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

From: Barry Song
Date: Tue Feb 06 2024 - 19:43:25 EST

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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 behavior needs more discussion.
>
> Chris
>
>
> Chris
>
>
> > /* skip swapcache */
> > folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
> > vma, vmf->address, false);
> > @@ -4116,6 +4126,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > unlock:
> > if (vmf->pte)
> > pte_unmap_unlock(vmf->pte, vmf->ptl);
> > + /* Clear the swap cache pin for direct swapin after PTL unlock */
> > + if (folio && !swapcache)
> > + swapcache_clear(si, entry);
> > out:
> > if (si)
> > put_swap_device(si);
> > @@ -4124,6 +4137,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > if (vmf->pte)
> > pte_unmap_unlock(vmf->pte, vmf->ptl);
> > out_page:
> > + if (!swapcache)
> > + swapcache_clear(si, entry);
> > folio_unlock(folio);
> > out_release:
> > folio_put(folio);
> > diff --git a/mm/swap.h b/mm/swap.h
> > index 758c46ca671e..fc2f6ade7f80 100644
> > --- a/mm/swap.h
> > +++ b/mm/swap.h
> > @@ -41,6 +41,7 @@ void __delete_from_swap_cache(struct folio *folio,
> > void delete_from_swap_cache(struct folio *folio);
> > void clear_shadow_from_swap_cache(int type, unsigned long begin,
> > unsigned long end);
> > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry);
> > struct folio *swap_cache_get_folio(swp_entry_t entry,
> > struct vm_area_struct *vma, unsigned long addr);
> > struct folio *filemap_get_incore_folio(struct address_space *mapping,
> > @@ -97,6 +98,10 @@ static inline int swap_writepage(struct page *p, struct writeback_control *wbc)
> > return 0;
> > }
> >
> > +static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
> > +{
> > +}
> > +
> > static inline struct folio *swap_cache_get_folio(swp_entry_t entry,
> > struct vm_area_struct *vma, unsigned long addr)
> > {
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 556ff7347d5f..746aa9da5302 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -3365,6 +3365,19 @@ int swapcache_prepare(swp_entry_t entry)
> > return __swap_duplicate(entry, SWAP_HAS_CACHE);
> > }
> >
> > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
> > +{
> > + struct swap_cluster_info *ci;
> > + unsigned long offset = swp_offset(entry);
> > + unsigned char usage;
> > +
> > + ci = lock_cluster_or_swap_info(si, offset);
> > + usage = __swap_entry_free_locked(si, offset, SWAP_HAS_CACHE);
> > + unlock_cluster_or_swap_info(si, ci);
> > + if (!usage)
> > + free_swap_slot(entry);
> > +}
> > +
> > struct swap_info_struct *swp_swap_info(swp_entry_t entry)
> > {
> > return swap_type_to_swap_info(swp_type(entry));
> > --
> > 2.43.0

Thanks
Barry

Next message: Wentong Wu: "[PATCH] mei: Add Meteor Lake support for IVSC device"
Previous message: Stephen Rothwell: "linux-next: manual merge of the vfs-brauner tree with the nfsd tree"
In reply to: Chris Li: "Re: [PATCH v2] mm/swap: fix race when skipping swapcache"
Next in thread: Chris Li: "Re: [PATCH v2] mm/swap: fix race when skipping swapcache"
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