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

[Barry Song <21cnbao@gmail.com>]

On Sun, Feb 18, 2024 at 9:02 PM Huang, Ying <ying.huang@intel.com> wrote:
>
> David Hildenbrand <david@redhat.com> writes:
>
> > On 16.02.24 10:51, Kairui Song wrote:
> >> From: Kairui Song <kasong@tencent.com>
> >> 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@intel.com>
> >> Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@yhuang6-desk2.ccr.corp.intel.com/
> >> Signed-off-by: Kairui Song <kasong@tencent.com>
> >> Cc: stable@vger.kernel.org
> >> ---
> >> 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@gmail.com/
> >> 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@gmail.com/
> >>   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?

>
> --
> 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?
>