Re: [PATCH] mm: always respect QUEUE_FLAG_STABLE_WRITES on the block device

[Christoph Hellwig <hch@lst.de>]

On Thu, May 04, 2023 at 12:56:24PM +0200, Ilya Dryomov wrote:
> Commit 1cb039f3dc16 ("bdi: replace BDI_CAP_STABLE_WRITES with a queue
> and a sb flag") introduced a regression for the raw block device use
> case.  Capturing QUEUE_FLAG_STABLE_WRITES flag in set_bdev_super() has
> the effect of respecting it only when there is a filesystem mounted on
> top of the block device.  If a filesystem is not mounted, block devices
> that do integrity checking return sporadic checksum errors.

With "If a file system is not mounted" you want to say "when accessing
a block device directly" here, right?  The two are not exclusive..

> Additionally, this commit made the corresponding sysfs knob writeable
> for debugging purposes.  However, because QUEUE_FLAG_STABLE_WRITES flag
> is captured when the filesystem is mounted and isn't consulted after
> that anywhere outside of swap code, changing it doesn't take immediate
> effect even though dumping the knob shows the new value.  With no way
> to dump SB_I_STABLE_WRITES flag, this is needlessly confusing.

But very much intentional.  s_bdev often is not the only device
in a file system, and we should never reference if from core
helpers.

So I think we should go with something like this:

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index db794399900734..aa36cc2a4530c1 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -3129,7 +3129,11 @@ EXPORT_SYMBOL_GPL(folio_wait_writeback_killable);
  */
 void folio_wait_stable(struct folio *folio)
 {
-	if (folio_inode(folio)->i_sb->s_iflags & SB_I_STABLE_WRITES)
+	struct inode *inode = folio_inode(folio);
+	struct super_block *sb = inode->i_sb;
+
+	if ((sb->s_iflags & SB_I_STABLE_WRITES) ||
+	    (sb_is_blkdev_sb(sb) && bdev_stable_writes(I_BDEV(inode))))
 		folio_wait_writeback(folio);
 }
 EXPORT_SYMBOL_GPL(folio_wait_stable);