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Sun, 18 Feb 2024 01:30:49 -0800 (PST) MIME-Version: 1.0 References: <20240216095105.14502-1-ryncsn@gmail.com> <871q9atd6o.fsf@yhuang6-desk2.ccr.corp.intel.com> <87sf1qrwsw.fsf@yhuang6-desk2.ccr.corp.intel.com> In-Reply-To: From: Kairui Song Date: Sun, 18 Feb 2024 17:30:31 +0800 Message-ID: Subject: Re: [PATCH v3] mm/swap: fix race when skipping swapcache To: Barry Song <21cnbao@gmail.com> Cc: "Huang, Ying" , David Hildenbrand , 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-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 02525140006 X-Stat-Signature: up7ry95d5os7reyosth98dk8iwmd5b7j X-HE-Tag: 1708248651-158144 X-HE-Meta: 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 xXavoVyd 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 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 4:47=E2=80=AFPM Barry Song <21cnbao@gmail.com> wrot= e: > > On Sun, Feb 18, 2024 at 9:41=E2=80=AFPM Huang, Ying wrote: > > > > Barry Song <21cnbao@gmail.com> writes: > > > > > 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) beca= use > > >> >> 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 ent= ry > > >> >> > > >> >> > > >> >> swap_read_folio() <- read to page A swap_read_folio() <- read to= page B > > >> >> > > >> >> ... set_pte_at() > > >> >> swap_free() <- entry is fre= e > > >> >> > > >> >> > > >> >> 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 disc= ard > > >> >> 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 entr= y > > >> >> 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 g= ood > > >> >> idea after some tests, that will cause racers to fall back to use= the > > >> >> swap cache again. Parallel swapin using different methods leads t= o > > >> >> 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 l= oss! > > >> >> 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 dedic= ated > > >> >> 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 rou= nds > > >> >> and no data loss observed. > > >> >> Performance overhead is minimal, microbenchmark swapin 10G from 3= 2G > > >> >> 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-de= sk2.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 wasti= ng CPU > > >> >> and add noise to perf statistics. > > >> >> - Use a bool to state the special case instead of reusing existin= g > > >> >> variables fixing error handling [Minchan Kim]. > > >> >> V2: > > >> >> https://lore.kernel.org/all/20240206182559.32264-1-ryncsn@gmail.c= om/ > > >> >> Update from V1: > > >> >> - Add some words on ZRAM case, it will discard swap content on sw= ap_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=3Dn built [Seong= Jae 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.co= m/ > > >> >> 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 *vm= f) > > >> >> 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 *v= mf) > > >> >> if (!folio) { > > >> >> if (data_race(si->flags & SWP_SYNCHRONOUS_IO) && > > >> >> __swap_count(entry) =3D=3D 1) { > > >> >> + /* > > >> >> + * Prevent parallel swapin from proceeding w= ith > > >> >> + * 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 reus= e. > > >> >> + */ > > >> >> + if (swapcache_prepare(entry)) { > > >> >> + /* Relax a bit to prevent rapid repe= ated page faults */ > > >> >> + schedule(); > > >> >> + goto out; > > >> >> + } > > >> >> + need_clear_cache =3D true; > > >> >> + > > >> > > > >> > I took a closer look at __read_swap_cache_async() and it essential= ly > > >> > 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_CA= CHE > > >> > * has not yet been cleared. Or race against anothe= r > > >> > * __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 swapcac= he, to > > >> > * serialize against concurrent threads wanting to do the sa= me. > > >> > * [more from your description] > > >> > */ > > >> > ret =3D swapcache_prepare(entry); > > >> > if (likely(!ret) > > >> > /* > > >> > * Move forward with swapin, we'll recheck if the PT= E hasn'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? > > > > They may happen in parallel. That is, one task is busy looping, while > > another task read the swap entry into swap cache. > > do_swap_page isn't busy looping swapcache_prepare, if it fails, it exits, > then we have a completely new page fault. this new page fault will > lookup swapcache and find it, going into the path to set swapcache > to ptes. so the new page fault won't do swapcache_prepare any more. > Hi Barry The issue here with this code snip is that we could have swapcache_prepare(entry) =3D=3D -EEXIST and pte_same =3D=3D true, not necessarily WILLNEED, a concurrent fault and swapout could cause that. Then we are stuck with while(true) here. It can be fixed, but still there are other potential issues, we end up with a similar loop in swap_state.c, may still need to bail out the fault for some cases, things seem not improved here.