Re: [PATCH v2] mm/swap: fix race when skipping swapcache
From: Barry Song
Date: Tue Feb 06 2024 - 23:24:49 EST
On Wed, Feb 7, 2024 at 5:18 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> 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@gmailcom> 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.
>
you are right. i thought the actual probability of the race this patch is trying
to fix is 0%. so we fix the final place of setting pte, we stop setting pte once
we detect one race. but i was wrong because the one who fails on
swapcache_prepare could be the one who first gets PTL.
> --
> 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