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

["Huang, Ying" <ying.huang@intel.com>]

Kairui Song <ryncsn@gmail.com> writes:

> 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?

cpu_relax() only makes sense in tight loop.

--
Best Regards,
Huang, Ying