> -----Original Message-----
> From: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> Sent: Tuesday, January 28, 2025 10:40 PM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>; Yosry Ahmed
> <yosryahmed@xxxxxxxxxx>; Nhat Pham <nphamcs@xxxxxxxxx>; Chengming
> Zhou <chengming.zhou@xxxxxxxxx>; Andrew Morton <akpm@linux-
> foundation.org>; linux-mm@xxxxxxxxx; stable@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v2 mm-hotfixes] mm/zswap: fix inconsistent charging
> when zswap_store_page() fails
>
> On Wed, Jan 29, 2025 at 4:09 AM Sridhar, Kanchana P
> <kanchana.p.sridhar@xxxxxxxxx> wrote:
> >
> > Hi Hyeonggon,
> >
> > > -----Original Message-----
> > > From: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> > > Sent: Tuesday, January 28, 2025 10:55 AM
> > > To: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>; Johannes
> Weiner
> > > <hannes@xxxxxxxxxxx>; Yosry Ahmed <yosryahmed@xxxxxxxxxx>; Nhat
> > > Pham <nphamcs@xxxxxxxxx>; Chengming Zhou
> > > <chengming.zhou@xxxxxxxxx>; Andrew Morton <akpm@linux-
> > > foundation.org>
> > > Cc: linux-mm@xxxxxxxxx; Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>;
> > > stable@xxxxxxxxxxxxxxx
> > > 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@xxxxxxxxxxxxxxx
> > > Signed-off-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> > > ---
> > >
> > > 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: ?
Yes, please go ahead Hyeonggon.
Thanks!
Kanchana
> 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
> >
