On Wed, Feb 07, 2024 at 02:25:59AM +0800, Kairui Song wrote:
> From: Kairui Song <[email protected]>
>
> 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
^^^
nit: From the recent code, I see swap_free is called earlier than set_pte_at
> <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.
Thanks for catching the issue, folks!
>
> 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" <[email protected]>
> Closes: https://lore.kernel.org/lkml/[email protected]/
> Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-race [1]
> Signed-off-by: Kairui Song <[email protected]>
> Reviewed-by: "Huang, Ying" <[email protected]>
> Acked-by: Yu Zhao <[email protected]>
>
> ---
> 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;
> +
> /* 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);
What happens?
do_swap_page
..
swapcache_prepare() <- tured the cache flag on
folio = vma_alloc_folio <- failed to allocate the folio
page = &foio->page; <- crash but it's out of scope from this patch
..
if (!folio)
goto unlock;
.
unlock:
swapcache_clear(si, entry) <- it's skipped this time.
Can we simply introduce a boolean flag to state the special case and
clear the cache state based on the flag?
if (swapcache_prepare())
goto out;
need_clear_cache = true;
out_path:
if (need_clear_cache)
swapcache_clear