On 19.02.24 09:20, Kairui Song wrote:
From: Kairui Song <kasong@xxxxxxxxxxx>
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@xxxxxxxxx>
Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
V3: https://lore.kernel.org/all/20240216095105.14502-1-ryncsn@xxxxxxxxx/
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@xxxxxxxxx/
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@xxxxxxxxx/
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@xxxxxxxxxx>
--
Cheers,
David / dhildenb
