Re: [PATCH] block: loop: set discard_granularity as PAGE_SIZE if sb->s_blocksize is 0

Re: [PATCH] block: loop: set discard_granularity as PAGE_SIZE if sb->s_blocksize is 0

[Vivek Goyal]

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@redhat.com>
> Reported-by: Pei Zhang <pezhang@redhat.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>

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

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
> 

Re: [PATCH] block: loop: set discard_granularity as PAGE_SIZE if sb->s_blocksize is 0

[Vivek Goyal]

On Mon, Jan 24, 2022 at 06:05:07AM -0800, Christoph Hellwig wrote:
> 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.
> > 
> > Fix the issue by setting discard_granularity as PAGE_SIZE in this case
> > since PAGE_SIZE is the most common data unit for FS.
> 
> sb->s_blocksize really does not mean anything.  kstat.blksize might
> be a better choice, even if it someimes errs on the too large side.

[ CC linux-unionfs, Miklos ]

This should work well for overlayfs too. I see it just passes the query
to underlying filesystem and that should report optimal I/O size.

On my overlayfs instance, I see.

# stat -c '%o' foo.txt
4096

Vivek