On 19.02.24 09:20, Kairui Song wrote:
> From: Kairui Song <[email protected]>
>
> 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" <[email protected]>
> Closes: https://lore.kernel.org/lkml/[email protected]/
> Signed-off-by: Kairui Song <[email protected]>
> Cc: [email protected]
>
> ---
> V3: https://lore.kernel.org/all/[email protected]/
> 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/[email protected]/
> 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/[email protected]/
> 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 <[email protected]>
--
Cheers,
David / dhildenb