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Tue, 10 Dec 2024 14:21:26 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id xan0KmZOWGcSEwAAD6G6ig (envelope-from ); Tue, 10 Dec 2024 14:21:26 +0000 Message-ID: <6b29b2a5-c244-4930-a5a0-1a24a04e7e35@suse.cz> Date: Tue, 10 Dec 2024 15:21:26 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 4/6] mm: make vma cache SLAB_TYPESAFE_BY_RCU Content-Language: en-US To: Suren Baghdasaryan , akpm@linux-foundation.org Cc: willy@infradead.org, liam.howlett@oracle.com, lorenzo.stoakes@oracle.com, mhocko@suse.com, hannes@cmpxchg.org, mjguzik@gmail.com, oliver.sang@intel.com, mgorman@techsingularity.net, david@redhat.com, peterx@redhat.com, oleg@redhat.com, dave@stgolabs.net, paulmck@kernel.org, brauner@kernel.org, dhowells@redhat.com, hdanton@sina.com, hughd@google.com, minchan@google.com, jannh@google.com, shakeel.butt@linux.dev, souravpanda@google.com, pasha.tatashin@soleen.com, corbet@lwn.net, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com References: <20241206225204.4008261-1-surenb@google.com> <20241206225204.4008261-5-surenb@google.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 6531EA0007 X-Stat-Signature: wszhmx9tu9gmq73w5331whjd9wfnmuer X-HE-Tag: 1733840466-812962 X-HE-Meta: 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 hSHGvCXU KHK9ajQmRsUCeU1Iz4QiWmCsKlYajixe28F9hsVo6frEItULIKIaEATsRAmzK0MZ0HlbNsQ73pw5Xjcknk5qjl+0DhFvnnrqylkiRi9m4Lce5SxpWPmrevP3DyvLAVPLZ6e74SjywIZJA9xO/MFwFi0mSnwwA0j30IoEv4g/jd/Yiss5VMsS1AhonxQyxtlBT8z+KqDKCEA7mqOHrIcMF4FxgiKqghQCML8uYO3Rhslz7amdJVL573mnz9YSPOmehG7JM3iijOdsKV91KG4P0cs8p4al5DWb67n74NGiSRwuHWc6s6+e+QzxFr+JhTJlzrrEUqGsnQ7siWwULdMUj/w+Rwrd8/o9JvoRYNl52NzXaniE= 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 12/6/24 23:52, Suren Baghdasaryan wrote: > To enable SLAB_TYPESAFE_BY_RCU for vma cache we need to ensure that > object reuse before RCU grace period is over will be detected inside > lock_vma_under_rcu(). > lock_vma_under_rcu() enters RCU read section, finds the vma at the > given address, locks the vma and checks if it got detached or remapped > to cover a different address range. These last checks are there > to ensure that the vma was not modified after we found it but before > locking it. > vma reuse introduces several new possibilities: > 1. vma can be reused after it was found but before it is locked; > 2. vma can be reused and reinitialized (including changing its vm_mm) > while being locked in vma_start_read(); > 3. vma can be reused and reinitialized after it was found but before > it is locked, then attached at a new address or to a new mm while > read-locked; > For case #1 current checks will help detecting cases when: > - vma was reused but not yet added into the tree (detached check) > - vma was reused at a different address range (address check); > We are missing the check for vm_mm to ensure the reused vma was not > attached to a different mm. This patch adds the missing check. > For case #2, we pass mm to vma_start_read() to prevent access to > unstable vma->vm_mm. This might lead to vma_start_read() returning > a false locked result but that's not critical if it's rare because > it will only lead to a retry under mmap_lock. > For case #3, we ensure the order in which vma->detached flag and > vm_start/vm_end/vm_mm are set and checked. vma gets attached after > vm_start/vm_end/vm_mm were set and lock_vma_under_rcu() should check > vma->detached before checking vm_start/vm_end/vm_mm. This is required > because attaching vma happens without vma write-lock, as opposed to > vma detaching, which requires vma write-lock. This patch adds memory > barriers inside is_vma_detached() and vma_mark_attached() needed to > order reads and writes to vma->detached vs vm_start/vm_end/vm_mm. > After these provisions, SLAB_TYPESAFE_BY_RCU is added to vm_area_cachep. > This will facilitate vm_area_struct reuse and will minimize the number > of call_rcu() calls. > > Signed-off-by: Suren Baghdasaryan I'm wondering about the vma freeing path. Consider vma_complete(): vma_mark_detached(vp->remove); vma->detached = true; - plain write vm_area_free(vp->remove); vma->vm_lock_seq = UINT_MAX; - plain write kmem_cache_free(vm_area_cachep) ... potential reallocation against: lock_vma_under_rcu() - mas_walk finds a stale vma due to race vma_start_read() if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(mm->mm_lock_seq.sequence)) - can be false, the vma was not being locked on the freeing side? down_read_trylock(&vma->vm_lock.lock) - suceeds, wasn't locked this is acquire, but was there any release? is_vma_detached() - false negative as the write above didn't propagate here yet; a read barrier but where is the write barrier? checks for vma->vm_mm, vm_start, vm_end - nobody reset them yet so false positive, or they got reset on reallocation but writes didn't propagate Am I missing something that would prevent lock_vma_under_rcu() falsely succeeding here?