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Sun, 18 Feb 2024 00:19:22 -0800 (PST) MIME-Version: 1.0 References: <20240216095105.14502-1-ryncsn@gmail.com> <871q9atd6o.fsf@yhuang6-desk2.ccr.corp.intel.com> In-Reply-To: <871q9atd6o.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Barry Song <21cnbao@gmail.com> Date: Sun, 18 Feb 2024 21:19:10 +1300 Message-ID: Subject: Re: [PATCH v3] mm/swap: fix race when skipping swapcache To: "Huang, Ying" Cc: David Hildenbrand , Kairui Song , linux-mm@kvack.org, Andrew Morton , Chris Li , Minchan Kim , Yu Zhao , Barry Song , SeongJae Park , Hugh Dickins , Johannes Weiner , Matthew Wilcox , Michal Hocko , Yosry Ahmed , stable@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: CE9F014000F X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: srdpdutpt51ruj3e8rqbgz3wrykxxdtb X-HE-Tag: 1708244363-18748 X-HE-Meta: U2FsdGVkX190V+ZmaMP3AxEqzN833yIENqEy2wEBA97hTkizihIpfdfSP8juFCKENGeZs+vjsALRAl8t+eD8MMtbXN5RKQZTzKYpXMh/BysUFrfVaV0CxktwTV1dx/UIgFLfupfHIe7gxgXneBiX68+Q/JaobDBQIGcwE3j7G0tqaC/mBQDSUqHXkCCBzrHVbRNGRQO8BsQl3p5yhx/mSdiKODwB9E8zCaTCnrz5Rx70W6An1+KHQojrrC/F5O1jy2gHBw9gIWhDHpD4nDV70r3Jp6lXPgY2bYitn9Gg1XpilbWntEQYWv1OThpu7o9szibRrM20ICbyg6XRc/oZJTZTUlVGF9ORqy/Q2wFBDcnxSjfOzKiiH8pjPjhYm4ddztpNdfi85+5Lz66sgbkqozVU2VUO17gkTHzxwNtHQ5evDLlgFwjYPtEkrZ03Xn0ooB6jVyKTFa+4wE2QogBy/7ch74aYFSl6o9NLT4VROBiZXFR0aCxSK/o0Zf5Tk5N+Kw6CR1UbcGV4pM/Vpnwosnj2xyM2Ln4LckpkIv5mzMBZeHSSSz/h96XGpB1TFYn//cNTV4WBKrnw06dPxv6eI6ARC6xchYdhw0YHFSqm8gq3mpBcfTGkIZigqPfdMvfLQL2e8B3pw39nuiaMsr8mrtIeMWZEfCXE8SUbx6Xq87qVD2nHmQci0JA8jqGSLEkCRd9QeawjFJ7GRHcGK6pMKUZU2GLeHNCM/EbK7kibn3vspAXd+9gwR2IFV36yjCqNQcwGMoun+3x91pSFUVc1mzvbfCVA2UUMWtM9pCkflaFCqMtvbAJLetkyqWSHYFC1uSPY/xZp+tEh8H0VR4EAiy+jhcp9AhQiKB8Rlacd8uB3hZ3T00KVZjnZPn8HctXYqJR8yVGMcOLH+atD7rvRwEdG4vqMTklO7VcE0OUkGDvx779eXtiHJAihNyAfhIczgVP43JtAmp24sEh4MZP hplgDVHx RfjgEhABr84Mbci+CItRhsuRezBu8XuUFMpBt 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: On Sun, Feb 18, 2024 at 9:02=E2=80=AFPM Huang, Ying = wrote: > > David Hildenbrand writes: > > > On 16.02.24 10:51, Kairui Song wrote: > >> 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. > >> 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-stre= ss-race [1] > >> Reported-by: "Huang, Ying" > >> Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@yhuang6-desk2.c= cr.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 CP= U > >> 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_fr= ee 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=3Dn built [SeongJae P= ark] > >> - Update the commit message and summary, refer to SWP_SYNCHRONOUS_IO i= nstead 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 =3D NULL; > >> rmap_t rmap_flags =3D RMAP_NONE; > >> + bool need_clear_cache =3D false; > >> bool exclusive =3D 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) =3D=3D 1) { > >> + /* > >> + * Prevent parallel swapin from proceeding with > >> + * the cache flag. Otherwise, another thread may > >> + * finish swapin first, free the entry, and swapo= ut > >> + * 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 =3D 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_CACH= E > > * 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 s= wap > > 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, t= o > > * serialize against concurrent threads wanting to do the same. > > * [more from your description] > > */ > > ret =3D swapcache_prepare(entry); > > if (likely(!ret) > > /* > > * Move forward with swapin, we'll recheck if the PTE has= n't > > * changed later. > > */ > > break; > > else if (ret !=3D -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 ag= ainst > > * another thread, or the entry could have been freed and reuse= d in the > > * meantime. Make sure that the PTE did not change, to detect fre= eing. > > */ > > vmf->pte =3D 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() doe= s 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 sc= hedule() 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? >