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Tue, 10 Dec 2024 17:25:34 +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 fhkHG455WGccTwAAD6G6ig (envelope-from ); Tue, 10 Dec 2024 17:25:34 +0000 Message-ID: <5036d089-0774-4863-88c5-eaaea1265ac7@suse.cz> Date: Tue, 10 Dec 2024 18:25:34 +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> <643beb6c-4226-46ca-b7e9-292467479aea@suse.cz> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Action: no action X-Stat-Signature: ebx1mq1opfja7j43iwcueofyoich37qf X-Rspam-User: X-Rspamd-Queue-Id: 8A07C1C0007 X-Rspamd-Server: rspam08 X-HE-Tag: 1733851494-110081 X-HE-Meta: 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 IlVHFSy+ ueC5dmFyVbel9Ci3YZjmWlZH+gunm7GDfSGEKZi3ZWqVLoabKEeVsvv6XUoYBiwOi7UGvMxgiLwMC42L8wWVN5kS9Bow5hwuz4XloHOvM2l9aYLFlkmTuJh2ghtsmQJb0G9PfZYgSoj6fuG8CqKOBU3RxFXv5fL/crro09q1JNS7rHCpGdkoHLHJ3A+j/xtsIYTdFt3nToixCm/Yu6xkaYYS6TA4WYbzp9D8LOvcj+GTsj+j97MY09jZebZ4XDJ91CrTeGW2rkt4tv323edwkSnAmkp8FC2SQ2ZjLMXfmQ6H8gKNLWlIrNCbvi3ryzdRQGFL83t25finkhTXQm70ZNefSP8sPNbGQcGlCEcWsBpsOUCelPgGasVz1SA2bOTsoXzFY 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 18:16, Suren Baghdasaryan wrote: > On Tue, Dec 10, 2024 at 8:32 AM Vlastimil Babka wrote: >> >> 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. > > Ah, you are right. With the possibility of reuse, even a freed VMA > should be kept write-locked until it is unlocked by > mmap_write_unlock(). I think the fix for this is simply to not reset > vma->vm_lock_seq inside vm_area_free(). I'll also need to add a But even if we don't reset vm_lock_seq to UINT_MAX, then whover reallocated it can proceed and end up doing a vma_start_write() and rewrite it there anyway, no? > comment for vm_lock_seq explaining these requirements. > Do you agree that such a change would resolve the issue? > >> >> > 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? >> >> >>