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

Chengming Zhou
2024-03-25T07:33:33Z

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()?