Re: [PATCH v2 mm-hotfixes] mm/zswap: fix inconsistent charging when zswap_store_page() fails

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

On Wed, Jan 29, 2025 at 4:09 AM Sridhar, Kanchana P
<kanchana.p.sridhar@intel.com> wrote:
>
> Hi Hyeonggon,
>
> > -----Original Message-----
> > From: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> > Sent: Tuesday, January 28, 2025 10:55 AM
> > To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>; Johannes Weiner
> > <hannes@cmpxchg.org>; Yosry Ahmed <yosryahmed@google.com>; Nhat
> > Pham <nphamcs@gmail.com>; Chengming Zhou
> > <chengming.zhou@linux.dev>; Andrew Morton <akpm@linux-
> > foundation.org>
> > Cc: linux-mm@kvack.org; Hyeonggon Yoo <42.hyeyoo@gmail.com>;
> > stable@vger.kernel.org
> > Subject: [PATCH v2 mm-hotfixes] mm/zswap: fix inconsistent charging when
> > zswap_store_page() fails
> >
> > Commit b7c0ccdfbafd ("mm: zswap: support large folios in zswap_store()")
> > skips charging any zswapped base pages when it failed to zswap the entire
> > folio.
> >
> > However, when some base pages are zswapped but it failed to zswap
> > the entire folio, the zswap operation is rolled back.
> > When freeing zswap entries for those pages, zswap_entry_free() uncharges
> > the pages that were not previously charged, causing zswap charging to
> > become inconsistent.
> >
> > This inconsistency triggers two warnings with following 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))
> >
> > While objcg events should only be accounted for when the entire folio is
> > zswapped, objcg charging should be performed regardlessly.
> > Fix accordingly.
> >
> > After resolving the inconsistency, these warnings disappear.
> >
> > Fixes: b7c0ccdfbafd ("mm: zswap: support large folios in zswap_store()")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> > ---
> >
> > v1->v2:
> >
> >  Fixed objcg events being accounted for on zswap failure.
> >
> >  Fixed the incorrect description. I misunderstood that the base pages are
> >  going to be stored in zswap, but their zswap entries are freed immediately.
> >
> >  Added a comment on why it charges pages that are going to be removed
> >  from zswap.
> >
> >  mm/zswap.c | 14 ++++++++++----
> >  1 file changed, 10 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 6504174fbc6a..10b30ac46deb 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -1568,20 +1568,26 @@ bool zswap_store(struct folio *folio)
> >
> >               bytes = zswap_store_page(page, objcg, pool);
> >               if (bytes < 0)
> > -                     goto put_pool;
> > +                     goto charge_zswap;
> >               compressed_bytes += bytes;
> >       }
> >
> > -     if (objcg) {
> > -             obj_cgroup_charge_zswap(objcg, compressed_bytes);
> > +     if (objcg)
> >               count_objcg_events(objcg, ZSWPOUT, nr_pages);
> > -     }
> >
> >       atomic_long_add(nr_pages, &zswap_stored_pages);
> >       count_vm_events(ZSWPOUT, nr_pages);
> >
> >       ret = true;
> >
> > +charge_zswap:
> > +     /*
> > +      * Charge zswapped pages even when it failed to zswap the entire
> > folio,
> > +      * because zswap_entry_free() will uncharge them anyway.
> > +      * Otherwise zswap charging will become inconsistent.
> > +      */
> > +     if (objcg)
> > +             obj_cgroup_charge_zswap(objcg, compressed_bytes);
>
> Thanks for finding this bug! I am thinking it might make sense to charge
> and increment the zswap_stored_pages counter in zswap_store_page().
> Something like:
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index b84c20d889b1..fd2a72598a8a 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1504,11 +1504,14 @@ static ssize_t zswap_store_page(struct page *page,
>         entry->pool = pool;
>         entry->swpentry = page_swpentry;
>         entry->objcg = objcg;
> +       if (objcg)
> +               obj_cgroup_charge_zswap(objcg, entry->length);
>         entry->referenced = true;
>         if (entry->length) {
>                 INIT_LIST_HEAD(&entry->lru);
>                 zswap_lru_add(&zswap_list_lru, entry);
>         }
> +       atomic_long_inc(&zswap_stored_pages);
>
>         return entry->length;
>
> @@ -1526,7 +1529,6 @@ bool zswap_store(struct folio *folio)
>         struct obj_cgroup *objcg = NULL;
>         struct mem_cgroup *memcg = NULL;
>         struct zswap_pool *pool;
> -       size_t compressed_bytes = 0;
>         bool ret = false;
>         long index;
>
> @@ -1569,15 +1571,11 @@ bool zswap_store(struct folio *folio)
>                 bytes = zswap_store_page(page, objcg, pool);
>                 if (bytes < 0)
>                         goto put_pool;
> -               compressed_bytes += bytes;
>         }
>
> -       if (objcg) {
> -               obj_cgroup_charge_zswap(objcg, compressed_bytes);
> +       if (objcg)
>                 count_objcg_events(objcg, ZSWPOUT, nr_pages);
> -       }
>
> -       atomic_long_add(nr_pages, &zswap_stored_pages);
>         count_vm_events(ZSWPOUT, nr_pages);
>
>         ret = true;

Hi Sridhar, It looks much clearer!
And we can optimize if it turns out to be worth the complexity.

May I ask your permission to add your Signed-off-by: and Co-developed-by: ?
I'm afraid to use this without your confirmation due to the
Developer's Certificate of Origin.

Best,
Hyeonggon

> >  put_pool:
> >       zswap_pool_put(pool);
> >  put_objcg:
> > --
> > 2.47.1
>