linux-kernel - Re: [PATCH v3 4/4] ceph: assert writeback loop invariants

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Message-ID: <CAOi1vP_tqdHGhwJSgRU5xbNWYBQ8aUkQuQ33jHknHcuqs7hSCQ@mail.gmail.com>
Date: Wed, 11 Feb 2026 19:04:22 +0100
From: Ilya Dryomov <idryomov@...il.com>
To: Sam Edwards <cfsworks@...il.com>
Cc: Viacheslav Dubeyko <Slava.Dubeyko@....com>, Xiubo Li <xiubli@...hat.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 v3 4/4] ceph: assert writeback loop invariants

On Thu, Jan 29, 2026 at 1:14 AM Sam Edwards <cfsworks@...il.com> wrote:
>
> On Mon, Jan 26, 2026 at 2:54 PM Viacheslav Dubeyko
> <Slava.Dubeyko@....com> wrote:
> >
> > On Sun, 2026-01-25 at 18:30 -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 two
> > > recent patches which fix oopses caused by `pages` persisting into
> > > the next loop iteration:
> > > - "ceph: do not propagate page array emplacement errors as batch errors"
> > > - "ceph: free page array when ceph_submit_write() fails"
> > >
> > > 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.
> > >
> > > This patch does not change invariants, behavior, or failure modes.
> > > The added BUG_ON() lines catch conditions that would already trigger oops,
> > > but do so earlier for easier debugging and programmer clarity.
> > >
> > > Signed-off-by: Sam Edwards <CFSworks@...il.com>
> > > ---
> > >  fs/ceph/addr.c | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> > > index cdf11288d6b7..4e392fc70d33 100644
> > > --- a/fs/ceph/addr.c
> > > +++ b/fs/ceph/addr.c
> > > @@ -1663,7 +1663,9 @@ 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);
> >
> > My complains are still the same. I would like not have BUG_ON() here.
>
> Hey Slava,
>
> I understand your preference, but as discussed before, the BUG_ON() is
> defensive programming to catch an invariant violation earlier than the
> existing BUG_ON() in ceph_allocate_page_array(). If the invariant is
> broken, we'll oops anyway; this just makes the oops happen sooner and
> easier to debug, and reduces cognitive load for future programmers
> trying to understand the loop invariants.

Hi Sam,

I agree with your argument, these BUG_ONs seem on point to me.

Thanks,

                Ilya