst b/Documentation/filesystems/dax.rst
> index 9a1b8fd9e82b..e3b30429d703 100644
> --- a/Documentation/filesystems/dax.rst
> +++ b/Documentation/filesystems/dax.rst
> @@ -23,8 +23,8 @@ on it as usual.  The `DAX` code currently only supports files with a block
>  size equal to your kernel's `PAGE_SIZE`, so you may need to specify a block
>  size when creating the filesystem.
>  
> -Currently 3 filesystems support `DAX`: ext2, ext4 and xfs.  Enabling `DAX` on them
> -is different.
> +Currently 4 filesystems support `DAX`: ext2, ext4, xfs and virtiofs.
> +Enabling `DAX` on them is different.
>  
>  Enabling DAX on ext2
>  --------------------
> @@ -168,6 +168,22 @@ if the underlying media does not support dax and/or the filesystem is
>  overridden with a mount option.
>  
>  
> +Enabling DAX on virtiofs
> +----------------------------
> +The semantic of DAX on virtiofs is basically equal to that on ext4 and xfs,
> +except that when '-o dax=inode' is specified, virtiofs client derives the hint
> +whether DAX shall be enabled or not from virtiofs server through FUSE protocol,
> +rather than the persistent `FS_XFLAG_DAX` flag. That is, whether DAX shall be
> +enabled or not is completely determined by virtiofs server, while virtiofs
> +server itself may deploy various algorithm making this decision, e.g. depending
> +on the persistent `FS_XFLAG_DAX` flag on the host.
> +
> +It is still supported to set or clear persistent `FS_XFLAG_DAX` flag inside
> +guest, but it is not guaranteed that DAX will be enabled or disabled for
> +corresponding file then. Users inside guest still need to call statx(2) and
> +check the statx flag `STATX_ATTR_DAX` to see if DAX is enabled for this file.
> +
> +
>  Implementation Tips for Block Driver Writers
>  --------------------------------------------
>  
> -- 
> 2.27.0
>