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 0D33539D6F2; Mon, 10 Aug 2026 08:52:24 +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=1786351946; cv=none; b=Jw6/z1Zkp0ydZVwKWHmuP5j2yYc1a8Osf7gJiikUrGtAThAS7rVGVlslY3JXIU48JHHIv9vwGkMmj8uLdv0aJKoqQtmzphTqqeFmo1btb122JoCvxr4UrC48BtlIto8agT/gyVhLPFnCJTIGOksn89GthoUkI4/sgLMbbVRsjGw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786351946; c=relaxed/simple; bh=9fXe0wPPrPf7LEsugMCL6cx+2x0oY+kpAVnhj5p+61g=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=uKnhT0uUdCjMKAZ3TdDRHRyvg0zZc5GhtW+Dt3Cu7kG39RjHlnNlgPFfSbK06xREpiZqR54aj0xc5Pav0buooYsiYqT+hV3Q/32wHhwoprOOO+aDptKLrzMbxhUb2XZnerRChy5OA5ztToEaO41sfiRmmer7l+lWtq1tFSv6GXs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZvEc+zMN; 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="ZvEc+zMN" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 66EA11F000E9; Mon, 10 Aug 2026 08:52:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786351944; bh=ja+bbx5/k6Ur1P7gZyvdceXvb5QS4sd8f7Hohxvn9oM=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=ZvEc+zMNW/8pT1cKRhb+nZuoMlg8vdkHd5yuLE0Zg2dtbFRoUlPsDVJq/FLUCnzRF La41lPi+VTuopR4OysD0A1dMw2M42MuSZZUFs+qq6QRzoAVJ0mUkjB/RIUGzYzIxsh 8HnQlkioNc97hH//K4APckBNNLx4SsIxHr6LHABqM86ZmR4h3L29Y8b2RA2UHQvlOC H2+Lzf7M60DlULqEK8kb6Tp2a/r4+MEgoErg2PEfESuaO9CQH0rdoZUZ3CBqBcWZMi HLwID1l/RyAc/ZSHUbV8BPFjJ1B3yYzPWmWzkQOMDS7ZpAmVDruN/Q8TbJ3gaIqXO6 zlnsg5aWbiGiQ== Date: Mon, 10 Aug 2026 09:52:04 +0100 From: "Lorenzo Stoakes (ARM)" To: Matthew Wilcox Cc: Suren Baghdasaryan , akpm@linux-foundation.org, dave.hansen@linux.intel.com, Liam.Howlett@oracle.com, david@kernel.org, shakeel.butt@linux.dev, vbabka@kernel.org, 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 Subject: Re: [PATCH v4 1/5] mm: Make per-VMA locks available universally Message-ID: References: <20260806200548.3124802-1-surenb@google.com> <20260806200548.3124802-2-surenb@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Sat, Aug 08, 2026 at 02:12:50AM +0100, Matthew Wilcox wrote: > On Thu, Aug 06, 2026 at 01:05:44PM -0700, Suren Baghdasaryan wrote: > > +++ b/kernel/bpf/stackmap.c > > @@ -272,13 +272,8 @@ struct stack_map_vma_lock { > > /* > > * Acquire a stable read-side reference on the VMA covering @ip. > > * > > - * With CONFIG_PER_VMA_LOCK=y this returns a VMA with its per-VMA read > > - * lock held and mmap_lock dropped, so the caller may sleep. > > - * > > - * With CONFIG_PER_VMA_LOCK=n it returns a VMA with mmap_lock still > > - * held; the caller must snapshot any fields it needs and pin vm_file > > - * with get_file() before stack_map_unlock_vma() drops mmap_lock, as > > - * the VMA may be split, merged, or freed after that. > > + * This returns a VMA with its per-VMA read lock held and mmap_lock > > + * dropped, so the caller may sleep. > > I don't know if BPF is compatible with !MMU or not, but the comment > is inconsistent with the code. How about: I do think there are components that simply don't think to depend on CONFIG_MMU even though they do. In fact more than think - have run into exactly that before. It's another thing that speaks to nommu being a legacy barnacle that bashes us on the head fairly regularly for little to no gain (and nobody is testing it for tip kernel AFAICT). > > * On NOMMU configurations, returns with the mmap_lock held. If the MMU > * is enabled, the per-VMA lock will be held instead. The lock > * should be released with stack_map_unlock_vma() which will release the > * appropriate lock. Once the lock is released, the VMA may be freed. I mean I suppose it's accurate but I don't love the idea of essentially implying nommu+bpf is a thing and also treating it as so important that it must be called out here. I'd rather it be inaccurate for nommu as are most comments in mm and mm-adjacent components, it's kinda implied in general. Those who care can look at the code. > > > * Returns NULL on failure, in which case no lock is held. > > */ > > @@ -288,7 +283,6 @@ stack_map_lock_vma(struct stack_map_vma_lock *lock, unsigned long ip) > > struct mm_struct *mm = lock->mm; > > struct vm_area_struct *vma; > > > > - /* noop under !CONFIG_PER_VMA_LOCK */ > > vma = lock_vma_under_rcu(mm, ip); > > if (vma) { > > lock->vma = vma; > > @@ -308,21 +302,20 @@ stack_map_lock_vma(struct stack_map_vma_lock *lock, unsigned long ip) > > return NULL; > > } > > > > -#ifdef CONFIG_PER_VMA_LOCK > > +#ifdef CONFIG_MMU > > if (!vma_start_read_locked(vma)) { > > mmap_read_unlock(mm); > > return NULL; > > } > > mmap_read_unlock(mm); > > #endif > > - > > lock->vma = vma; > > return vma; > > } -- Cheers, Lorenzo