Re: [PATCH] mm/swap: fix race condition in direct swapin path

[Kairui Song]

On Tue, Feb 6, 2024 at 9:35 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Kairui Song <ryncsn@xxxxxxxxx> writes:
>
> > From: Kairui Song <kasong@xxxxxxxxxxx>
> >
> > In the direct swapin path, when two or more threads swapin the same entry
> > at the same time, they get different pages (A, B) because swap cache is
> > skipped. 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 modify and swap-out the page again, using the
> > same entry. It break the pte_same check because PTE value is unchanged,
> > causing ABA problem. Then thread (T0) will then install the stalled page
> > (A) into the PTE so new data in page (B) is lost, 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_readpage() <- read to page A   swap_readpage() <- read to page B
> > <slow on later locks or interrupt>  <finished swapin first>
> > ...                                 set_pte_at()
> >                                     swap_free() <- Now the entry is freed.
> >                                     <write to page B, now page A stalled>
> >                                     <swap out page B using 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!
> >
> > 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 the
> > cached swapin path, two swapin path being used at the same time
> > 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]
> > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
>
> Reported-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

Of course :), wasn't sure about how to add your credit, will add this to V2.

> > ---
> > Huge thanks to Huang Ying and Chris Li for help finding this issue!
> >
> >  mm/memory.c   | 19 +++++++++++++++++++
> >  mm/swap.h     |  5 +++++
> >  mm/swapfile.c | 16 ++++++++++++++++
> >  3 files changed, 40 insertions(+)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 7e1f4849463a..fd7c55a292f1 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3867,6 +3867,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       if (!folio) {
> >               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
> >                   __swap_count(entry) == 1) {
> > +                     /*
> > +                      * With swap count == 1, after we read the entry,
> > +                      * other threads could finish swapin first, free
> > +                      * the entry, then swapout the modified page using
> > +                      * the same entry. Now the content we just read is
> > +                      * stalled, and it's undetectable as pte_same()
> > +                      * returns true due to entry reuse.
> > +                      *
> > +                      * So pin the swap entry using the cache flag even
>
> "pin" doesn't sound intuitive here.  I know that the swap entry will not
> be freed with this.  But now, the parallel swap in will busy waiting.
> So, I suggest to say,
>
> Prevent parallel swapin from proceeding with the cache flag.  Otherwise,
> it may swapin first, free the entry, then swapout the modified page
> using the same entry ...

Good suggestion.

>
> > +                      * cache is not used.
> > +                      */
> > +                     if (swapcache_prepare(entry))
> > +                             goto out;
> > +
> >                       /* skip swapcache */
> >                       folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
> >                                               vma, vmf->address, false);
> > @@ -4116,6 +4130,9 @@ 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 +4141,8 @@ 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);
> > diff --git a/mm/swap.h b/mm/swap.h
> > index 758c46ca671e..fc2f6ade7f80 100644
> > --- a/mm/swap.h
> > +++ b/mm/swap.h
> > @@ -41,6 +41,7 @@ void __delete_from_swap_cache(struct folio *folio,
> >  void delete_from_swap_cache(struct folio *folio);
> >  void clear_shadow_from_swap_cache(int type, unsigned long begin,
> >                                 unsigned long end);
> > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry);
> >  struct folio *swap_cache_get_folio(swp_entry_t entry,
> >               struct vm_area_struct *vma, unsigned long addr);
> >  struct folio *filemap_get_incore_folio(struct address_space *mapping,
> > @@ -97,6 +98,10 @@ static inline int swap_writepage(struct page *p, struct writeback_control *wbc)
> >       return 0;
> >  }
> >
> > +static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
> > +{
> > +}
> > +
> >  static inline struct folio *swap_cache_get_folio(swp_entry_t entry,
> >               struct vm_area_struct *vma, unsigned long addr)
> >  {
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 556ff7347d5f..f7d4ad152a7f 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -3365,6 +3365,22 @@ int swapcache_prepare(swp_entry_t entry)
> >       return __swap_duplicate(entry, SWAP_HAS_CACHE);
> >  }
> >
> > +/*
> > + * Clear the cache flag and release pinned entry.
>
> Even if we will keep "pin" in above comments, this is hard to be
> understood for reader.  Need a little more explanation like "release
> pinned entry for device with SWP_SYNCHRONOUS_IO.
>
> Or, just remove the comments.  We have comments in calling site already.

Then I prefer to remove this, there is only one caller, it should be
easy to understand.

> > + */
> > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
> > +{
> > +     struct swap_cluster_info *ci;
> > +     unsigned long offset = swp_offset(entry);
> > +     unsigned char usage;
> > +
> > +     ci = lock_cluster_or_swap_info(si, offset);
> > +     usage = __swap_entry_free_locked(si, offset, SWAP_HAS_CACHE);
> > +     unlock_cluster_or_swap_info(si, ci);
> > +     if (!usage)
> > +             free_swap_slot(entry);
> > +}
> > +
> >  struct swap_info_struct *swp_swap_info(swp_entry_t entry)
> >  {
> >       return swap_type_to_swap_info(swp_type(entry));
>
> Otherwise it looks good for me, Thanks!
>
> Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

Thanks for the review.