Re: [PATCH v3] mm/swap: fix race when skipping swapcache
From: Kairui Song
Date: Sun Feb 18 2024 - 04:20:54 EST
On Sun, Feb 18, 2024 at 4:34 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Kairui Song <ryncsn@xxxxxxxxx> writes:
>
> > 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 wait since it's a rare and very
> > short event. A schedule() call is added to avoid wasting too much CPU
> > or adding too much noise to perf statistics
> >
> > 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.
>
> The swap entry may be put in swap cache by some parallel code path
> anyway. So, we always need to consider that when reasoning the code.
>
> > 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]
> > Reported-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> > Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> >
> > ---
> > Update from V2:
> > - Add a schedule() if raced to prevent repeated page faults wasting CPU
> > and add noise to perf statistics.
> > - Use a bool to state the special case instead of reusing existing
> > variables fixing error handling [Minchan Kim].
> >
> > V2: https://lore.kernel.org/all/20240206182559.32264-1-ryncsn@xxxxxxxxx/
> >
> > 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.
> >
> > V1: https://lore.kernel.org/all/20240205110959.4021-1-ryncsn@xxxxxxxxx/
> >
> > include/linux/swap.h | 5 +++++
> > mm/memory.c | 20 ++++++++++++++++++++
> > mm/swap.h | 5 +++++
> > mm/swapfile.c | 13 +++++++++++++
> > 4 files changed, 43 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..7059230d0a54 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3799,6 +3799,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > struct page *page;
> > struct swap_info_struct *si = NULL;
> > rmap_t rmap_flags = RMAP_NONE;
> > + bool need_clear_cache = false;
> > bool exclusive = false;
> > swp_entry_t entry;
> > pte_t pte;
> > @@ -3867,6 +3868,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > if (!folio) {
> > if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
> > __swap_count(entry) == 1) {
> > + /*
> > + * 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)) {
> > + /* Relax a bit to prevent rapid repeated page faults */
> > + schedule();
>
> The current task may be chosen in schedule(). So, I think that we
> should use cond_resched() here.
>
I think if we are worried about current task got chosen again we can
use schedule_timeout_uninterruptible(1) here. Isn't cond_resched still
__schedule() and and it can even get omitted, so it should be "weaker"
IIUC.