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

From: Huang, Ying
Date: Sun Feb 18 2024 - 21:36:03 EST

"Huang, Ying" <ying.huang@xxxxxxxxx> writes:

> 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.

Just found that the cond_sched() in __read_swap_cache_async() has been
changed to schedule_timeout_uninterruptible(1) to fix some live lock.
Details are in the description of commit 029c4628b2eb ("mm: swap: get
rid of livelock in swapin readahead"). I think the similar issue may
happen here too. So, we must use schedule_timeout_uninterruptible(1)
here until some other better idea becomes available.

--
Best Regards,
Huang, Ying