Re: [PATCH v3] mm/swap: fix race when skipping swapcache

[David Hildenbrand <david@redhat.com>]

On 18.02.24 08:59, Huang, Ying wrote:
> David Hildenbrand <david@redhat.com> writes:
> 
>> On 16.02.24 10:51, Kairui Song wrote:
>>> From: Kairui Song <kasong@tencent.com>
>>> 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.
>>> 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@intel.com>
>>> Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@yhuang6-desk2.ccr.corp.intel.com/
>>> Signed-off-by: Kairui Song <kasong@tencent.com>
>>> Cc: stable@vger.kernel.org
>>> ---
>>> 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@gmail.com/
>>> 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@gmail.com/
>>>    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();
>>> +				goto out;
>>> +			}
>>> +			need_clear_cache = true;
>>> +
>>
>> I took a closer look at __read_swap_cache_async() and it essentially
>> does something similar.
>>
>> Instead of returning, it keeps retrying until it finds that
>> swapcache_prepare() fails for another reason than -EEXISTS (e.g.,
>> freed concurrently) or it finds the entry in the swapcache.
>>
>> So if you would succeed here on a freed+reused swap entry,
>> __read_swap_cache_async() would simply retry.
>>
>> It spells that out:
>>
>> 		/*
>> 		 * We might race against __delete_from_swap_cache(), and
>> 		 * stumble across a swap_map entry whose SWAP_HAS_CACHE
>> 		 * has not yet been cleared.  Or race against another
>> 		 * __read_swap_cache_async(), which has set SWAP_HAS_CACHE
>> 		 * in swap_map, but not yet added its folio to swap cache.
>> 		 */
>>
>> Whereby we could not race against this code here as well where we
>> speculatively set SWAP_HAS_CACHE and might never add something to the swap
>> cache.
>>
>>
>> I'd probably avoid the wrong returns and do something even closer to
>> __read_swap_cache_async().
>>
>> while (true) {
>> 	/*
>> 	 * Fake that we are trying to insert a page into the swapcache, to
>> 	 * serialize against concurrent threads wanting to do the same.
>> 	 * [more from your description]
>> 	 */
>> 	ret = swapcache_prepare(entry);
>> 	if (likely(!ret)
>> 		/*
>> 		 * Move forward with swapin, we'll recheck if the PTE hasn't
>> 		 * changed later.
>> 		 */
>> 		break;
>> 	else if (ret != -EEXIST)
>> 		goto out;
> 
> The swap entry may be kept in swap cache for long time.  For example, it
> may be read into swap cache via MADV_WILLNEED.

Right, we'd have to check for the swapcache.

I briefly thought about just factoring out what we have in 
__read_swap_cache_async() and reusing here. Similar problem to solve, 
and quite a lot of duplicate code.

But not worth the churn in a simple fix. We could explore that option
as a cleanup on top.

-- 
Cheers,

David / dhildenb