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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAH5Ym4jsBiEHoyPnJh+dffGzuKuvaF9JOV2WX9_sU6V2NLqXxA@mail.gmail.com>
Date: Wed, 28 Jan 2026 16:14:13 -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 v3 4/4] ceph: assert writeback loop invariants

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.

Regards,
Sam

>
> Thanks,
> Slava.
>
> > +
> >               ceph_wbc.max_pages = ceph_wbc.wsize >> PAGE_SHIFT;
> >
> >  get_more_pages: