Re: [PATCH v4] mm/swap: fix race when skipping swapcache

From: Huang, Ying
Date: Mon Feb 19 2024 - 20:11:16 EST

Barry Song <21cnbao@xxxxxxxxx> writes:

> On Mon, Feb 19, 2024 at 9:21 PM 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 swap it in, also prevent
>> any parallel code from putting the entry in the cache. Release the pin
>> after PT unlocked.
>>
>> Racers just loop and wait since it's a rare and very short event.
>> A schedule_timeout_uninterruptible(1) call is added to avoid repeated
>> page faults wasting too much CPU, causing livelock or adding too much
>> noise to perf statistics. A similar livelock issue was described in
>> commit 029c4628b2eb ("mm: swap: get rid of livelock in swapin readahead")
>>
>> 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
>> Cached: 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_-_@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
>> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx
>>
>> ---
>> V3: https://lore.kernel.org/all/20240216095105.14502-1-ryncsn@xxxxxxxxx/
>> Update from V3:
>> - Use schedule_timeout_uninterruptible(1) for now instead of schedule() to
>> prevent the busy faulting task holds CPU and livelocks [Huang, Ying]
>>
>> V2: https://lore.kernel.org/all/20240206182559.32264-1-ryncsn@xxxxxxxxx/
>> 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].
>>
>> V1: https://lore.kernel.org/all/20240205110959.4021-1-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.
>>
>> 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..a99f5e7be9a5 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_timeout_uninterruptible(1);
>
> Not a ideal model, imaging two tasks,
>
> task A - low priority running on a LITTLE core
> task B - high priority and have real-time requirements such as audio,
> graphics running on a big core.
>
> The original code will make B win even if it is a bit later than A as its CPU is
> much faster to finish swap_read_folio for example from zRAM. task B's
> swap-in can finish very soon.
>
> With the patch, B will wait a tick and its real-time performance will be
> negatively affected from time to time once low priority and high priority
> tasks fault in the same PTE and high priority tasks are a bit later than
> low priority tasks. This is a kind of priority inversion.
>
> When we support large folio swap-in, things can get worse. For example,
> to swap-in 16 or even more pages in one do_swap_page, the chance for
> task A and task B located in the same range of 16 PTEs will increase
> though they are not located in the same PTE.
>
> Please consider this is not a blocker for this patch. But I will put the problem
> in my list and run some real tests on Android phones later.

Yes. This may hurt performance. But this is necessary to solve a
livelock problem similar as commit 029c4628b2eb ("mm: swap: get rid of
livelock in swapin readahead"). Please consider that too.

--
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));
>> --
>> 2.43.0
>>
>>
>
> Thanks
> Barry