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

From: Huang, Ying
Date: Sun Feb 18 2024 - 03:41:37 EST

Barry Song <21cnbao@xxxxxxxxx> writes:

> On Sun, Feb 18, 2024 at 9:02 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> David Hildenbrand <david@xxxxxxxxxx> writes:
>>
>> > On 16.02.24 10:51, 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 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.
>> >> 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();
>> >> + goto out;
>> >> + }
>> >> + need_clear_cache = true;
>> >> +
>> >
>> > I took a closer look at __read_swap_cache_async() and it essentially
>> > does something similar.
>> >
>> > Instead of returning, it keeps retrying until it finds that
>> > swapcache_prepare() fails for another reason than -EEXISTS (e.g.,
>> > freed concurrently) or it finds the entry in the swapcache.
>> >
>> > So if you would succeed here on a freed+reused swap entry,
>> > __read_swap_cache_async() would simply retry.
>> >
>> > It spells that out:
>> >
>> > /*
>> > * We might race against __delete_from_swap_cache(), and
>> > * stumble across a swap_map entry whose SWAP_HAS_CACHE
>> > * has not yet been cleared. Or race against another
>> > * __read_swap_cache_async(), which has set SWAP_HAS_CACHE
>> > * in swap_map, but not yet added its folio to swap cache.
>> > */
>> >
>> > Whereby we could not race against this code here as well where we
>> > speculatively set SWAP_HAS_CACHE and might never add something to the swap
>> > cache.
>> >
>> >
>> > I'd probably avoid the wrong returns and do something even closer to
>> > __read_swap_cache_async().
>> >
>> > while (true) {
>> > /*
>> > * Fake that we are trying to insert a page into the swapcache, to
>> > * serialize against concurrent threads wanting to do the same.
>> > * [more from your description]
>> > */
>> > ret = swapcache_prepare(entry);
>> > if (likely(!ret)
>> > /*
>> > * Move forward with swapin, we'll recheck if the PTE hasn't
>> > * changed later.
>> > */
>> > break;
>> > else if (ret != -EEXIST)
>> > goto out;
>>
>> The swap entry may be kept in swap cache for long time. For example, it
>> may be read into swap cache via MADV_WILLNEED.
>
> This seems fine.
>
> if swapcache has data from WILLNEED, the new page fault will hit it. Thus,
> we won't go into the SYNC_IO path any more?

They may happen in parallel. That is, one task is busy looping, while
another task read the swap entry into swap cache.

--
Best Regards,
Huang, Ying

>>
>> --
>> Best Regards,
>> Huang, Ying
>>
>> >
>> > /*
>> > * See __read_swap_cache_async(). We might either have raced against
>> > * another thread, or the entry could have been freed and reused in the
>> > * meantime. Make sure that the PTE did not change, to detect freeing.
>> > */
>> > vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>> > vmf->address, &vmf->ptl);
>> > if (!vmf->pte || !pte_same(ptep_get(vmf->pte), vmf->orig_pte))
>> > goto unlock;
>> >
>> >
>> > schedule();
>> > }
>> >
>> >
>> >
>> > I was skeptical about the schedule(), but __read_swap_cache_async() does it
>> > already because there is no better way to wait for the event to happen.
>> >
>> > With something like above you would no longer depend on the speed of schedule() to
>> > determine how often you would retry the fault, which would likely make sense.
>> >
>> > I do wonder about the schedule() vs. schedule_timeout_uninterruptible(), though.
>> > No expert on that area, do you have any idea?
>>