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

[Matthew Wilcox <willy@infradead.org>]

On Thu, May 04, 2023 at 03:55:15PM +0200, Christoph Hellwig wrote:
> 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);

I hate both of these patches ;-)  What we should do is add
AS_STABLE_WRITES, have the appropriate places call
mapping_set_stable_writes() and then folio_wait_stable() becomes

	if (mapping_test_stable_writes(folio->mapping))
		folio_wait_writeback(folio);

and we remove all the dereferences (mapping->host->i_sb->s_iflags, plus
whatever else is going on there)