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Tue, 10 Dec 2024 16:32:00 +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 SjIdLQBtWGdXPQAAD6G6ig (envelope-from ); Tue, 10 Dec 2024 16:32:00 +0000 Message-ID: <643beb6c-4226-46ca-b7e9-292467479aea@suse.cz> Date: Tue, 10 Dec 2024 17:32:00 +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 Cc: akpm@linux-foundation.org, 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> <6b29b2a5-c244-4930-a5a0-1a24a04e7e35@suse.cz> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 971AE40005 X-Rspam-User: X-Stat-Signature: d4prhm759h1fb7n1t94xreg3tc95gd7n X-HE-Tag: 1733848297-825175 X-HE-Meta: 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 +T+bc8Fe 8rXYM5/mYwo5HKJxWPzRXjju6qvg5hmbItGLGo5jk3agjAY6Dn/EYnzITJT/VVisU4r1tgJGfAwx5rZUSyAl+GhQbBusLshaJf6i0kk8Ej7Kgjud2xaQBCcIJacO5BJzuboFysCAnMvbMGbsG9PIeQx3F7C7munf9jeD8MoIBHgaJxSXV5f3fpjukXdkbuQ5smyI9PMh2FY9UU2gqBOfYH05LgyuApc59NpMt4q2bVlaJWJrYQ/B6/ykChoQheq0ghTlEgqqNOeUDpMzvfGk+Kk/BuvDasSB+mXImYXkFvuSbwjTPOFk+5oRMhU94gTUjh2TJo7rCTB3MtwtOD3kCVszisVVcEetbwGL+i1OIFpCBXJOtxf1cC1I+D0oOggQJtpWR 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 12/10/24 17:20, Suren Baghdasaryan wrote: > On Tue, Dec 10, 2024 at 6:21 AM Vlastimil Babka wrote: >> >> 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? > > Yes, there was a release. I think what you missed is that > vma_mark_detached() that is called from vma_complete() requires VMA to > be write-locked (see vma_assert_write_locked() in > vma_mark_detached()). The rule is that a VMA can be attached without > write-locking but only a write-locked VMA can be detached. So, after OK but write unlocking means the mm's seqcount is bumped and becomes non-equal with vma's vma->vm_lock_seq, right? Yet in the example above we happily set it to UINT_MAX and thus effectively false unlock it for vma_start_read()? And this is all done before the vma_complete() side would actually reach mmap_write_unlock(), AFAICS. > vma_mark_detached() and before down_read_trylock(&vma->vm_lock.lock) > in vma_start_read() the VMA write-lock should have been released by > mmap_write_unlock() and therefore vma->detached=false should be > visible to the reader when it executed lock_vma_under_rcu(). > >> 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? >>