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

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Mar 25, 2024 at 12:33 AM Chengming Zhou
<chengming.zhou@linux.dev> wrote:
>
> On 2024/3/25 15:06, Yosry Ahmed wrote:
> > 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.
> >
>
> This makes me wonder who is responsible for marking folio dirty in this swapcache
> bypass case? Should we call folio_mark_dirty() after the swap_read_folio()?

In shrink_folio_list(), we try to add anonymous folios to the
swapcache if they are not there before checking if they are dirty.
add_to_swap() calls folio_mark_dirty(), so this should take care of
it. There is an interesting comment there though. It says that PTE
should be dirty, so unmapping the folio should have already marked it
as dirty by the time we are adding it to the swapcache, except for the
MADV_FREE case.

However, I think we actually unmap the folio after we add it to the
swapcache in shrink_folio_list(). Also, I don't immediately see why
the PTE would be dirty. In do_swap_page(), making the PTE dirty seems
to be conditional on the fault being a write fault, but I didn't look
thoroughly, maybe I missed it. It is also possible that the comment is
just outdated.