Re: [PATCH v2 1/1] fs/writeback: skip AS_NO_DATA_INTEGRITY mappings in wait_sb_inodes()

[Joanne Koong <joannelkoong@gmail.com>]

On Sun, Jan 4, 2026 at 10:54 AM David Hildenbrand (Red Hat)
<david@kernel.org> wrote:
>
> On 1/3/26 19:03, Andrew Morton wrote:
> > On Sun, 14 Dec 2025 19:00:43 -0800 Joanne Koong <joannelkoong@gmail.com> wrote:
> >
> >> Skip waiting on writeback for inodes that belong to mappings that do not
> >> have data integrity guarantees (denoted by the AS_NO_DATA_INTEGRITY
> >> mapping flag).
> >>
> >> This restores fuse back to prior behavior where syncs are no-ops. This
> >> is needed because otherwise, if a system is running a faulty fuse
> >> server that does not reply to issued write requests, this will cause
> >> wait_sb_inodes() to wait forever.
> >>
> >> Fixes: 0c58a97f919c ("fuse: remove tmp folio for writebacks and internal rb tree")
> >> Reported-by: Athul Krishna <athul.krishna.kr@protonmail.com>
> >> Reported-by: J. Neuschäfer <j.neuschaefer@gmx.net>
> >> Cc: stable@vger.kernel.org
> >> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> >>
> >> ..
> >>
> >> --- a/fs/fs-writeback.c
> >> +++ b/fs/fs-writeback.c
> >> @@ -2751,7 +2751,8 @@ static void wait_sb_inodes(struct super_block *sb)
> >>               * do not have the mapping lock. Skip it here, wb completion
> >>               * will remove it.
> >>               */
> >> -            if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
> >> +            if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK) ||
> >> +                mapping_no_data_integrity(mapping))
> >>                      continue;
> >
> > It's not obvious why a no-data-integrity mapping would want to skip
> > writeback - what do these things have to do with each other?
> >
> > So can we please have a v2 which has a comment here explaining this to the
> > reader?
>
> Sorry for not replying earlier, I missed a couple of mails sent to my
> @redhat address due to @gmail being force-unsubscribed from linux-mm ...
>
> Probably sufficient to add at the beginning of the commit:
>
> "Above the while() loop in wait_sb_inodes(), we document that we must
> wait for all pages under writeback for data integrity. Consequently, if
> a mapping, like fuse, traditionally does not have data integrity
> semantics, there is no need to wait at all; we can simply skip these inodes.
>
> So skip ..."

Sounds good, I'll send out v3 with these changes. Thanks for the
feedback, Andrew and David.

>
> --
> Cheers
>
> David