Re: [PATCH v4] mm/swap: fix race when skipping swapcache

[David Hildenbrand <david@redhat.com>]

On 19.02.24 09:20, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> When skipping swapcache for SWP_SYNCHRONOUS_IO, if two or more threads
> swapin the same entry at the same time, they get different pages (A, B).
> Before one thread (T0) finishes the swapin and installs page (A)
> to the PTE, another thread (T1) could finish swapin of page (B),
> swap_free the entry, then swap out the possibly modified page
> reusing the same entry. It breaks the pte_same check in (T0) because
> PTE value is unchanged, causing ABA problem. Thread (T0) will
> install a stalled page (A) into the PTE and cause data corruption.
> 
> One possible callstack is like this:
> 
> CPU0                                 CPU1
> ----                                 ----
> do_swap_page()                       do_swap_page() with same entry
> <direct swapin path>                 <direct swapin path>
> <alloc page A>                       <alloc page B>
> swap_read_folio() <- read to page A  swap_read_folio() <- read to page B
> <slow on later locks or interrupt>   <finished swapin first>
> ...                                  set_pte_at()
>                                       swap_free() <- entry is free
>                                       <write to page B, now page A stalled>
>                                       <swap out page B to same swap entry>
> pte_same() <- Check pass, PTE seems
>                unchanged, but page A
>                is stalled!
> swap_free() <- page B content lost!
> set_pte_at() <- staled page A installed!
> 
> And besides, for ZRAM, swap_free() allows the swap device to discard
> the entry content, so even if page (B) is not modified, if
> swap_read_folio() on CPU0 happens later than swap_free() on CPU1,
> it may also cause data loss.
> 
> To fix this, reuse swapcache_prepare which will pin the swap entry using
> the cache flag, and allow only one thread to swap it in, also prevent
> any parallel code from putting the entry in the cache. Release the pin
> after PT unlocked.
> 
> Racers just loop and wait since it's a rare and very short event.
> A schedule_timeout_uninterruptible(1) call is added to avoid repeated
> page faults wasting too much CPU, causing livelock or adding too much
> noise to perf statistics. A similar livelock issue was described in
> commit 029c4628b2eb ("mm: swap: get rid of livelock in swapin readahead")
> 
> Reproducer:
> 
> This race issue can be triggered easily using a well constructed
> reproducer and patched brd (with a delay in read path) [1]:
> 
> With latest 6.8 mainline, race caused data loss can be observed easily:
> $ gcc -g -lpthread test-thread-swap-race.c && ./a.out
>    Polulating 32MB of memory region...
>    Keep swapping out...
>    Starting round 0...
>    Spawning 65536 workers...
>    32746 workers spawned, wait for done...
>    Round 0: Error on 0x5aa00, expected 32746, got 32743, 3 data loss!
>    Round 0: Error on 0x395200, expected 32746, got 32743, 3 data loss!
>    Round 0: Error on 0x3fd000, expected 32746, got 32737, 9 data loss!
>    Round 0 Failed, 15 data loss!
> 
> This reproducer spawns multiple threads sharing the same memory region
> using a small swap device. Every two threads updates mapped pages one by
> one in opposite direction trying to create a race, with one dedicated
> thread keep swapping out the data out using madvise.
> 
> The reproducer created a reproduce rate of about once every 5 minutes,
> so the race should be totally possible in production.
> 
> After this patch, I ran the reproducer for over a few hundred rounds
> and no data loss observed.
> 
> Performance overhead is minimal, microbenchmark swapin 10G from 32G
> zram:
> 
> Before:     10934698 us
> After:      11157121 us
> Cached:     13155355 us (Dropping SWP_SYNCHRONOUS_IO flag)
> 
> Fixes: 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous device")
> Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1]
> Reported-by: "Huang, Ying" <ying.huang@intel.com>
> Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@yhuang6-desk2.ccr.corp.intel.com/
> Signed-off-by: Kairui Song <kasong@tencent.com>
> Cc: stable@vger.kernel.org
> 
> ---
> V3: https://lore.kernel.org/all/20240216095105.14502-1-ryncsn@gmail.com/
> Update from V3:
> - Use schedule_timeout_uninterruptible(1) for now instead of schedule() to
>    prevent the busy faulting task holds CPU and livelocks [Huang, Ying]
> 
> V2: https://lore.kernel.org/all/20240206182559.32264-1-ryncsn@gmail.com/
> Update from V2:
> - Add a schedule() if raced to prevent repeated page faults wasting CPU
>    and add noise to perf statistics.
> - Use a bool to state the special case instead of reusing existing
>    variables fixing error handling [Minchan Kim].
> 
> V1: https://lore.kernel.org/all/20240205110959.4021-1-ryncsn@gmail.com/
> Update from V1:
> - Add some words on ZRAM case, it will discard swap content on swap_free
>    so the race window is a bit different but cure is the same. [Barry Song]
> - Update comments make it cleaner [Huang, Ying]
> - Add a function place holder to fix CONFIG_SWAP=n built [SeongJae Park]
> - Update the commit message and summary, refer to SWP_SYNCHRONOUS_IO
>    instead of "direct swapin path" [Yu Zhao]
> - Update commit message.
> - Collect Review and Acks.
> 

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb