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X-IronPort-AV: E=McAfee;i="6600,9927,10976"; a="18309830" X-IronPort-AV: E=Sophos;i="6.05,248,1701158400"; d="scan'208";a="18309830" Received: from orviesa007.jf.intel.com ([10.64.159.147]) by orvoesa102.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Feb 2024 18:10:50 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.05,248,1701158400"; d="scan'208";a="1508747" Received: from yhuang6-desk2.sh.intel.com (HELO yhuang6-desk2.ccr.corp.intel.com) ([10.238.208.55]) by orviesa007-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Feb 2024 18:10:45 -0800 From: "Huang, Ying" To: Barry Song <21cnbao@gmail.com> Cc: Chris Li , Kairui Song , linux-mm@kvack.org, Andrew Morton , 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 v2] mm/swap: fix race when skipping swapcache In-Reply-To: (Barry Song's message of "Wed, 7 Feb 2024 07:40:12 +0800") References: <20240206182559.32264-1-ryncsn@gmail.com> Date: Wed, 07 Feb 2024 10:08:49 +0800 Message-ID: <87sf25yqj2.fsf@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: A3D7C120002 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 4trqnfassquwkxr59pzf7wfh8b9xr4nh X-HE-Tag: 1707271851-592795 X-HE-Meta: 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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: Barry Song <21cnbao@gmail.com> writes: > On Wed, Feb 7, 2024 at 7:18=E2=80=AFAM Chris Li wrote: >> >> Hi Kairui, >> >> Sorry replying to your patch V1 late, I will reply on the V2 thread. >> >> On Tue, Feb 6, 2024 at 10:28=E2=80=AFAM Kairui Song w= rote: >> > >> > 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 usi= ng >> > the cache flag, and allow only one thread to pin it. Release the pin >> > after PT unlocked. Racers will simply busy wait since it's a rare >> > and very short event. >> > >> > 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 synchrono= us device") >> > Reported-by: "Huang, Ying" >> > Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@yhuang6-desk2.c= cr.corp.intel.com/ >> > Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stre= ss-race [1] >> > Signed-off-by: Kairui Song >> > Reviewed-by: "Huang, Ying" >> > Acked-by: Yu Zhao >> > >> > --- >> > 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. >> > >> > include/linux/swap.h | 5 +++++ >> > mm/memory.c | 15 +++++++++++++++ >> > mm/swap.h | 5 +++++ >> > mm/swapfile.c | 13 +++++++++++++ >> > 4 files changed, 38 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..1749c700823d 100644 >> > --- a/mm/memory.c >> > +++ b/mm/memory.c >> > @@ -3867,6 +3867,16 @@ 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 m= ay >> > + * finish swapin first, free the entry, and sw= apout >> > + * reusing the same entry. It's undetectable as >> > + * pte_same() returns true due to entry reuse. >> > + */ >> > + if (swapcache_prepare(entry)) >> > + goto out; >> > + >> >> I am puzzled by this "goto out". If I understand this correctly, you >> have two threads CPU1 and CPU2 racing to set the flag SWAP_HAS_CACHE. >> The CPU1 will succeed in adding the flag and the CPU2 will get >> "-EEXIST" from "swapcache_prepare(entry)". Am I understanding it >> correctly so far? >> >> Then the goto out seems wrong to me. For the CPU2, the page fault will >> return *unhandled*. Even worse, the "-EEXIST" error is not preserved, >> CPU2 does not even know the page fault is not handled, it will resume >> from the page fault instruction, possibly generate another page fault >> at the exact same location. That page fault loop will repeat until >> CPU1 install the new pte on that faulting virtual address and pick up >> by CPU2. >> >> Am I missing something obvious there? > > I feel you are right. any concurrent page faults at the same pte > will increase the count of page faults for a couple of times now. > >> >> I just re-read your comment: "Racers will simply busy wait since it's >> a rare and very short event." That might be referring to the above >> CPU2 page fault looping situation. I consider the page fault looping >> on CPU2 not acceptable. For one it will mess up the page fault >> statistics. >> In my mind, having an explicit loop for CPU2 waiting for the PTE to >> show up is still better than this page fault loop. You can have more >> CPU power friendly loops. > > I assume you mean something like > > while(!pte_same()) > cpu_relax(); > > then we still have a chance to miss the change of B. > > For example, another thread is changing pte to A->B->A, our loop can > miss B. Thus we will trap into an infinite loop. this is even worse. > > is it possible to loop for the success of swapcache_prepare(entry) > instead? This doesn't work too. The swap count can increase to > 1 and be put in swap cache for long time. Another possibility is to move swapcache_prepare() after vma_alloc_folio() to reduce the race window. -- Best Regards, Huang, Ying >> >> This behavior needs more discussion. >> >> Chris >> >> >> Chris >> >> >> > /* skip swapcache */ >> > folio =3D vma_alloc_folio(GFP_HIGHUSER_MOVABLE= , 0, >> > vma, vmf->address, fal= se); >> > @@ -4116,6 +4126,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >> > unlock: >> > if (vmf->pte) >> > pte_unmap_unlock(vmf->pte, vmf->ptl); >> > + /* Clear the swap cache pin for direct swapin after PTL unlock= */ >> > + if (folio && !swapcache) >> > + swapcache_clear(si, entry); >> > out: >> > if (si) >> > put_swap_device(si); >> > @@ -4124,6 +4137,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >> > if (vmf->pte) >> > pte_unmap_unlock(vmf->pte, vmf->ptl); >> > out_page: >> > + if (!swapcache) >> > + swapcache_clear(si, entry); >> > folio_unlock(folio); >> > out_release: >> > folio_put(folio); >> > 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, st= ruct writeback_control *wbc) >> > return 0; >> > } >> > >> > +static inline void swapcache_clear(struct swap_info_struct *si, swp_e= ntry_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 =3D swp_offset(entry); >> > + unsigned char usage; >> > + >> > + ci =3D lock_cluster_or_swap_info(si, offset); >> > + usage =3D __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)); >> > -- >> > 2.43.0 > > Thanks > Barry