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. 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? > > -- > Best Regards, > Huang, Ying > > > > > /* > > * See __read_swap_cache_async(). We might either have raced against > > * another thread, or the entry could have been freed and reused in the > > * meantime. Make sure that the PTE did not change, to detect freeing. > > */ > > vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, > > vmf->address, &vmf->ptl); > > if (!vmf->pte || !pte_same(ptep_get(vmf->pte), vmf->orig_pte)) > > goto unlock; > > > > > > schedule(); > > } > > > > > > > > I was skeptical about the schedule(), but __read_swap_cache_async() does it > > already because there is no better way to wait for the event to happen. > > > > With something like above you would no longer depend on the speed of schedule() to > > determine how often you would retry the fault, which would likely make sense. > > > > I do wonder about the schedule() vs. schedule_timeout_uninterruptible(), though. > > No expert on that area, do you have any idea? >