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Date: Mon, 19 Feb 2024 10:01:17 +0100
From: David Hildenbrand <david@...hat.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Kairui Song <kasong@...cent.com>, linux-mm@...ck.org,
Andrew Morton <akpm@...ux-foundation.org>, Chris Li <chrisl@...nel.org>,
Minchan Kim <minchan@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
Barry Song <v-songbaohua@...o.com>, SeongJae Park <sj@...nel.org>,
Hugh Dickins <hughd@...gle.com>, Johannes Weiner <hannes@...xchg.org>,
Matthew Wilcox <willy@...radead.org>, Michal Hocko <mhocko@...e.com>,
Yosry Ahmed <yosryahmed@...gle.com>, stable@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/swap: fix race when skipping swapcache
On 18.02.24 08:59, Huang, Ying wrote:
> David Hildenbrand <david@...hat.com> writes:
>
>> On 16.02.24 10:51, Kairui Song wrote:
>>> From: Kairui Song <kasong@...cent.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@...el.com>
>>> Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@yhuang6-desk2.ccr.corp.intel.com/
>>> Signed-off-by: Kairui Song <kasong@...cent.com>
>>> Cc: stable@...r.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
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