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

From: Barry Song
Date: Thu Feb 15 2024 - 19:57:10 EST

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On Fri, Feb 16, 2024 at 11:58 AM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, 15 Feb 2024 12:55:11 -0800 Minchan Kim <minchan@xxxxxxxxxx> wrote:
>
> > Only concern of the approach is that it would be harder to have the fix
> > in the stable tree. If there isn't strong objection, I prefer the
> > Kairui's orginal solution(with some tweak of scheduler if it's
> > necessary) first and then pursue your idea on latest tree.
>
> Do we agree that this fix is needed in -stable? For some reason I
> don't have a cc:stable in the changelog.
>
> I'd like to move this patch into mm-hotfixes-stable in order to get it
> upstream very soon. Are we all agreeable with that? I don't have an
> Acked-by: David?
>
> I have a note (which dates back to an earlier version) that Barry would
> be performing runtime testing. Has that occurred?

yep. i wanted to check if Kairui's patch can also take a positive affection
on my problem, a race between large folios swap-in and madvise pageout
i reported here[1]

but unfortunately I failed as Kairui's patch can only handle normal folio
with one page as if (swapcache_prepare(entry)) supports only one
entry.

so please consider my test irrelevant for this moment. if Kairui's approach
is accepted, I think I need to somehow make a batched API like
swapcache_prepare_nr(entries, nr) afterwards.

[1] https://lore.kernel.org/linux-mm/20240205095155.7151-1-v-songbaohua@xxxxxxxx/

>
> Thanks.
>
>
> Here's what is presently in mm-hotfixes-unstable:
>
>
>
> From: Kairui Song <kasong@xxxxxxxxxxx>
> Subject: mm/swap: fix race when skipping swapcache
> Date: Wed, 7 Feb 2024 02:25:59 +0800
>
> 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)
>
> Link: https://lkml.kernel.org/r/20240206182559.32264-1-ryncsn@xxxxxxxxx
> 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>
> Cc: Chris Li <chrisl@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> Cc: Barry Song <21cnbao@xxxxxxxxx>
> Cc: SeongJae Park <sj@xxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>
> include/linux/swap.h | 5 +++++
> mm/memory.c | 15 +++++++++++++++
> mm/swap.h | 5 +++++
> mm/swapfile.c | 13 +++++++++++++
> 4 files changed, 38 insertions(+)
>
> --- a/include/linux/swap.h~mm-swap-fix-race-when-skipping-swapcache
> +++ a/include/linux/swap.h
> @@ -549,6 +549,11 @@ static inline int swap_duplicate(swp_ent
> return 0;
> }
>
> +static inline int swapcache_prepare(swp_entry_t swp)
> +{
> + return 0;
> +}
> +
> static inline void swap_free(swp_entry_t swp)
> {
> }
> --- a/mm/memory.c~mm-swap-fix-race-when-skipping-swapcache
> +++ a/mm/memory.c
> @@ -3867,6 +3867,16 @@ vm_fault_t do_swap_page(struct vm_fault
> 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;
> +
> /* 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
> 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 @@ out_nomap:
> 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);
> --- a/mm/swapfile.c~mm-swap-fix-race-when-skipping-swapcache
> +++ a/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));
> --- a/mm/swap.h~mm-swap-fix-race-when-skipping-swapcache
> +++ a/mm/swap.h
> @@ -41,6 +41,7 @@ void __delete_from_swap_cache(struct fol
> 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
> 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)
> {
> _
>
>

Thanks
Barry

Next message: Palmer Dabbelt: "Re: [PATCH v4 0/4] membarrier: riscv: Core serializing command"
Previous message: Andrew Morton: "Re: [PATCH v3 13/35] lib: add allocation tagging support for memory allocation profiling"
In reply to: Andrew Morton: "Re: [PATCH v2] mm/swap: fix race when skipping swapcache"
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