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[79.242.52.182]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-439b02f252dsm111389435e9.22.2025.02.24.10.03.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 Feb 2025 10:04:00 -0800 (PST) Message-ID: Date: Mon, 24 Feb 2025 19:03:58 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache To: Peter Xu , Barry Song <21cnbao@gmail.com> Cc: Liam.Howlett@oracle.com, aarcange@redhat.com, akpm@linux-foundation.org, axelrasmussen@google.com, bgeffon@google.com, brauner@kernel.org, hughd@google.com, jannh@google.com, kaleshsingh@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lokeshgidra@google.com, mhocko@suse.com, ngeoffray@google.com, rppt@kernel.org, ryan.roberts@arm.com, shuah@kernel.org, surenb@google.com, v-songbaohua@oppo.com, viro@zeniv.linux.org.uk, willy@infradead.org, zhangpeng362@huawei.com, zhengtangquan@oppo.com, yuzhao@google.com, stable@vger.kernel.org References: <69dbca2b-cf67-4fd8-ba22-7e6211b3e7c4@redhat.com> <20250220092101.71966-1-21cnbao@gmail.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 24.02.25 18:50, Peter Xu wrote: > On Sun, Feb 23, 2025 at 10:31:37AM +1300, Barry Song wrote: >> On Fri, Feb 21, 2025 at 2:49 PM Peter Xu wrote: >>> >>> On Fri, Feb 21, 2025 at 01:07:24PM +1300, Barry Song wrote: >>>> On Fri, Feb 21, 2025 at 12:32 PM Peter Xu wrote: >>>>> >>>>> On Thu, Feb 20, 2025 at 10:21:01PM +1300, Barry Song wrote: >>>>>> 2. src_anon_vma and its lock – swapcache doesn’t require it（folio is not mapped） >>>>> >>>>> Could you help explain what guarantees the rmap walk not happen on a >>>>> swapcache page? >>>>> >>>>> I'm not familiar with this path, though at least I see damon can start a >>>>> rmap walk on PageAnon almost with no locking.. some explanations would be >>>>> appreciated. >>>> >>>> I am observing the following in folio_referenced(), which the anon_vma lock >>>> was originally intended to protect. >>>> >>>> if (!pra.mapcount) >>>> return 0; >>>> >>>> I assume all other rmap walks should do the same? >>> >>> Yes normally there'll be a folio_mapcount() check, however.. >>> >>>> >>>> int folio_referenced(struct folio *folio, int is_locked, >>>> struct mem_cgroup *memcg, unsigned long *vm_flags) >>>> { >>>> >>>> bool we_locked = false; >>>> struct folio_referenced_arg pra = { >>>> .mapcount = folio_mapcount(folio), >>>> .memcg = memcg, >>>> }; >>>> >>>> struct rmap_walk_control rwc = { >>>> .rmap_one = folio_referenced_one, >>>> .arg = (void *)&pra, >>>> .anon_lock = folio_lock_anon_vma_read, >>>> .try_lock = true, >>>> .invalid_vma = invalid_folio_referenced_vma, >>>> }; >>>> >>>> *vm_flags = 0; >>>> if (!pra.mapcount) >>>> return 0; >>>> ... >>>> } >>>> >>>> By the way, since the folio has been under reclamation in this case and >>>> isn't in the lru, this should also prevent the rmap walk, right? >>> >>> .. I'm not sure whether it's always working. >>> >>> The thing is anon doesn't even require folio lock held during (1) checking >>> mapcount and (2) doing the rmap walk, in all similar cases as above. I see >>> nothing blocks it from a concurrent thread zapping that last mapcount: >>> >>> thread 1 thread 2 >>> -------- -------- >>> [whatever scanner] >>> check folio_mapcount(), non-zero >>> zap the last map.. then mapcount==0 >>> rmap_walk() >>> >>> Not sure if I missed something. >>> >>> The other thing is IIUC swapcache page can also have chance to be faulted >>> in but only if a read not write. I actually had a feeling that your >>> reproducer triggered that exact path, causing a read swap in, reusing the >>> swapcache page, and hit the sanity check there somehow (even as mentioned >>> in the other reply, I don't yet know why the 1st check didn't seem to >>> work.. as we do check folio->index twice..). >>> >>> Said that, I'm not sure if above concern will happen in this specific case, >>> as UIFFDIO_MOVE is pretty special, that we check exclusive bit first in swp >>> entry so we know it's definitely not mapped elsewhere, meanwhile if we hold >>> pgtable lock so maybe it can't get mapped back.. it is just still tricky, >>> at least we do some dances all over releasing and retaking locks. >>> >>> We could either justify that's safe, or maybe still ok and simpler if we >>> could take anon_vma write lock, making sure nobody will be able to read the >>> folio->index when it's prone to an update. >> >> What prompted me to do the former is that folio_get_anon_vma() returns >> NULL for an unmapped folio. As for the latter, we need to carefully evaluate >> whether the change below is safe. >> >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -505,7 +505,7 @@ struct anon_vma *folio_get_anon_vma(const struct >> folio *folio) >> anon_mapping = (unsigned long)READ_ONCE(folio->mapping); >> if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) >> goto out; >> >> - if (!folio_mapped(folio)) >> + if (!folio_mapped(folio) && !folio_test_swapcache(folio)) >> goto out; >> >> anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); >> @@ -521,7 +521,7 @@ struct anon_vma *folio_get_anon_vma(const struct >> folio *folio) >> * SLAB_TYPESAFE_BY_RCU guarantees that - so the atomic_inc_not_zero() >> * above cannot corrupt). >> */ > > [1] > >> >> - if (!folio_mapped(folio)) { >> + if (!folio_mapped(folio) && !folio_test_swapcache(folio)) { >> rcu_read_unlock(); >> put_anon_vma(anon_vma); >> return NULL; > > Hmm, this let me go back read again on how we manage anon_vma lifespan, > then I just noticed this may not work. > > See the comment right above [1], here's a full version: > > /* > * If this folio is still mapped, then its anon_vma cannot have been > * freed. But if it has been unmapped, we have no security against the > * anon_vma structure being freed and reused (for another anon_vma: > * SLAB_TYPESAFE_BY_RCU guarantees that - so the atomic_inc_not_zero() > * above cannot corrupt). > */ > > So afaiu that means we pretty much very rely upon folio_mapped() check to > make sure anon_vma being valid at all that we fetched from folio->mapping, > not to mention the rmap walk later afterwards. > > Then above diff in folio_get_anon_vma() should be problematic, as when > "folio_mapped()==false && folio_test_swapcache()==true", above change will > start to return anon_vma pointer even if the anon_vma could have been freed > and reused by other VMAs. When splitting a folio, we use folio_get_anon_vma(). That seems to work as long as we have the folio locked. -- Cheers, David / dhildenb