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.
--
Best Regards,
Huang, Ying
> + goto out;
> + }
> + need_clear_cache = true;
> +
> /* skip swapcache */
> folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
> vma, vmf->address, false);
> @@ -4117,6 +4132,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> if (vmf->pte)
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> out:
> + /* Clear the swap cache pin for direct swapin after PTL unlock */
> + if (need_clear_cache)
> + swapcache_clear(si, entry);
> if (si)
> put_swap_device(si);
> return ret;
> @@ -4131,6 +4149,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> folio_unlock(swapcache);
> folio_put(swapcache);
> }
> + if (need_clear_cache)
> + swapcache_clear(si, entry);
> if (si)
> put_swap_device(si);
> return ret;
> 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));
