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

[Kairui Song <ryncsn@gmail.com>]

On Wed, Feb 7, 2024 at 10:10 AM Huang, Ying <ying.huang@intel.com> wrote:
>
> Barry Song <21cnbao@gmail.com> writes:
>
> > On Wed, Feb 7, 2024 at 7:18 AM Chris Li <chrisl@kernel.org> wrote:
> >>
> >> Hi Kairui,
> >>
> >> Sorry replying to your patch V1 late, I will reply on the V2 thread.
> >>
> >> On Tue, Feb 6, 2024 at 10:28 AM Kairui Song <ryncsn@gmail.com> 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 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;
> >> > +
> >>
> >> I am puzzled by this "goto out". If I understand this correctly, you
> >> have two threads CPU1 and CPU2 racing to set the flag SWAP_HAS_CACHE.
> >> The CPU1 will succeed in adding the flag and  the CPU2 will get
> >> "-EEXIST" from "swapcache_prepare(entry)".  Am I understanding it
> >> correctly so far?
> >>
> >> Then the goto out seems wrong to me. For the CPU2, the page fault will
> >> return *unhandled*. Even worse, the "-EEXIST" error is not preserved,
> >> CPU2 does not even know the page fault is not handled, it will resume
> >> from the page fault instruction, possibly generate another page fault
> >> at the exact same location. That page fault loop will repeat until
> >> CPU1 install the new pte on that faulting virtual address and pick up
> >> by CPU2.
> >>
> >> Am I missing something obvious there?
> >
> > I feel you are right. any concurrent page faults at the same pte
> > will increase the count of page faults for a couple of times now.
> >
> >>
> >> I just re-read your comment: "Racers will simply busy wait since it's
> >> a rare and very short event." That might be referring to the above
> >> CPU2 page fault looping situation. I consider the page fault looping
> >> on CPU2 not acceptable. For one it will mess up the page fault
> >> statistics.
> >> In my mind, having an explicit loop for CPU2 waiting for the PTE to
> >> show up is still better than this page fault loop. You can have more
> >> CPU power friendly loops.
> >
> > I assume you mean something like
> >
> > while(!pte_same())
> >    cpu_relax();
> >
> > then we still have a chance to miss the change of B.
> >
> > For example, another thread is changing pte to A->B->A, our loop can
> > miss B. Thus we will trap into an infinite loop. this is even worse.
> >
> > is it possible to loop for the success of swapcache_prepare(entry)
> > instead?
>
> This doesn't work too.  The swap count can increase to > 1 and be put in
> swap cache for long time.
>
> Another possibility is to move swapcache_prepare() after
> vma_alloc_folio() to reduce the race window.

Reducing the race window seems like a good way. Or maybe we can just
add a cpu_relax() so raced swapins will just slow down, and won't loop
too much time and so the side effect (counter or power consumption)
should be much smaller?