Re: [PATCH v2] mm/swap: fix race when skipping swapcache

[Kairui Song <ryncsn@gmail.com>]

On Wed, Feb 7, 2024 at 7:02 AM Minchan Kim <minchan@kernel.org> wrote:
>
> On Wed, Feb 07, 2024 at 02:25:59AM +0800, 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
>
>                                        ^^^
> nit: From the recent code, I see swap_free is called earlier than set_pte_at

Thanks, will update the message.

>
>
> >                                      <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.
>
> Thanks for catching the issue, folks!
>
> >
> > 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 busy wait since it's a rare
> > and very short event.
> >
> > 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")
> > Reported-by: "Huang, Ying" <ying.huang@intel.com>
> > Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@yhuang6-desk2.ccr.corp.intel.com/
> > Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1]
> > Signed-off-by: Kairui Song <kasong@tencent.com>
> > Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
> > Acked-by: Yu Zhao <yuzhao@google.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.
> >
> >  include/linux/swap.h |  5 +++++
> >  mm/memory.c          | 15 +++++++++++++++
> >  mm/swap.h            |  5 +++++
> >  mm/swapfile.c        | 13 +++++++++++++
> >  4 files changed, 38 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..1749c700823d 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3867,6 +3867,16 @@ 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))
> > +                             goto out;
> > +
> >                       /* skip swapcache */
> >                       folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
> >                                               vma, vmf->address, false);
> > @@ -4116,6 +4126,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >  unlock:
> >       if (vmf->pte)
> >               pte_unmap_unlock(vmf->pte, vmf->ptl);
> > +     /* Clear the swap cache pin for direct swapin after PTL unlock */
> > +     if (folio && !swapcache)
> > +             swapcache_clear(si, entry);
> >  out:
> >       if (si)
> >               put_swap_device(si);
> > @@ -4124,6 +4137,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       if (vmf->pte)
> >               pte_unmap_unlock(vmf->pte, vmf->ptl);
> >  out_page:
> > +     if (!swapcache)
> > +             swapcache_clear(si, entry);
> >       folio_unlock(folio);
> >  out_release:
> >       folio_put(folio);
>
> What happens?
>
> do_swap_page
>   ..
>   swapcache_prepare() <- tured the cache flag on
>
>   folio = vma_alloc_folio <- failed to allocate the folio
>   page = &foio->page; <- crash but it's out of scope from this patch
>
>   ..
>   if (!folio)
>      goto unlock;
>
> ..
> unlock:
>   swapcache_clear(si, entry) <- it's skipped this time.
>
>
> Can we simply introduce a boolean flag to state the special case and
> clear the cache state based on the flag?

Good idea, that should make the code easier to understand.