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

From: Chris Li
Date: Sun Feb 18 2024 - 13:37:27 EST

On Sun, Feb 18, 2024 at 12:41 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Barry Song <21cnbao@xxxxxxxxx> writes:
>
> > On Sun, Feb 18, 2024 at 9:02 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> >>
> >> David Hildenbrand <david@xxxxxxxxxx> writes:
> >>
> >> > On 16.02.24 10:51, Kairui Song 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 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@xxxxxxxxx>
> >> >> Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> >> >> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> >> >> Cc: stable@xxxxxxxxxxxxxxx
> >> >> ---
> >> >> 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@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.
> >> >> V1:
> >> >> https://lore.kernel.org/all/20240205110959.4021-1-ryncsn@xxxxxxxxx/
> >> >> 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.

I am trying to find the alternative path which can cause the swap
fault to bring in the swap cache in page fault while the SYNC IO is
looping for HAS_SWAP_CACHE. Kairui was able to identify the in the
current code in do_page_fault() path, the rmap and fork case wouldn't
be able to modify the swap cache causing a problem. The MADV_WILLNEED
is an excellent example. Thank you for finding this example.

> >
> > This seems fine.
> >
> > if swapcache has data from WILLNEED, the new page fault will hit it. Thus,
> > we won't go into the SYNC_IO path any more?
>
> They may happen in parallel. That is, one task is busy looping, while
> another task read the swap entry into swap cache.

Agree.

Chris