linux-kernel - Re: [PATCH 4/5] ceph: Assert writeback loop invariants

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Message-ID: <CAH5Ym4ig7uBdereXpL8T3Cjn1zqzRxG1VwXb59rwHQjTQKWrPw@mail.gmail.com>
Date: Mon, 5 Jan 2026 22:53:17 -0800
From: Sam Edwards <cfsworks@...il.com>
To: Viacheslav Dubeyko <Slava.Dubeyko@....com>
Cc: Xiubo Li <xiubli@...hat.com>, "idryomov@...il.com" <idryomov@...il.com>, 
	Milind Changire <mchangir@...hat.com>, 
	"ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>, "brauner@...nel.org" <brauner@...nel.org>, 
	"jlayton@...nel.org" <jlayton@...nel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] ceph: Assert writeback loop invariants

On Mon, Jan 5, 2026 at 2:29 PM Viacheslav Dubeyko <Slava.Dubeyko@....com> wrote:
>
> On Tue, 2025-12-30 at 18:43 -0800, Sam Edwards wrote:
> > If `locked_pages` is zero, the page array must not be allocated:
> > ceph_process_folio_batch() uses `locked_pages` to decide when to
> > allocate `pages`, and redundant allocations trigger
> > ceph_allocate_page_array()'s BUG_ON(), resulting in a worker oops (and
> > writeback stall) or even a kernel panic. Consequently, the main loop in
> > ceph_writepages_start() assumes that the lifetime of `pages` is confined
> > to a single iteration.
> >
> > This expectation is currently not clear enough, as evidenced by the
> > previous two patches which fix oopses caused by `pages` persisting into
> > the next loop iteration.
> >
> > Use an explicit BUG_ON() at the top of the loop to assert the loop's
> > preexisting expectation that `pages` is cleaned up by the previous
> > iteration. Because this is closely tied to `locked_pages`, also make it
> > the previous iteration's responsibility to guarantee its reset, and
> > verify with a second new BUG_ON() instead of handling (and masking)
> > failures to do so.
> >
> > Signed-off-by: Sam Edwards <CFSworks@...il.com>
> > ---
> >  fs/ceph/addr.c | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> > index 91cc43950162..b3569d44d510 100644
> > --- a/fs/ceph/addr.c
> > +++ b/fs/ceph/addr.c
> > @@ -1669,7 +1669,9 @@ static int ceph_writepages_start(struct address_space *mapping,
> >               tag_pages_for_writeback(mapping, ceph_wbc.index, ceph_wbc.end);
> >
> >       while (!has_writeback_done(&ceph_wbc)) {
> > -             ceph_wbc.locked_pages = 0;
> > +             BUG_ON(ceph_wbc.locked_pages);
> > +             BUG_ON(ceph_wbc.pages);
> > +
>

Hi Slava,

> It's not good idea to introduce BUG_ON() in write pages logic. I am definitely
> against these two BUG_ON() here.

I share your distaste for BUG_ON() in writeback. However, the
BUG_ON(ceph_wbc.pages); already exists in ceph_allocate_page_array().
This patch is trying to catch that earlier, where it's easier to
troubleshoot. If I changed these to WARN_ON(), it would not prevent
the oops.

And the writeback code has a lot of BUG_ON() checks already (so I am
not "introducing" BUG_ON), but I suppose you could be saying "it's
already a problem, please don't make it worse."

If I introduce a ceph_discard_page_array() function (as discussed on
patch 4), I could call it at the top of the loop (to *ensure* a clean
state) instead of using BUG_ON() (to *assert* a clean state). I'd like
to hear from other reviewers which approach they'd prefer.

>
> >               ceph_wbc.max_pages = ceph_wbc.wsize >> PAGE_SHIFT;
> >
> >  get_more_pages:
> > @@ -1703,11 +1705,10 @@ static int ceph_writepages_start(struct address_space *mapping,
> >               }
> >
> >               rc = ceph_submit_write(mapping, wbc, &ceph_wbc);
> > -             if (rc)
> > -                     goto release_folios;
> > -
>
> Frankly speaking, its' completely not clear the from commit message why we move
> this check. What's the problem is here? How this move can fix the issue?

The diff makes it a little unclear, but I'm actually moving
ceph_wbc.{locked_pages,strip_unit_end} = 0; *above* the check (see
commit message: "also make it the previous iteration's responsibility
to guarantee [locked_pages is] reset") so that they happen
unconditionally. Git just happens to see it in terms of the if/goto
moving downward, not the assignments moving up.

Warm regards,
Sam


>
> Thanks,
> Slava.
>
> >               ceph_wbc.locked_pages = 0;
> >               ceph_wbc.strip_unit_end = 0;
> > +             if (rc)
> > +                     goto release_folios;
> >
> >               if (folio_batch_count(&ceph_wbc.fbatch) > 0) {
> >                       ceph_wbc.nr_folios =