From nobody Fri Dec 19 04:53:44 2025 Received: from mail-pl1-f202.google.com (mail-pl1-f202.google.com [209.85.214.202]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7A370270EB3 for ; Thu, 13 Feb 2025 22:47:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.214.202 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739486858; cv=none; b=jJaGcUavdoJvxShkrDiWY1y+fL2Opi/nOnjVX5SDvtGQyrpKu8Sxk01nUDBEgMqboszs2yozjRRFvOO869kHmJH+kNB3FbpbARS2JeZgCJxyMF9y7Y9RUSQVW5FyjpOZdQiP0+2WTUKiBGOc9w9tSVnkIZt/EPpFQhE9EGHwaFw= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739486858; c=relaxed/simple; bh=ZS+q2FOJ4YzNtq2i12+Nw5i6j6+GhULqmdiyVBu4ei8=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=mp+fwSOteYt5pkkjdUgHlepxJOSASIKGbjlMVt2WwKslj24OWapInYiUF8K3K1a6DprGEKix0B65106CC1kmT4i/n9JWOlAfWmmhktMQ1l3MVd+remGckGl+2o1coWQPRdc/sKR5Z9zn+nxIE+QYbbZAMW19JDk65GwV/ZzJip4= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--surenb.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=3SEA9xGr; arc=none smtp.client-ip=209.85.214.202 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--surenb.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="3SEA9xGr" Received: by mail-pl1-f202.google.com with SMTP id d9443c01a7336-220d8aa893dso18006555ad.3 for ; Thu, 13 Feb 2025 14:47:36 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1739486856; x=1740091656; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=HXs3qm65Mw4fWBObt4HCubdoJZ7UfWSFf9bCMyPuZ5Q=; b=3SEA9xGr6Q/n7UkACnVx2iR20sFY+wieZepwr7u/JP5jeuTk4d8TJ09tVGGe6ewyGV FORnsQxWRRSkWgaBL+4egQugrCFZsoCp2OeHOcJ/CDkyNkFlw/q5iMus28/NMlkxc8Xy jEx1P7XLJpW8o8vGDCAaWVq6UnKDaR7I9WgUtnP0PSM+MrrtPHwTWKfCu68xdyG5kX7I VhtrOcU+LvgQcnbvp150kxaymTKkyXUxdMayvs4rRAmZgNkECvSMtgdrc5+Ufmn45kMO MjB2qGf4RITKgSXeNZzRVx2sEVIFgGN3f7CVv6qB+Qh51Zi7j923lmxM185YsQS2Y2+z wpPw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1739486856; x=1740091656; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=HXs3qm65Mw4fWBObt4HCubdoJZ7UfWSFf9bCMyPuZ5Q=; b=Mun1G1RyIi0Uc7C8dQ0rhURS/BPoSQVe+OuWKI6stJgp3odFL3rmALo+QcSWYmp0S4 qFTp1G9BwfEP4ioOBHn+fW7Rh2/imZBDw4Y9XDDNBEpMyfqgWazxo3oHwU3OGj2752wF njWdHu7+on+3pLAk1K1mxQz2lLRP6rzNV0S5x2pzj8yVwoxfvHWcBCoFPGExgb6W2Tvj MAn51YrRIBaNkEsdZVfG8TotAVFs0eJDVz2mCquNV4f1rxcBtXoq7OWHd4jiNZuwOUrz KjIAeT4NRe3StnHu3MILcw8RE4ujnJcc7GZAQwuF9gll/iswH7BNGC9wBpBmDp4Dor25 0SPw== X-Forwarded-Encrypted: i=1; AJvYcCWKv1ZjbS9Z3OcoFwi3r0TXBeDElC2asTZTEr7Wkw6241PYmaoHs6Ekc17NlHtWEkP+3CMQ73BuqHoiCSE=@vger.kernel.org X-Gm-Message-State: AOJu0YzuYwzuspUZcn3ZqoQ0ImZ1AGHBdKfb7v9mhjdLObhb6HLuTolc fUVs/n3qp3fK23GNQ3YiNyZvzLTy/xzQ8XfxHrUfX/6z2139YVt2HPcYtuVAWM8M+TH4O8Ja5sH hCA== X-Google-Smtp-Source: AGHT+IFEeHfewJkwAt0Xqt/tC1plnftNTUCWIl49bO9ckcrGnetyW/ghDc1KfOz7C3qqfgR140G3l01rzEw= X-Received: from plblc14.prod.google.com ([2002:a17:902:fa8e:b0:212:48d4:bf16]) (user=surenb job=prod-delivery.src-stubby-dispatcher) by 2002:a17:902:dac4:b0:21f:9107:fca3 with SMTP id d9443c01a7336-220d20e90a3mr71286775ad.30.1739486856011; Thu, 13 Feb 2025 14:47:36 -0800 (PST) Date: Thu, 13 Feb 2025 14:46:55 -0800 In-Reply-To: <20250213224655.1680278-1-surenb@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250213224655.1680278-1-surenb@google.com> X-Mailer: git-send-email 2.48.1.601.g30ceb7b040-goog Message-ID: <20250213224655.1680278-19-surenb@google.com> Subject: [PATCH v10 18/18] 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, david.laight.linux@gmail.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, richard.weiyang@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, "Liam R. Howlett" 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 Reviewed-by: Liam R. Howlett Reviewed-by: Lorenzo Stoakes --- Changes since v9 [1]: - Updated documenation, per Lorenzo Stoakes - Add Reviewed-by, per Lorenzo Stoakes [1] https://lore.kernel.org/all/20250111042604.3230628-18-surenb@google.com/ 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..e6756e78b476 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, the VMA's sequence number is set to match that = of +the 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 2.48.1.601.g30ceb7b040-goog