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

[Yosry Ahmed <yosryahmed@google.com>]

On Sun, Mar 24, 2024 at 9:54 PM Barry Song <21cnbao@gmail.com> wrote:
>
> 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.

It actually worked out quite well that Kairui's fix landed shortly
before this bug was reported, as this fix wouldn't have been possible
without it as far as I can tell.

>
> 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.

That would be interesting. I am curious if it would be as
straightforward in zram to just mark the folio as dirty in this case
like zswap does, given its implementation as a block device.