Re: [PATCH v3] mm/swap: fix race when skipping swapcache

From: Huang, Ying
Date: Sun Feb 18 2024 - 19:41:20 EST

Kairui Song <ryncsn@xxxxxxxxx> writes:

> On Sun, Feb 18, 2024 at 4:34 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Kairui Song <ryncsn@xxxxxxxxx> writes:
>>
>> > 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 pin it. Release the pin
>> > after PT unlocked. Racers will simply wait since it's a rare and very
>> > short event. A schedule() call is added to avoid wasting too much CPU
>> > or adding too much noise to perf statistics
>> >
>> > 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 use the
>> > swap cache again. Parallel swapin using different methods leads to
>> > a much more complex scenario.
>>
>> The swap entry may be put in swap cache by some parallel code path
>> anyway. So, we always need to consider that when reasoning the code.
>>
>> > 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]
>> > 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
>> >
>> > ---
>> > 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].
>> >
>> > V2: https://lore.kernel.org/all/20240206182559.32264-1-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.
>> >
>> > V1: https://lore.kernel.org/all/20240205110959.4021-1-ryncsn@xxxxxxxxx/
>> >
>> > include/linux/swap.h | 5 +++++
>> > mm/memory.c | 20 ++++++++++++++++++++
>> > mm/swap.h | 5 +++++
>> > mm/swapfile.c | 13 +++++++++++++
>> > 4 files changed, 43 insertions(+)
>> >
>> > diff --git a/include/linux/swap.h b/include/linux/swap.h
>> > index 4db00ddad261..8d28f6091a32 100644
>> > --- a/include/linux/swap.h
>> > +++ b/include/linux/swap.h
>> > @@ -549,6 +549,11 @@ static inline int swap_duplicate(swp_entry_t swp)
>> > return 0;
>> > }
>> >
>> > +static inline int swapcache_prepare(swp_entry_t swp)
>> > +{
>> > + return 0;
>> > +}
>> > +
>> > static inline void swap_free(swp_entry_t swp)
>> > {
>> > }
>> > diff --git a/mm/memory.c b/mm/memory.c
>> > index 7e1f4849463a..7059230d0a54 100644
>> > --- a/mm/memory.c
>> > +++ b/mm/memory.c
>> > @@ -3799,6 +3799,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> > struct page *page;
>> > struct swap_info_struct *si = NULL;
>> > rmap_t rmap_flags = RMAP_NONE;
>> > + bool need_clear_cache = false;
>> > bool exclusive = false;
>> > swp_entry_t entry;
>> > pte_t pte;
>> > @@ -3867,6 +3868,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> > if (!folio) {
>> > if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
>> > __swap_count(entry) == 1) {
>> > + /*
>> > + * Prevent parallel swapin from proceeding with
>> > + * the cache flag. Otherwise, another thread may
>> > + * finish swapin first, free the entry, and swapout
>> > + * reusing the same entry. It's undetectable as
>> > + * pte_same() returns true due to entry reuse.
>> > + */
>> > + if (swapcache_prepare(entry)) {
>> > + /* Relax a bit to prevent rapid repeated page faults */
>> > + schedule();
>>
>> The current task may be chosen in schedule(). So, I think that we
>> should use cond_resched() here.
>>
>
> I think if we are worried about current task got chosen again we can
> use schedule_timeout_uninterruptible(1) here. Isn't cond_resched still
> __schedule() and and it can even get omitted, so it should be "weaker"
> IIUC.

schedule_timeout_uninterruptible(1) will introduce 1ms latency for the
second task. That may kill performance of some workloads.

--
Best Regards,
Huang, Ying