From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 9CB9D47A0C0; Fri, 7 Aug 2026 15:37:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786117031; cv=none; b=iqQkVXY+zrCHfVF8raGUN3/SDtU1wF7aJxt2dhm2z4S7QtqDMLgzNKgSYG3xzTDqvehE7KeonfXGIaWbz2cDE7f6F/Ao7PUx9f+NNrGe21WhhX8wBsoVf/5csUGl5426XAVcbAIqOGvd2CjY1cRorgMBfWz5VbXHmWtrwZAGGmM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786117031; c=relaxed/simple; bh=UCnGMXcxknMn2gpC7VCMq0/DzV6ohIC8YD5p2mCJbWc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Lw0MjdrfMezGctBOVwLCSTzvV39jWp4RUeFZVuORZx4SDzlp9RXtbR1WS+kiP0SbmO0DdZkgcly4sl17Mqqn26Vld5QdQCykidfB4rr8LK1pBKm3Blx6jUUUdpu9PwlAp+4OsmjPriwNxyu0TeaMGBhFiSF2hRLGW+1D7HqYpNw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NTVrrKZM; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="NTVrrKZM" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 00F0C1F00A3D; Fri, 7 Aug 2026 15:37:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786117030; bh=5RitbQenhrDoWq7k1wu9B70oWwVmOgYnYYYp4h7OS1E=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=NTVrrKZM1bG6Vvw5wY9rgG2SM3BG/ydY+6BoETIkSqLuw7GRzibg3KM8RNq7W8eO8 +IMp55X5+6WVBfj6gXSVowQZwjzKQ12cGdzf6QYRm7RyimoK4o2nm2neMs7fktzqrf Uf8Hg4wwnK3OHp2PznFVCGw3jn+RbMDSd+EJjT6yLmiwsuei9M0c6DJd02BCh2DfX1 QHiQhhBI/Sue9e3Ea8OJW+qDGDXsv8swxwsl+u90PpfoMlRC8LtqNqLYqaSa3JKZM1 oyoxWf1EEmZsrfzSjw9DdEHMHkvh4TyfS+TQi3SSUCSCi8lHyR8k0RalEGRMrdOkSU BTTfD6CUrr12w== Message-ID: <6df7ba6d-0479-4b37-8e93-bb0b7d1288f7@kernel.org> Date: Fri, 7 Aug 2026 17:37:03 +0200 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 1/5] mm: Make per-VMA locks available universally Content-Language: en-US To: Suren Baghdasaryan , akpm@linux-foundation.org Cc: dave.hansen@linux.intel.com, Liam.Howlett@oracle.com, ljs@kernel.org, david@kernel.org, willy@infradead.org, shakeel.butt@linux.dev, jannh@google.com, aliceryhl@google.com, arve@android.com, cmllamas@google.com, christian@brauner.io, tkjos@android.com, dsahern@kernel.org, davem@davemloft.net, gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, netdev@vger.kernel.org References: <20260806200548.3124802-1-surenb@google.com> <20260806200548.3124802-2-surenb@google.com> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 8/6/26 22:05, Suren Baghdasaryan wrote: > From: Dave Hansen > > The per-VMA locks have been around for several years. They've had some > bugs worked out of them and have seen quite wide use. However, they > are still only available when architectures explicitly enable them. > Remove the conditional compilation around the per-VMA locks, making > them available on all architectures and configs. > > The approach up to now seemed to be to add ARCH_SUPPORTS_PER_VMA_LOCK > when the architecture started using per-VMA locks in the fault > handler. But, contrary to the naming, the Kconfig option does not > really indicate whether the architecture supports per-VMA locks or > not. It is more of a marker for whether the architecture is likely to > benefit from per-VMA locks. > > To me, the most important thing side-effect of universal availability > is letting per-VMA locks be used in SMP=n configs. This lets us use > per-VMA locking in all x86 code without fallbacks. > > Overall, this just generally makes the kernel simpler. Just look at > the diffstat. It also opens the door to users that want to use the > per-VMA locks in common code. Doing *that* brings additional > simplifications. > > The downside of this is adding some fields to vm_area_struct and > mm_struct. There are likely ways to optimize this, especially for > things like SMP=n configs. For now, do the simplest thing: use the > same implementation everywhere. > > == Considerations for NOMMU config == > > NOMMU systems do not write-lock VMAs, therefore read-locking a VMA > would always succeed unless VMA is detached. Therefore for NOMMU > config we make vma_mark_attached() a NOOP, which keeps VMAs always in > detached state. This causes VMA read-locking to always fail and the > caller falls back to locking mmap_lock. > > The following functions will have a different implementation in NOMMU > config: > > - vma_mark_attached(), vma_mark_detached() are made NOOPs, keeping VMAs > always in a detached state and preventing assertions and refcount > underflows; > > - vma_start_write(), vma_start_write_killable() are made NOOPs to avoid > warnings in __vma_start_write() due to VMAs being detached. > These functions are not used in NOMMU code but __vma_start_write() > is an exported function, therefore might be used by drivers. > > - vma_assert_attached() is made NOOP because it's reachable from NOMMU > code via split_vma()->vma_iter_store_new()->vma_iter_store_overwrite(); > > - vma_assert_write_locked() is asserting vma->vm_mm is write-locked; > > The following functions work for both MMU and NOMMU configs: > > - vma_lock_init() performs the same initialization as for MMU config; > > - mm_lock_seqcount_init(), mm_lock_seqcount_begin(), mm_lock_seqcount_end() > are called from mmap_write_{lock|unlock} and update mm_lock_seq correctly. > > - mmap_lock_speculate_try_begin(), mmap_lock_speculate_retry() work as is > because mm_lock_seq is updated correctly; > > - vma_start_read(), vma_start_read_locked() will always fail because VMAs > are always detached; > > - vma_end_read() will never be called because vma_start_read() never > succeeds; > > - vma_is_attached() always return false because VMAs are always detached; > > - vma_assert_detached() will never trigger because VMAs are never attached; > > Changes in the following files are not affecting NOMMU config: > > task_mmu.c - not compiled when CONFIG_MMU=n; > pagewalk.c - not compiled when CONFIG_MMU=n; > userfaultfd.c - not compiled when CONFIG_MMU=n (CONFIG_USERFAULTFD depends > on CONFIG_MMU); > > The following changes are made to keep NOMMU config working like before: > > stack_map_lock_vma() - keeps mmap_lock in NOMMU config; > bpf_iter_task_vma_new() - bails out in NOMMU config; > > Signed-off-by: Dave Hansen > Signed-off-by: Suren Baghdasaryan > Cc: Suren Baghdasaryan > Cc: Andrew Morton > Cc: "Liam R. Howlett" > Cc: Lorenzo Stoakes > Cc: Vlastimil Babka > Cc: Shakeel Butt > Cc: linux-mm@kvack.org > Cc: Greg Kroah-Hartman > Cc: Arve Hjønnevåg > Cc: Todd Kjos > Cc: Christian Brauner > Cc: Carlos Llamas > Cc: Alice Ryhl > Cc: "David S. Miller" > Cc: David Ahern > Cc: netdev@vger.kernel.org Acked-by: Vlastimil Babka (SUSE)