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b=TMCUVEggzvVanx+e/Uu4EqSf3yXVWaVGoPvI4NlSXwK1cv7CeMOyYJ/8z1WaDCTrt BhqnJN9isbF8uhQt3EbidJJJYlD2SKHwRcbBZjnNq8l3wbVAlNllqr941cMeVQR9+7 tAehqQ43fWSDn2x0sz8jvzH+zQJSTgviAAk2QnAxAiA5MtFg6Lc/5wDC5rhPeRb8Yf SRAY7H27yrmig3AJZR70T0uFR6uo08v1d9GRP2w/+6Ef/+dFpWAfrWGLoDRMsaLwn/ edxYTRZHXKbSStBQQA8bubM5X2eCbwZpDOgw4ph7fJ7DOFBX+/uH+5YissW1lWjFIx SxLJ3Pon5ZUdQ== Received: by mail-il1-f174.google.com with SMTP id e9e14a558f8ab-363d953a6c5so20445ab.1 for ; Tue, 06 Feb 2024 15:10:16 -0800 (PST) X-Gm-Message-State: AOJu0Ywk16s8ehmK5KL6B5iaFtJ+b02nVqhL6imTJUGAlYjRaV9DCo3r xN7Qnqp42UUHxf6G4QMPtQMXDqXXqKAcxFpNB5fjC5N39XfKanUuVcqSkaJoLR22yYnjfx+TXeo KTHbjv3aH+y2oVZOdw2DxOC7UYHT5jnbOAR8q X-Google-Smtp-Source: AGHT+IHV3jN4FC+kIzyEfyDAHrgmmaKZzwZWDknKMhHOe89mGhU8ReTYcaUpTN9+YVa12xZRpbqsJEhSypyO47A2sGI= X-Received: by 2002:a05:6e02:144:b0:363:b21d:9d2d with SMTP id j4-20020a056e02014400b00363b21d9d2dmr4167280ilr.11.1707261016008; Tue, 06 Feb 2024 15:10:16 -0800 (PST) MIME-Version: 1.0 References: <20240206182559.32264-1-ryncsn@gmail.com> In-Reply-To: <20240206182559.32264-1-ryncsn@gmail.com> From: Chris Li Date: Tue, 6 Feb 2024 15:10:04 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2] mm/swap: fix race when skipping swapcache To: Kairui Song Cc: linux-mm@kvack.org, Andrew Morton , "Huang, Ying" , 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 201EE20012 X-Stat-Signature: 9o3r53t3ppg1676sgw1kqaetij3wc1g6 X-HE-Tag: 1707261018-775523 X-HE-Meta: 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 6Kw9YjZU NnEWDRgyb74BG+o72OHhtgvMrY4+sXjIKLoMPSy0b6mVAMlRhd9DJYimjbo/apldxU5ABHgan0e5AjFzpDxr6Lq07Hqo1oRwbD6e3RUs80m3qbpjKBNu39H6jxT/hzyJ+ZXdtibraeSxf1lFXo6e9okYc57oe2CBk/IGnnoVGnOxHeKl443+aTVjh2BA+9mUt8gmCrsp3kDcM5Sc++2FbTgzXcIN1FIanFlL6tlHF4ZHl8ixUpSqBgp9MOGBZWTGKqgCnRHp+1KPa0Cw6KR+XEa6nkLD2LfNVYVb7z/g4PwP/EhR0sJ/jZjJAzw4Eva9SONCC1cX097jwWpuNagK9duMzyiqVZJTDrU9cqjVgHXfhS44DDPMODJmgjrhtyQP1kPdthrlMK+Kv4RTPBPSUzlT4XMwSjO/y7rjus7NTAfNCh+2UWRbTO9YjuK9zDzRx8myV03JVaQKh12Y= 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: 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 wrot= e: > > 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 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 synchronous = device") > Reported-by: "Huang, Ying" > Closes: https://lore.kernel.org/lkml/87bk92gqpx.fsf_-_@yhuang6-desk2.ccr.= corp.intel.com/ > Link: https://github.com/ryncsn/emm-test-project/tree/master/swap-stress-= 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_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 [SeongJae Park= ] > - Update the commit message and summary, refer to SWP_SYNCHRONOUS_IO inst= ead 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 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)) > + 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 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. This behavior needs more discussion. Chris Chris > /* skip swapcache */ > folio =3D vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0= , > vma, vmf->address, false)= ; > @@ -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, struc= t writeback_control *wbc) > return 0; > } > > +static inline void swapcache_clear(struct swap_info_struct *si, swp_entr= y_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 > >