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AJvYcCW1jWIfoS0i72ooz+3zbke+0/zktYhvSqVyvVUA+ob3YVOf5iy8LFKJejJLfoc+h2wb5DR0jWZN5613nuQ=@vger.kernel.org X-Gm-Message-State: AOJu0YwGlbXCsaA9hSH842bj4PFkvt84Ua8tVBzyMWbekTRsbES5xf63 6bWp4cUUl/Li49RHgiU3/mW0ONbGSazKJw8DXCTDdgEhqSFxeZPfyJmsMuponrWoyCFedbqMOg0 h/w== X-Google-Smtp-Source: AGHT+IFCRcSRmnDxCEXNS618C+s8AbCbrJ3sqm2+B/Vq6ZMmO1in0EJV2noWuvjJbPv4qSgauLdf86FSJXo= X-Received: from pfbfb35.prod.google.com ([2002:a05:6a00:2da3:b0:728:b8e3:9934]) (user=surenb job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a00:4b54:b0:729:49a:2db0 with SMTP id d2e1a72fcca58-7290c273bfemr20412405b3a.25.1734377095966; Mon, 16 Dec 2024 11:24:55 -0800 (PST) Date: Mon, 16 Dec 2024 11:24:19 -0800 In-Reply-To: <20241216192419.2970941-1-surenb@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20241216192419.2970941-1-surenb@google.com> X-Mailer: git-send-email 2.47.1.613.gc27f4b7a9f-goog Message-ID: <20241216192419.2970941-17-surenb@google.com> Subject: [PATCH v6 16/16] docs/mm: document latest changes to vm_lock From: Suren Baghdasaryan To: akpm@linux-foundation.org Cc: peterz@infradead.org, willy@infradead.org, liam.howlett@oracle.com, lorenzo.stoakes@oracle.com, mhocko@suse.com, vbabka@suse.cz, 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, surenb@google.com Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Change the documentation to reflect that vm_lock is integrated into vma and replaced with vm_refcnt. Document newly introduced vma_start_read_locked{_nested} functions. Signed-off-by: Suren Baghdasaryan --- Documentation/mm/process_addrs.rst | 44 ++++++++++++++++++------------ 1 file changed, 26 insertions(+), 18 deletions(-) diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_= addrs.rst index 81417fa2ed20..f573de936b5d 100644 --- a/Documentation/mm/process_addrs.rst +++ b/Documentation/mm/process_addrs.rst @@ -716,9 +716,14 @@ calls :c:func:`!rcu_read_lock` to ensure that the VMA = is looked up in an RCU critical section, then attempts to VMA lock it via :c:func:`!vma_start_rea= d`, before releasing the RCU lock via :c:func:`!rcu_read_unlock`. =20 -VMA read locks hold the read lock on the :c:member:`!vma->vm_lock` semapho= re for -their duration and the caller of :c:func:`!lock_vma_under_rcu` must releas= e it -via :c:func:`!vma_end_read`. +In cases when the user already holds mmap read lock, :c:func:`!vma_start_r= ead_locked` +and :c:func:`!vma_start_read_locked_nested` can be used. These functions d= o not +fail due to lock contention but the caller should still check their return= values +in case they fail for other reasons. + +VMA read locks increment :c:member:`!vma.vm_refcnt` reference counter for = their +duration and the caller of :c:func:`!lock_vma_under_rcu` must drop it via +:c:func:`!vma_end_read`. =20 VMA **write** locks are acquired via :c:func:`!vma_start_write` in instanc= es where a VMA is about to be modified, unlike :c:func:`!vma_start_read` the lock is = always @@ -726,9 +731,9 @@ acquired. An mmap write lock **must** be held for the d= uration of the VMA write lock, releasing or downgrading the mmap write lock also releases the VMA w= rite lock so there is no :c:func:`!vma_end_write` function. =20 -Note that a semaphore write lock is not held across a VMA lock. Rather, a -sequence number is used for serialisation, and the write semaphore is only -acquired at the point of write lock to update this. +Note that when write-locking a VMA lock, the :c:member:`!vma.vm_refcnt` is= temporarily +modified so that readers can detect the presense of a writer. The referenc= e counter is +restored once the vma sequence number used for serialisation is updated. =20 This ensures the semantics we require - VMA write locks provide exclusive = write access to the VMA. @@ -738,7 +743,7 @@ Implementation details =20 The VMA lock mechanism is designed to be a lightweight means of avoiding t= he use of the heavily contended mmap lock. It is implemented using a combination = of a -read/write semaphore and sequence numbers belonging to the containing +reference counter and sequence numbers belonging to the containing :c:struct:`!struct mm_struct` and the VMA. =20 Read locks are acquired via :c:func:`!vma_start_read`, which is an optimis= tic @@ -779,28 +784,31 @@ release of any VMA locks on its release makes sense, = as you would never want to keep VMAs locked across entirely separate write operations. It also mainta= ins correct lock ordering. =20 -Each time a VMA read lock is acquired, we acquire a read lock on the -:c:member:`!vma->vm_lock` read/write semaphore and hold it, while checking= that -the sequence count of the VMA does not match that of the mm. +Each time a VMA read lock is acquired, we increment :c:member:`!vma.vm_ref= cnt` +reference counter and check that the sequence count of the VMA does not ma= tch +that of the mm. =20 -If it does, the read lock fails. If it does not, we hold the lock, excludi= ng -writers, but permitting other readers, who will also obtain this lock unde= r RCU. +If it does, the read lock fails and :c:member:`!vma.vm_refcnt` is dropped. +If it does not, we keep the reference counter raised, excluding writers, b= ut +permitting other readers, who can also obtain this lock under RCU. =20 Importantly, maple tree operations performed in :c:func:`!lock_vma_under_r= cu` are also RCU safe, so the whole read lock operation is guaranteed to funct= ion correctly. =20 -On the write side, we acquire a write lock on the :c:member:`!vma->vm_lock` -read/write semaphore, before setting the VMA's sequence number under this = lock, -also simultaneously holding the mmap write lock. +On the write side, we set a bit in :c:member:`!vma.vm_refcnt` which can't = be +modified by readers and wait for all readers to drop their reference count. +Once there are no readers, VMA's sequence number is set to match that of t= he +mm. During this entire operation mmap write lock is held. =20 This way, if any read locks are in effect, :c:func:`!vma_start_write` will= sleep until these are finished and mutual exclusion is achieved. =20 -After setting the VMA's sequence number, the lock is released, avoiding -complexity with a long-term held write lock. +After setting the VMA's sequence number, the bit in :c:member:`!vma.vm_ref= cnt` +indicating a writer is cleared. From this point on, VMA's sequence number = will +indicate VMA's write-locked state until mmap write lock is dropped or down= graded. =20 -This clever combination of a read/write semaphore and sequence count allow= s for +This clever combination of a reference counter and sequence count allows f= or fast RCU-based per-VMA lock acquisition (especially on page fault, though utilised elsewhere) with minimal complexity around lock ordering. =20 --=20 2.47.1.613.gc27f4b7a9f-goog