Re: [PATCH] mm/swap: fix race condition in direct swapin path

[Huang, Ying]

Kairui Song <ryncsn@xxxxxxxxx> writes:

> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> In the direct swapin path, when two or more threads swapin the same entry
> at the same time, they get different pages (A, B) because swap cache is
> skipped. 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 modify and swap-out the page again, using the
> same entry. It break the pte_same check because PTE value is unchanged,
> causing ABA problem. Then thread (T0) will then install the stalled page
> (A) into the PTE so new data in page (B) is lost, 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_readpage() <- read to page A   swap_readpage() <- read to page B
> <slow on later locks or interrupt>  <finished swapin first>
> ...                                 set_pte_at()
>                                     swap_free() <- Now the entry is freed.
>                                     <write to page B, now page A stalled>
>                                     <swap out page B using 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!
>
> 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 the
> cached swapin path, two swapin path being used at the same time
> 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")
> Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1]
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>

Reported-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

> ---
> Huge thanks to Huang Ying and Chris Li for help finding this issue!
>
>  mm/memory.c   | 19 +++++++++++++++++++
>  mm/swap.h     |  5 +++++
>  mm/swapfile.c | 16 ++++++++++++++++
>  3 files changed, 40 insertions(+)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 7e1f4849463a..fd7c55a292f1 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3867,6 +3867,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	if (!folio) {
>  		if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
>  		    __swap_count(entry) == 1) {
> +			/*
> +			 * With swap count == 1, after we read the entry,
> +			 * other threads could finish swapin first, free
> +			 * the entry, then swapout the modified page using
> +			 * the same entry. Now the content we just read is
> +			 * stalled, and it's undetectable as pte_same()
> +			 * returns true due to entry reuse.
> +			 *
> +			 * So pin the swap entry using the cache flag even

"pin" doesn't sound intuitive here.  I know that the swap entry will not
be freed with this.  But now, the parallel swap in will busy waiting.
So, I suggest to say,

Prevent parallel swapin from proceeding with the cache flag.  Otherwise,
it may swapin first, free the entry, then swapout the modified page
using the same entry ...

> +			 * cache is not used.
> +			 */
> +			if (swapcache_prepare(entry))
> +				goto out;
> +
>  			/* skip swapcache */
>  			folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
>  						vma, vmf->address, false);
> @@ -4116,6 +4130,9 @@ 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 +4141,8 @@ 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..f7d4ad152a7f 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3365,6 +3365,22 @@ int swapcache_prepare(swp_entry_t entry)
>  	return __swap_duplicate(entry, SWAP_HAS_CACHE);
>  }
>  
> +/*
> + * Clear the cache flag and release pinned entry.

Even if we will keep "pin" in above comments, this is hard to be
understood for reader.  Need a little more explanation like "release
pinned entry for device with SWP_SYNCHRONOUS_IO.

Or, just remove the comments.  We have comments in calling site already.

> + */
> +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));

Otherwise it looks good for me, Thanks!

Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>