Re: [PATCH] mm: zswap: fix data loss on SWP_SYNCHRONOUS_IO devices

Barry Song
2024-03-25T04:54:20Z

On Mon, Mar 25, 2024 at 10:23 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Sun, Mar 24, 2024 at 2:04 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > Zhongkun He reports data corruption when combining zswap with zram.
> >
> > The issue is the exclusive loads we're doing in zswap. They assume
> > that all reads are going into the swapcache, which can assume
> > authoritative ownership of the data and so the zswap copy can go.
> >
> > However, zram files are marked SWP_SYNCHRONOUS_IO, and faults will try
> > to bypass the swapcache. This results in an optimistic read of the
> > swap data into a page that will be dismissed if the fault fails due to
> > races. In this case, zswap mustn't drop its authoritative copy.
> >
> > Link: https://lore.kernel.org/all/CACSyD1N+dUvsu8=zV9P691B9bVq33erwOXNTmEaUbi9DrDeJzw@mail.gmail.com/
> > Reported-by: Zhongkun He <hezhongkun.hzk@bytedance.com>
> > Fixes: b9c91c43412f ("mm: zswap: support exclusive loads")
> > Cc: stable@vger.kernel.org [6.5+]
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > Tested-by: Zhongkun He <hezhongkun.hzk@bytedance.com>

Acked-by: Barry Song <baohua@kernel.org>

>
> Do we also want to mention somewhere (commit log or comment) that
> keeping the entry in the tree is fine because we are still protected
> from concurrent loads/invalidations/writeback by swapcache_prepare()
> setting SWAP_HAS_CACHE or so?

It seems that Kairui's patch comprehensively addresses the issue at hand.
Johannes's solution, on the other hand, appears to align zswap behavior
more closely with that of a traditional swap device, only releasing an entry
when the corresponding swap slot is freed, particularly in the sync-io case.

Johannes' patch has inspired me to consider whether zRAM could achieve
a comparable outcome by immediately releasing objects in swap cache
scenarios. When I have the opportunity, I plan to experiment with zRAM.

>
> Anyway, this LGTM.
>
> Acked-by: Yosry Ahmed <yosryahmed@google.com>
>
> > ---
> > mm/zswap.c | 23 +++++++++++++++++++----
> > 1 file changed, 19 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 535c907345e0..41a1170f7cfe 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -1622,6 +1622,7 @@ bool zswap_load(struct folio *folio)
> > swp_entry_t swp = folio->swap;
> > pgoff_t offset = swp_offset(swp);
> > struct page *page = &folio->page;
> > + bool swapcache = folio_test_swapcache(folio);
> > struct zswap_tree *tree = swap_zswap_tree(swp);
> > struct zswap_entry *entry;
> > u8 *dst;
> > @@ -1634,7 +1635,20 @@ bool zswap_load(struct folio *folio)
> > spin_unlock(&tree->lock);
> > return false;
> > }
> > - zswap_rb_erase(&tree->rbroot, entry);
> > + /*
> > + * When reading into the swapcache, invalidate our entry. The
> > + * swapcache can be the authoritative owner of the page and
> > + * its mappings, and the pressure that results from having two
> > + * in-memory copies outweighs any benefits of caching the
> > + * compression work.
> > + *
> > + * (Most swapins go through the swapcache. The notable
> > + * exception is the singleton fault on SWP_SYNCHRONOUS_IO
> > + * files, which reads into a private page and may free it if
> > + * the fault fails. We remain the primary owner of the entry.)
> > + */
> > + if (swapcache)
> > + zswap_rb_erase(&tree->rbroot, entry);
> > spin_unlock(&tree->lock);
> >
> > if (entry->length)
> > @@ -1649,9 +1663,10 @@ bool zswap_load(struct folio *folio)
> > if (entry->objcg)
> > count_objcg_event(entry->objcg, ZSWPIN);
> >
> > - zswap_entry_free(entry);
> > -
> > - folio_mark_dirty(folio);
> > + if (swapcache) {
> > + zswap_entry_free(entry);
> > + folio_mark_dirty(folio);
> > + }
> >
> > return true;
> > }
> > --
> > 2.44.0

Thanks
Barry