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X-IronPort-AV: E=McAfee;i="6600,9927,10987"; a="27783324" X-IronPort-AV: E=Sophos;i="6.06,168,1705392000"; d="scan'208";a="27783324" Received: from orviesa006.jf.intel.com ([10.64.159.146]) by fmvoesa101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 18 Feb 2024 00:34:21 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.06,168,1705392000"; d="scan'208";a="4583888" Received: from yhuang6-desk2.sh.intel.com (HELO yhuang6-desk2.ccr.corp.intel.com) ([10.238.208.55]) by orviesa006-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 18 Feb 2024 00:34:17 -0800 From: "Huang, Ying" To: Kairui Song Cc: linux-mm@kvack.org, Kairui Song , Andrew Morton , Chris Li , Minchan Kim , Yu Zhao , Barry Song , SeongJae Park , Hugh Dickins , Johannes Weiner , Matthew Wilcox , Michal Hocko , Yosry Ahmed , David Hildenbrand , stable@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] mm/swap: fix race when skipping swapcache In-Reply-To: <20240216095105.14502-1-ryncsn@gmail.com> (Kairui Song's message of "Fri, 16 Feb 2024 17:51:05 +0800") References: <20240216095105.14502-1-ryncsn@gmail.com> Date: Sun, 18 Feb 2024 16:32:21 +0800 Message-ID: <87wmr2rx4a.fsf@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii X-Rspamd-Queue-Id: 4C695140014 X-Rspam-User: X-Stat-Signature: ofkzshcdqq4qwhft4ja8ggfooxyxsq3x X-Rspamd-Server: rspam03 X-HE-Tag: 1708245262-43193 X-HE-Meta: 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 eW3o7tM/ S+BXtMox0enQJfY7CrCbeVcQ1G6HmKJMaVOzjne+jr/x9xqPP38NA3m/+fJiaF6xYeycYuPS+uS3vLydZ50YhHlGDHhoo42pMADotV/9WbCPyDno9c9RyOq/+EEvug8pGIMSZ99DcYa6OHUVpplyhUXR5OmjdHwB5dHUDYXOM143lO0dFwuTYxLP0E69AI8xCCDOO8gZjGtig1vLfz02kS41PI2QAxZVEJowqdlnQat5lghHraosfO7CfnoK+4VAzerzX2NnyzJ8REREwzBvnxYoh6l6JjapSLEjUqdZpwkURN2DkfHIpw76ud55HwBO4fqg/KgnCpbdkhuwqKviAlZtCbrkH1ANc18LM8aA5oNjfP34= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Kairui Song writes: > From: Kairui Song > > 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 > > > swap_read_folio() <- read to page A swap_read_folio() <- read to page B > > ... set_pte_at() > swap_free() <- entry is free > > > 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. The swap entry may be put in swap cache by some parallel code path anyway. So, we always need to consider that when reasoning the code. > 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" > Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@yhuang6-desk2.ccr.corp.intel.com/ > Signed-off-by: Kairui Song > 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(); The current task may be chosen in schedule(). So, I think that we should use cond_resched() here. -- 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));