[PATCH V2] block: loop:use kstatfs.f_bsize of backing file to set discard granularity

[PATCH V2] block: loop:use kstatfs.f_bsize of backing file to set discard granularity

[Ming Lei]

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.

Christoph suggested to pass kstatfs.f_bsize as discard granularity, and
this way is fine because kstatfs.f_bsize means 'Optimal transfer block
size', which still matches with definition of discard granularity.

So fix the issue by setting discard_granularity as kstatfs.f_bsize if it
is available, otherwise fackback to PAGE_SIZE.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Vivek Goyal <vgoyal@redhat.com>
Reported-by: Pei Zhang <pezhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
V2:
	- take Christoph's suggestion to use kstatfs.f_bsize

 drivers/block/loop.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index b1b05c45c07c..0991f08f79d7 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -79,6 +79,7 @@
 #include <linux/ioprio.h>
 #include <linux/blk-cgroup.h>
 #include <linux/sched/mm.h>
+#include <linux/statfs.h>
 
 #include "loop.h"
 
@@ -774,8 +775,13 @@ static void loop_config_discard(struct loop_device *lo)
 		granularity = 0;
 
 	} else {
+		struct kstatfs sbuf;
+
 		max_discard_sectors = UINT_MAX >> 9;
-		granularity = inode->i_sb->s_blocksize;
+		if (!vfs_statfs(&file->f_path, &sbuf))
+			granularity = sbuf.f_bsize;
+		else
+			granularity = PAGE_SIZE;
 	}
 
 	if (max_discard_sectors) {
-- 
2.31.1

Re: [PATCH V2] block: loop:use kstatfs.f_bsize of backing file to set discard granularity

[Christoph Hellwig]

On Tue, Jan 25, 2022 at 12:40:05PM +0800, Ming Lei wrote:
>  	} else {
> +		struct kstatfs sbuf;
> +
>  		max_discard_sectors = UINT_MAX >> 9;
> -		granularity = inode->i_sb->s_blocksize;
> +		if (!vfs_statfs(&file->f_path, &sbuf))
> +			granularity = sbuf.f_bsize;
> +		else
> +			granularity = PAGE_SIZE;

If vfs_statfs fails we're pretty much toast and there isn't really any
point in continuing here.

Re: [PATCH V2] block: loop:use kstatfs.f_bsize of backing file to set discard granularity

[Ming Lei]

On Tue, Jan 25, 2022 at 07:10:57AM +0100, Christoph Hellwig wrote:
> On Tue, Jan 25, 2022 at 12:40:05PM +0800, Ming Lei wrote:
> >  	} else {
> > +		struct kstatfs sbuf;
> > +
> >  		max_discard_sectors = UINT_MAX >> 9;
> > -		granularity = inode->i_sb->s_blocksize;
> > +		if (!vfs_statfs(&file->f_path, &sbuf))
> > +			granularity = sbuf.f_bsize;
> > +		else
> > +			granularity = PAGE_SIZE;
> 
> If vfs_statfs fails we're pretty much toast and there isn't really any
> point in continuing here.

But it is configure code path, even though vfs_statfs() fails,
loop_config_discard() still need to keep discard setting consistent.

Setting granularity as PAGE_SIZE is just for making discard granularity
matched with max_discard_sectors. Or you have better/simpler handling?


Thanks,
Ming

Re: [PATCH V2] block: loop:use kstatfs.f_bsize of backing file to set discard granularity

[Christoph Hellwig]

On Tue, Jan 25, 2022 at 05:27:36PM +0800, Ming Lei wrote:
> 
> But it is configure code path, even though vfs_statfs() fails,
> loop_config_discard() still need to keep discard setting consistent.
> 
> Setting granularity as PAGE_SIZE is just for making discard granularity
> matched with max_discard_sectors. Or you have better/simpler handling?

I'd fail the entire configuration preferably. Or at least not support
discard at all.