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Wed, 8 Jan 2025 11:52:50 +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 3RiFKRJnfmebFQAAD6G6ig (envelope-from ); Wed, 08 Jan 2025 11:52:50 +0000 Message-ID: Date: Wed, 8 Jan 2025 12:52:50 +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 v7 12/17] mm: replace vm_lock and detached flag with a reference count Content-Language: en-US To: Suren Baghdasaryan , akpm@linux-foundation.org Cc: peterz@infradead.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, lokeshgidra@google.com, minchan@google.com, jannh@google.com, shakeel.butt@linux.dev, souravpanda@google.com, pasha.tatashin@soleen.com, klarasmodin@gmail.com, corbet@lwn.net, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com References: <20241226170710.1159679-1-surenb@google.com> <20241226170710.1159679-13-surenb@google.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Spam-Score: -4.30 X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_TLS_ALL(0.00)[]; RCPT_COUNT_TWELVE(0.00)[32]; MID_RHS_MATCH_FROM(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com,sina.com]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[infradead.org,oracle.com,suse.com,cmpxchg.org,gmail.com,intel.com,techsingularity.net,redhat.com,stgolabs.net,kernel.org,sina.com,google.com,linux.dev,soleen.com,lwn.net,vger.kernel.org,kvack.org,android.com]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; FUZZY_BLOCKED(0.00)[rspamd.com]; R_RATELIMIT(0.00)[to_ip_from(RLumbhs4xhzuuihrchnpuyb6qu)] X-Spam-Flag: NO X-Spam-Level: On 12/26/24 18:07, Suren Baghdasaryan wrote: > rw_semaphore is a sizable structure of 40 bytes and consumes > considerable space for each vm_area_struct. However vma_lock has > two important specifics which can be used to replace rw_semaphore > with a simpler structure: > 1. Readers never wait. They try to take the vma_lock and fall back to > mmap_lock if that fails. > 2. Only one writer at a time will ever try to write-lock a vma_lock > because writers first take mmap_lock in write mode. > Because of these requirements, full rw_semaphore functionality is not > needed and we can replace rw_semaphore and the vma->detached flag with > a refcount (vm_refcnt). > When vma is in detached state, vm_refcnt is 0 and only a call to > vma_mark_attached() can take it out of this state. Note that unlike > before, now we enforce both vma_mark_attached() and vma_mark_detached() > to be done only after vma has been write-locked. vma_mark_attached() > changes vm_refcnt to 1 to indicate that it has been attached to the vma > tree. When a reader takes read lock, it increments vm_refcnt, unless the > top usable bit of vm_refcnt (0x40000000) is set, indicating presence of > a writer. When writer takes write lock, it both increments vm_refcnt and > sets the top usable bit to indicate its presence. If there are readers, > writer will wait using newly introduced mm->vma_writer_wait. Since all > writers take mmap_lock in write mode first, there can be only one writer > at a time. The last reader to release the lock will signal the writer > to wake up. > refcount might overflow if there are many competing readers, in which case > read-locking will fail. Readers are expected to handle such failures. > > Suggested-by: Peter Zijlstra > Suggested-by: Matthew Wilcox > Signed-off-by: Suren Baghdasaryan > */ > static inline bool vma_start_read(struct vm_area_struct *vma) > { > + int oldcnt; > + > /* > * Check before locking. A race might cause false locked result. > * We can use READ_ONCE() for the mm_lock_seq here, and don't need > @@ -720,13 +745,20 @@ static inline bool vma_start_read(struct vm_area_struct *vma) > if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence)) > return false; > > - if (unlikely(down_read_trylock(&vma->vm_lock.lock) == 0)) > + > + rwsem_acquire_read(&vma->vmlock_dep_map, 0, 0, _RET_IP_); I don't know much about lockdep, but I see that down_read() does rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_); down_read_trylock() does rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_); This is passing the down_read()-like variant but it behaves like a trylock, no? > + /* Limit at VMA_REF_LIMIT to leave one count for a writer */ It's mainly to not increase as much as VMA_LOCK_OFFSET bit could become false positively set set by readers, right? The "leave one count" sounds like an implementation detail of VMA_REF_LIMIT and will change if Liam's suggestion is proven feasible? > + if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt, > + VMA_REF_LIMIT))) { > + rwsem_release(&vma->vmlock_dep_map, _RET_IP_); > return false; > + } > + lock_acquired(&vma->vmlock_dep_map, _RET_IP_); > > /* > - * Overflow might produce false locked result. > + * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result. > * False unlocked result is impossible because we modify and check > - * vma->vm_lock_seq under vma->vm_lock protection and mm->mm_lock_seq > + * vma->vm_lock_seq under vma->vm_refcnt protection and mm->mm_lock_seq > * modification invalidates all existing locks. > * > * We must use ACQUIRE semantics for the mm_lock_seq so that if we are > @@ -734,10 +766,12 @@ static inline bool vma_start_read(struct vm_area_struct *vma) > * after it has been unlocked. > * This pairs with RELEASE semantics in vma_end_write_all(). > */ > - if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) { > - up_read(&vma->vm_lock.lock); > + if (unlikely(oldcnt & VMA_LOCK_OFFSET || > + vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) { > + vma_refcount_put(vma); > return false; > } > + > return true; > } > > @@ -749,8 +783,17 @@ static inline bool vma_start_read(struct vm_area_struct *vma) > */ > static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma, int subclass) > { > + int oldcnt; > + > mmap_assert_locked(vma->vm_mm); > - down_read_nested(&vma->vm_lock.lock, subclass); > + rwsem_acquire_read(&vma->vmlock_dep_map, subclass, 0, _RET_IP_); Same as above? > + /* Limit at VMA_REF_LIMIT to leave one count for a writer */ Also > + if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt, > + VMA_REF_LIMIT))) { > + rwsem_release(&vma->vmlock_dep_map, _RET_IP_); > + return false; > + } > + lock_acquired(&vma->vmlock_dep_map, _RET_IP_); > return true; > } >