Re: [PATCH v6.12 hotfix] mm/zswap: fix inconsistent charging when zswap_store_page() fails

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Tue, Jan 28, 2025 at 6:14 PM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Wed, Jan 29, 2025 at 02:49:38AM +0900, Hyeonggon Yoo wrote:
> > Commit b7c0ccdfbafd ("mm: zswap: support large folios in zswap_store()")
> > mistakenly skipped charging any zswapped pages when a single call to
> > zswap_store_page() failed, even if some pages in the folio are
> > successfully stored in zswap.
> >
> > Making things worse, these not-charged pages are uncharged in
> > zswap_entry_free(), making zswap charging inconsistent.
> >
> > This inconsistency triggers two warnings when following these steps:
> >   # On a machine with 64GiB of RAM and 36GiB of zswap
> >   $ stress-ng --bigheap 2 # wait until the OOM-killer kills stress-ng
> >   $ sudo reboot
> >
> >   Two warnings are:
> >     in mm/memcontrol.c:163, function obj_cgroup_release():
> >       WARN_ON_ONCE(nr_bytes & (PAGE_SIZE - 1));
> >
> >     in mm/page_counter.c:60, function page_counter_cancel():
> >       if (WARN_ONCE(new < 0, "page_counter underflow: %ld nr_pages=%lu\n",
> >         new, nr_pages))
> >
> > Charge zswapped pages even if some pages of the folio are not zswapped.
> > After resolving the inconsistency, these warnings disappear.
> >
> > Fixes: b7c0ccdfbafd ("mm: zswap: support large folios in zswap_store()")
>
> This commit is in 6.13, not 6.12, so your subject line is a bit
> confusing :(

Oh, thanks for catching. Will fix it.
Also, I noticed I incorrectly described the problem.

Will send v2 (for v6.13!) after adjusting them.

Best,
Hyeonggon