Re: [PATCH v2] mm/swap: fix race when skipping swapcache
From: Huang, Ying
Date: Tue Feb 06 2024 - 23:18:54 EST
Barry Song <21cnbao@xxxxxxxxx> writes:
> On Wed, Feb 7, 2024 at 3:29 PM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>>
>> On Wed, Feb 7, 2024 at 10:10 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>> >
>> > Barry Song <21cnbao@xxxxxxxxx> writes:
>> >
>> > > On Wed, Feb 7, 2024 at 7:18 AM Chris Li <chrisl@xxxxxxxxxx> 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@xxxxxxxxx> wrote:
>> > >> >
>> > >> > From: Kairui Song <kasong@xxxxxxxxxxx>
>> > >> >
>> > >> > 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@xxxxxxxxx>
>> > >> > Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
>> > >> > Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1]
>> > >> > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
>> > >> > Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>> > >> > Acked-by: Yu Zhao <yuzhao@xxxxxxxxxx>
>> > >> >
>> > >> > ---
>> > >> > 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.
>
> what about we make everything go as it is. I mean, we only need to
> record we have failed on swapcache_prepare, but we don't goto out.
>
> bool swapcache_prepare_failed = swapcache_prepare();
> .... // don't change any code
>
>
> but we only change the last code to set pte from the below
> ptl
> if(pte_same)
> set_pte
>
> to
>
> ptl
> if(pte_same && !swapcache_prepare_failed)
> set_pte
>
> as the chance is close to 0%, the increased count should be very minor.
IIUC, if (!swapcache_prepare_failed), it will always fail. If so, why
bother wasting CPU cycles? If you return directly, and the first thread
runs quickly enough, you can use the installed PTE directly.
--
Best Regards,
Huang, Ying
>>
>> 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?
>
> Thanks
> Barry