On Mon, Jan 24, 2022 at 06:06:28PM +0800, Ming Lei wrote:
> If backing file's filesystem has implemented ->fallocate(), we think the
> loop device can support discard, then pass sb->s_blocksize as
> discard_granularity. However, some underlying FS, such as overlayfs,
> doesn't set sb->s_blocksize, and causes discard_granularity to be set as
> zero, then the warning in __blkdev_issue_discard() is triggered.
[ Copying linux-unionfs and Miklos ]
Miklos mentioned that it might be ok to copy upper->s_blocksize into
ovl->s_blocksize in overlayfs as other anonymous filesystem set it to
some value (nfs, 9p, fuse, cifs) as well.
So it might be reasonable to fix overlayfs as well. But I think we need
a block layer fix as well to deal with any filesystem which supports
->fallocate() and does not advertize ->s_blocksize. Not advertizing
->s_blocksize will probably only lead to suboptimial performance and
nothing more.
>
> Fix the issue by setting discard_granularity as PAGE_SIZE in this case
> since PAGE_SIZE is the most common data unit for FS.
>
> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Reported-by: Pei Zhang <pezhang@xxxxxxxxxx>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
Acked-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
Vivek
> ---
> drivers/block/loop.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index b1b05c45c07c..8c15bfab7e1a 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -776,6 +776,10 @@ static void loop_config_discard(struct loop_device *lo)
> } else {
> max_discard_sectors = UINT_MAX >> 9;
> granularity = inode->i_sb->s_blocksize;
> +
> + /* Take PAGE_SIZE if the FS doesn't provide us one hint */
> + if (!granularity)
> + granularity = PAGE_SIZE;
> }
>
> if (max_discard_sectors) {
> --
> 2.31.1
>
