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

[Johannes Weiner <hannes@cmpxchg.org>]

On Sun, Mar 24, 2024 at 02:22:46PM -0700, Yosry Ahmed 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>
> 
> 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?

I don't think it's necessary, as zswap isn't doing anything special
here. It's up to the caller to follow the generic swap exclusion
protocol that zswap also adheres to. So IMO the relevant comment
should be, and is, above that swapcache_prepare() in do_swap_page().

> Anyway, this LGTM.
> 
> Acked-by: Yosry Ahmed <yosryahmed@google.com>

Thanks!