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Mon, 19 Feb 2024 14:10:16 -0800 (PST) MIME-Version: 1.0 References: <20240219082040.7495-1-ryncsn@gmail.com> In-Reply-To: <20240219082040.7495-1-ryncsn@gmail.com> From: Barry Song <21cnbao@gmail.com> Date: Tue, 20 Feb 2024 11:10:03 +1300 Message-ID: Subject: Re: [PATCH v4] mm/swap: fix race when skipping swapcache To: Kairui Song Cc: linux-mm@kvack.org, Andrew Morton , "Huang, Ying" , Chris Li , Minchan Kim , Barry Song , Yu Zhao , SeongJae Park , David Hildenbrand , Hugh Dickins , Johannes Weiner , Matthew Wilcox , Michal Hocko , Yosry Ahmed , Aaron Lu , 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: 4DF9140009 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: gmc3drank7hx6p1cqji9ns16o6gxe3xa X-HE-Tag: 1708380617-883664 X-HE-Meta: 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 9dNUfb3G GxKLUHLHsBpWxl+qItG8Tn2iVtj5SYGzcuVKY 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 Mon, Feb 19, 2024 at 9:21=E2=80=AFPM 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 swap it in, also prevent > any parallel code from putting the entry in the cache. Release the pin > after PT unlocked. > > Racers just loop and wait since it's a rare and very short event. > A schedule_timeout_uninterruptible(1) call is added to avoid repeated > page faults wasting too much CPU, causing livelock or adding too much > noise to perf statistics. A similar livelock issue was described in > commit 029c4628b2eb ("mm: swap: get rid of livelock in swapin readahead") > > 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 > Cached: 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 > > --- > V3: https://lore.kernel.org/all/20240216095105.14502-1-ryncsn@gmail.com/ > Update from V3: > - Use schedule_timeout_uninterruptible(1) for now instead of schedule() t= o > prevent the busy faulting task holds CPU and livelocks [Huang, Ying] > > V2: https://lore.kernel.org/all/20240206182559.32264-1-ryncsn@gmail.com/ > 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]. > > V1: https://lore.kernel.org/all/20240205110959.4021-1-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=3Dn 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. > > 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..a99f5e7be9a5 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_timeout_uninterruptible(1); Not a ideal model, imaging two tasks, task A - low priority running on a LITTLE core task B - high priority and have real-time requirements such as audio, graphics running on a big core. The original code will make B win even if it is a bit later than A as its C= PU is much faster to finish swap_read_folio for example from zRAM. task B's swap-in can finish very soon. With the patch, B will wait a tick and its real-time performance will be negatively affected from time to time once low priority and high priority tasks fault in the same PTE and high priority tasks are a bit later than low priority tasks. This is a kind of priority inversion. When we support large folio swap-in, things can get worse. For example, to swap-in 16 or even more pages in one do_swap_page, the chance for task A and task B located in the same range of 16 PTEs will increase though they are not located in the same PTE. Please consider this is not a blocker for this patch. But I will put the pr= oblem in my list and run some real tests on Android phones later. > + goto out; > + } > + need_clear_cache =3D true; > + > /* skip swapcache */ > folio =3D 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, 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 > > Thanks Barry