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 D112C476044; Fri, 7 Aug 2026 15:39:05 +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=1786117147; cv=none; b=WRikadG6rr8Vmp0QPq1P8Cx/eYNQRHft01fbf/1o7LDaTA3XkgA7fMsTAmKd8XKziAs/1fQUtSeCOO3jGnGux5yP/hzQvvhmKGyA4+E3MCD0sBcVvfWUrwOU1z4vR7Xd62TtdCbrxGkB5djqYEyp+gEYzv5PuWUyKkoii/B9ko4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786117147; c=relaxed/simple; bh=DDCIXrVx9ykV1HwQ9aZPqjf2Kv3Uq4axRuHBtrVxNNA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=YZIaZ3z+v7uaaj5k3BuOwKTJV12xsR+SXxUQ3ZU6pFGtIiSKscTKBhde7sdfbbt8qr4D4Uv6N/YhEeDSqJptAdxpA2VNKJmnyPFso8tk8PCbe+n34ImVFMvatDAzJeTw4TogE+k5aGptMM8dLAGTh2hcRKwwmE2dz+F3f6DoNh0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=eyVWYnmZ; 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="eyVWYnmZ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D36AB1F000E9; Fri, 7 Aug 2026 15:39:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786117145; bh=NAheNQzZrUL8VkT48h9rxz80c6O8mgo9DdVWeFvBsyc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=eyVWYnmZHpLCJKZPvOBM6jCVsS4y+LNltnyxjOuPsCFwzFoD3nuyLapTIz7+L9xXv +OO6+9pa6W/m2cXcIFi5O0tKeNXdPr5xZzL6hxYqldJJUNU1Pmjw5Aa3hO7WMFg8aw +/pOcQI8jgDpGTPX/H1MmebbqtpmHDcfIRTEpn9GB6Fc+msemNn7ZE6qHrBQt7PqOK pVA1kfntwF0hMRHyCHUsZnLvjQukofINMLYWhlYxhDfXr1FOz9Y5NX6ZWiXZ9JQcEB bnLLUJv9lJHMghYTbw+ZD9a4khlY7sCMG4cZ1jTOwlly6ktEKz8L60Meg7koDpKXTi U2DwLovoY7j4Q== Message-ID: Date: Fri, 7 Aug 2026 17:39:00 +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 3/5] mm: Add RCU-based VMA lookup helper that waits for writers 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> <20260806200548.3124802-4-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 > > == Background == > > There are basically two parallel ways to look up a VMA: the > traditional way, which is protected by mmap_read_lock, and the RCU-based > per-VMA lock way which is based on RCU and refcounts. > > == Problem == > > The mmap_lock one is more straightforward to use but it has a big > disadvantage in that it can not be mixed with page faults since those > can take mmap_lock for read, which can deadlock when mixed with nested > page faults and parallel writers. > For example: > > mmap_read_lock(mm); > // Another thread does mmap_write_lock(). > // New mmap_lock readers are blocked. > vma = vma_lookup(mm, address); > // This deadlocks on mmap_read_lock() if it faults: > copy_from_user(address); > mmap_read_unlock(mm); > > The per-VMA lock can be mixed with faults, but they can fail and need to > be able to fall back to the traditional way. > > == Solution == > > Add vma_start_read_unlocked() - a variant of the RCU-based lookup that > waits for writers. This is basically the same as the existing RCU-based > lookup, but on a failure to lock it temporarily takes mmap_lock for read > and waits for writers to finish before locking the VMA, dropping the > mmap_lock and returning the locked VMA. This has some advantages: > > 1. Callers do not need to have a fallback path for when they > collide with writers. > 2. It can be used in contexts where page faults can happen because > it can take the mmap_lock for read but never *holds* it. > 3. Its fast path does not require taking mmap_lock for read. > > Basically, when applied correctly, this approach results in faster > *and* simpler code. > > While at it, fix the comments for vma_start_read_locked(), > vma_start_read_locked_nested(), and uffd_lock_vma(). > > Suggested-by: Lorenzo Stoakes (ARM) > 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)