On Thu, May 4, 2023 at 3:55 PM Christoph Hellwig <hch@xxxxxx> 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..
Hi Christoph,
Right, I meant to say "when accessing a block device directly".
>
> > 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);
Heh, this is almost exactly what I came up with initially (|| arms were
swapped in that version), but decided to improve upon after noticing the
writeable thing.
Thanks,
Ilya
