Re: [PATCH] mm/swap: fix race condition in direct swapin path

[Yu Zhao]

On Mon, Feb 5, 2024 at 4:10 AM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> In the direct swapin path, when two or more threads swapin the same entry

There is no other places referring to that path as "direct" swapin.

I'd rephrase it as: "When skipping swapcache for SWP_SYNCHRONOUS_IO,
..", and similarly for the subject: "mm: fix race when skipping
swapcache".

> at the same time, they get different pages (A, B) because swap cache is
> skipped. 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 modify and swap-out the page again, using the
> same entry. It break the pte_same check because PTE value is unchanged,
> causing ABA problem. Then thread (T0) will then install the stalled page
> (A) into the PTE so new data in page (B) is lost, 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_readpage() <- read to page A   swap_readpage() <- read to page B
> <slow on later locks or interrupt>  <finished swapin first>
> ...                                 set_pte_at()
>                                     swap_free() <- Now the entry is freed.
>                                     <write to page B, now page A stalled>
>                                     <swap out page B using 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!
>
> To fix this, reuse swapcache_prepare which will pin the swap entry using
> the cache flag, and allow only one thread to pin it. Release the pin
> after PT unlocked. Racers will simply busy wait since it's a rare
> and very short event.
>
> Other methods like increasing the swap count don't seem to be a good
> idea after some tests, that will cause racers to fall back to the
> cached swapin path, two swapin path being used at the same time
> leads to a much more complex scenario.
>
> 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
> Non-direct: 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]
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>

Cc: stable@xxxxxxxxxxxxxxx

Acked-by: Yu Zhao <yuzhao@xxxxxxxxxx>