From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A1F0C215F7B for ; Mon, 17 Mar 2025 05:12:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1742188341; cv=none; b=QUWAwckma5IYMBPLQVV3Wmfa42Oitl70nHyavgfA07joYE8kfZ6yYP0EXnOSl3URuUEnhEs/HkcayRrj/I8Q+SxlDs48gip32YutGr6i1EhvwbSM7Dd9oqfZAQ6grRF8MGBUkDDI/ASYn8pIwY4UaHjxg/hUw995nZz0dw2pltE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1742188341; c=relaxed/simple; bh=uw1L0e7Y8yYdIMyJ0jcI4YXOCzTNvVW6zDyWWGiXFsE=; h=Date:To:From:Subject:Message-Id; b=qMpBhSSqWiH9O+L+Yb8IhGIA2bbTvf8nUoDWrVc/qU+pDuG9NxphILBNRxE1nX49qEP5AY9B97yVFCh7LcBV8tMIvAkX/Ro4SlI+ABZNM0lBV35aLMieDtt35O2PSNLTS6O004/IJl6ml1Bpflg/PTs0nO7jUxFPXjSMvXgjRzY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=0XsZNVJH; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="0XsZNVJH" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 720ADC4CEEC; Mon, 17 Mar 2025 05:12:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1742188341; bh=uw1L0e7Y8yYdIMyJ0jcI4YXOCzTNvVW6zDyWWGiXFsE=; h=Date:To:From:Subject:From; b=0XsZNVJHnvil7A3CZBhDbM2ToHmOTWHoE7Z3MyVCyKV5tks1XP15Ov1KbVRr4zfwr Q5UopsM/s0Rwml3RHEDuWN8yP2IqXiK+zzly95XG36uIdMVeqwaESGURPoFJMhOMTp cT4a8nRRpgji27FFkxXN6ZYg0OnneR5HNn8l9rCs= Date: Sun, 16 Mar 2025 22:12:20 -0700 To: mm-commits@vger.kernel.org,willy@infradead.org,will@kernel.org,vbabka@suse.cz,souravpanda@google.com,shivankg@amd.com,shakeel.butt@linux.dev,sfr@canb.auug.org.au,richard.weiyang@gmail.com,peterz@infradead.org,peterx@redhat.com,paulmck@kernel.org,pasha.tatashin@soleen.com,oleg@redhat.com,mjguzik@gmail.com,minchan@google.com,mhocko@suse.com,mgorman@techsingularity.net,lorenzo.stoakes@oracle.com,lokeshgidra@google.com,Liam.Howlett@Oracle.com,klarasmodin@gmail.com,jannh@google.com,hughd@google.com,hca@linux.ibm.com,hannes@cmpxchg.org,dhowells@redhat.com,david@redhat.com,dave@stgolabs.net,corbet@lwn.net,brauner@kernel.org,surenb@google.com,akpm@linux-foundation.org From: Andrew Morton Subject: [merged mm-stable] docs-mm-document-latest-changes-to-vm_lock.patch removed from -mm tree Message-Id: <20250317051221.720ADC4CEEC@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The quilt patch titled Subject: docs/mm: document latest changes to vm_lock has been removed from the -mm tree. Its filename was docs-mm-document-latest-changes-to-vm_lock.patch This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm ------------------------------------------------------ From: Suren Baghdasaryan Subject: docs/mm: document latest changes to vm_lock Date: Thu, 13 Feb 2025 14:46:55 -0800 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. Link: https://lkml.kernel.org/r/20250213224655.1680278-19-surenb@google.com Signed-off-by: Suren Baghdasaryan Reviewed-by: Liam R. Howlett Reviewed-by: Lorenzo Stoakes Tested-by: Shivank Garg Link: https://lkml.kernel.org/r/5e19ec93-8307-47c2-bb13-3ddf7150624e@amd.com Reviewed-by: Vlastimil Babka Cc: Christian Brauner Cc: David Hildenbrand Cc: David Howells Cc: Davidlohr Bueso Cc: Hugh Dickins Cc: Jann Horn Cc: Johannes Weiner Cc: Jonathan Corbet Cc: Klara Modin Cc: Lokesh Gidra Cc: Mateusz Guzik Cc: Matthew Wilcox Cc: Mel Gorman Cc: Michal Hocko Cc: Minchan Kim Cc: Oleg Nesterov Cc: Pasha Tatashin Cc: "Paul E . McKenney" Cc: Peter Xu Cc: Peter Zijlstra (Intel) Cc: Shakeel Butt Cc: Sourav Panda Cc: Suren Baghdasaryan Cc: Wei Yang Cc: Will Deacon Cc: Heiko Carstens Cc: Stephen Rothwell Signed-off-by: Andrew Morton --- Documentation/mm/process_addrs.rst | 46 +++++++++++++++------------ 1 file changed, 27 insertions(+), 19 deletions(-) --- a/Documentation/mm/process_addrs.rst~docs-mm-document-latest-changes-to-vm_lock +++ a/Documentation/mm/process_addrs.rst @@ -716,9 +716,14 @@ calls :c:func:`!rcu_read_lock` to ensure critical section, then attempts to VMA lock it via :c:func:`!vma_start_read`, before releasing the RCU lock via :c:func:`!rcu_read_unlock`. -VMA read locks hold the read lock on the :c:member:`!vma->vm_lock` semaphore for -their duration and the caller of :c:func:`!lock_vma_under_rcu` must release it -via :c:func:`!vma_end_read`. +In cases when the user already holds mmap read lock, :c:func:`!vma_start_read_locked` +and :c:func:`!vma_start_read_locked_nested` can be used. These functions do 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`. VMA **write** locks are acquired via :c:func:`!vma_start_write` in instances 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 lock, releasing or downgrading the mmap write lock also releases the VMA write lock so there is no :c:func:`!vma_end_write` function. -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 reference counter is +restored once the vma sequence number used for serialisation is updated. This ensures the semantics we require - VMA write locks provide exclusive write access to the VMA. @@ -738,7 +743,7 @@ Implementation details The VMA lock mechanism is designed to be a lightweight means of avoiding the 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. Read locks are acquired via :c:func:`!vma_start_read`, which is an optimistic @@ -779,28 +784,31 @@ release of any VMA locks on its release keep VMAs locked across entirely separate write operations. It also maintains correct lock ordering. -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. - -If it does, the read lock fails. If it does not, we hold the lock, excluding -writers, but permitting other readers, who will also obtain this lock under RCU. +Each time a VMA read lock is acquired, we increment :c:member:`!vma.vm_refcnt` +reference counter and check that the sequence count of the VMA does not match +that of the mm. + +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, but +permitting other readers, who can also obtain this lock under RCU. Importantly, maple tree operations performed in :c:func:`!lock_vma_under_rcu` are also RCU safe, so the whole read lock operation is guaranteed to function correctly. -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. 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. -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_refcnt` +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 downgraded. -This clever combination of a read/write semaphore and sequence count allows for +This clever combination of a reference counter and sequence count allows for fast RCU-based per-VMA lock acquisition (especially on page fault, though utilised elsewhere) with minimal complexity around lock ordering. _ Patches currently in -mm which might be from surenb@google.com are