From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mgamail.intel.com (mgamail.intel.com [192.198.163.13]) (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 17B531E8837; Thu, 30 Jan 2025 16:04:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=192.198.163.13 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738253091; cv=none; b=PPHIxHtVFp6n3V93Y0t6auG5dqTL6ehUkHbD8yVWQIw+XoS0uQufiHhxXpPFkowbYLhIV0YwNSdfJywvqAYZwS/KH/8GicjRjqpWqVyWOysVDOA7B5HlicNF84ahbJL8FKhJCiCUIiQllpPdFHxPHDm0Ng7SrGSAlvWSVAIfBAk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738253091; c=relaxed/simple; bh=4ZRTirV/qMXGv1lhHQu9Z2RIqCSCgFSwedbKpogvN08=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Se7ys2vVeEGpf7L+lO3hxMkSBq9DOyahCFIQv8IIWuukH/jePsKKQ09u/C5MwF1nqcRqGgbVCvX0N7XJNxxvCdLFeVXu57sQbOU4xOJhohs7G348xbpeneYVVlCtPYCB5H7W1GcPbKnJoUN5si7GwxdF3oB1m8zHYXRT8YJEdFs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com; spf=pass smtp.mailfrom=intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=mm8ukA87; arc=none smtp.client-ip=192.198.163.13 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="mm8ukA87" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1738253090; x=1769789090; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=4ZRTirV/qMXGv1lhHQu9Z2RIqCSCgFSwedbKpogvN08=; b=mm8ukA87Yc9p/oRcSNkT8uEQmAmZKONiuEffNztQPbaqWZjDO/tkG/0o LnYAyOuUP4AomQcJVGNKRwIP9MZLUWC5o623i5g3BtR+ZQdHOEbsahEP1 jSvXflKgNFg4m0Zc2ccPALg16KvbQ2xxlQx6NQjE4x5fHOoEyckDSsEE6 5fGU8IJJe7vxpYBCK5MNQyoTJJRIknN/4y7jF6nUbHa3ftdZxgJwBidXO 9fHhqlpGhDDdWTjIUqi2baw0h/fxhcCqm9KOtgyJ5tBI5deT0qnn2LxSC pQTo2uCvlHdhp6Xii2b6wcEqZJsCWYIetVKBSRuh7+MWhaYFRDLQayVFS A==; X-CSE-ConnectionGUID: UagTAG2ERnKWf1HOrwL2QA== X-CSE-MsgGUID: Or29WqiVTyuYcSTb3T1jmg== X-IronPort-AV: E=McAfee;i="6700,10204,11331"; a="41636911" X-IronPort-AV: E=Sophos;i="6.13,245,1732608000"; d="scan'208";a="41636911" Received: from fmviesa009.fm.intel.com ([10.60.135.149]) by fmvoesa107.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Jan 2025 08:04:49 -0800 X-CSE-ConnectionGUID: UwYGh57xR/yTBqzQLoyWlA== X-CSE-MsgGUID: RzBbKHHaRvSoK3Av6eDkLg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.13,245,1732608000"; d="scan'208";a="109965833" Received: from inaky-mobl1.amr.corp.intel.com (HELO [10.125.108.230]) ([10.125.108.230]) by fmviesa009-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Jan 2025 08:04:48 -0800 Message-ID: Date: Thu, 30 Jan 2025 08:04:49 -0800 Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/7] Move prefaulting into write slow paths To: Kent Overstreet , Dave Hansen Cc: linux-kernel@vger.kernel.org, Linus Torvalds , Ted Ts'o , Christian Brauner , "Darrick J. Wong" , Matthew Wilcox , Al Viro , linux-fsdevel@vger.kernel.org, almaz.alexandrovich@paragon-software.com, ntfs3@lists.linux.dev, miklos@szeredi.hu, linux-bcachefs@vger.kernel.org, clm@fb.com, josef@toxicpanda.com, dsterba@suse.com, linux-btrfs@vger.kernel.org, dhowells@redhat.com, jlayton@kernel.org, netfs@lists.linux.dev References: <20250129181749.C229F6F3@davehans-spike.ostc.intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; keydata= xsFNBE6HMP0BEADIMA3XYkQfF3dwHlj58Yjsc4E5y5G67cfbt8dvaUq2fx1lR0K9h1bOI6fC oAiUXvGAOxPDsB/P6UEOISPpLl5IuYsSwAeZGkdQ5g6m1xq7AlDJQZddhr/1DC/nMVa/2BoY 2UnKuZuSBu7lgOE193+7Uks3416N2hTkyKUSNkduyoZ9F5twiBhxPJwPtn/wnch6n5RsoXsb ygOEDxLEsSk/7eyFycjE+btUtAWZtx+HseyaGfqkZK0Z9bT1lsaHecmB203xShwCPT49Blxz VOab8668QpaEOdLGhtvrVYVK7x4skyT3nGWcgDCl5/Vp3TWA4K+IofwvXzX2ON/Mj7aQwf5W iC+3nWC7q0uxKwwsddJ0Nu+dpA/UORQWa1NiAftEoSpk5+nUUi0WE+5DRm0H+TXKBWMGNCFn c6+EKg5zQaa8KqymHcOrSXNPmzJuXvDQ8uj2J8XuzCZfK4uy1+YdIr0yyEMI7mdh4KX50LO1 pmowEqDh7dLShTOif/7UtQYrzYq9cPnjU2ZW4qd5Qz2joSGTG9eCXLz5PRe5SqHxv6ljk8mb ApNuY7bOXO/A7T2j5RwXIlcmssqIjBcxsRRoIbpCwWWGjkYjzYCjgsNFL6rt4OL11OUF37wL QcTl7fbCGv53KfKPdYD5hcbguLKi/aCccJK18ZwNjFhqr4MliQARAQABzUVEYXZpZCBDaHJp c3RvcGhlciBIYW5zZW4gKEludGVsIFdvcmsgQWRkcmVzcykgPGRhdmUuaGFuc2VuQGludGVs LmNvbT7CwXgEEwECACIFAlQ+9J0CGwMGCwkIBwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEGg1 lTBwyZKwLZUP/0dnbhDc229u2u6WtK1s1cSd9WsflGXGagkR6liJ4um3XCfYWDHvIdkHYC1t MNcVHFBwmQkawxsYvgO8kXT3SaFZe4ISfB4K4CL2qp4JO+nJdlFUbZI7cz/Td9z8nHjMcWYF IQuTsWOLs/LBMTs+ANumibtw6UkiGVD3dfHJAOPNApjVr+M0P/lVmTeP8w0uVcd2syiaU5jB aht9CYATn+ytFGWZnBEEQFnqcibIaOrmoBLu2b3fKJEd8Jp7NHDSIdrvrMjYynmc6sZKUqH2 I1qOevaa8jUg7wlLJAWGfIqnu85kkqrVOkbNbk4TPub7VOqA6qG5GCNEIv6ZY7HLYd/vAkVY E8Plzq/NwLAuOWxvGrOl7OPuwVeR4hBDfcrNb990MFPpjGgACzAZyjdmYoMu8j3/MAEW4P0z F5+EYJAOZ+z212y1pchNNauehORXgjrNKsZwxwKpPY9qb84E3O9KYpwfATsqOoQ6tTgr+1BR CCwP712H+E9U5HJ0iibN/CDZFVPL1bRerHziuwuQuvE0qWg0+0SChFe9oq0KAwEkVs6ZDMB2 P16MieEEQ6StQRlvy2YBv80L1TMl3T90Bo1UUn6ARXEpcbFE0/aORH/jEXcRteb+vuik5UGY 5TsyLYdPur3TXm7XDBdmmyQVJjnJKYK9AQxj95KlXLVO38lczsFNBFRjzmoBEACyAxbvUEhd GDGNg0JhDdezyTdN8C9BFsdxyTLnSH31NRiyp1QtuxvcqGZjb2trDVuCbIzRrgMZLVgo3upr MIOx1CXEgmn23Zhh0EpdVHM8IKx9Z7V0r+rrpRWFE8/wQZngKYVi49PGoZj50ZEifEJ5qn/H Nsp2+Y+bTUjDdgWMATg9DiFMyv8fvoqgNsNyrrZTnSgoLzdxr89FGHZCoSoAK8gfgFHuO54B lI8QOfPDG9WDPJ66HCodjTlBEr/Cwq6GruxS5i2Y33YVqxvFvDa1tUtl+iJ2SWKS9kCai2DR 3BwVONJEYSDQaven/EHMlY1q8Vln3lGPsS11vSUK3QcNJjmrgYxH5KsVsf6PNRj9mp8Z1kIG qjRx08+nnyStWC0gZH6NrYyS9rpqH3j+hA2WcI7De51L4Rv9pFwzp161mvtc6eC/GxaiUGuH BNAVP0PY0fqvIC68p3rLIAW3f97uv4ce2RSQ7LbsPsimOeCo/5vgS6YQsj83E+AipPr09Caj 0hloj+hFoqiticNpmsxdWKoOsV0PftcQvBCCYuhKbZV9s5hjt9qn8CE86A5g5KqDf83Fxqm/ vXKgHNFHE5zgXGZnrmaf6resQzbvJHO0Fb0CcIohzrpPaL3YepcLDoCCgElGMGQjdCcSQ+Ci FCRl0Bvyj1YZUql+ZkptgGjikQARAQABwsFfBBgBAgAJBQJUY85qAhsMAAoJEGg1lTBwyZKw l4IQAIKHs/9po4spZDFyfDjunimEhVHqlUt7ggR1Hsl/tkvTSze8pI1P6dGp2XW6AnH1iayn yRcoyT0ZJ+Zmm4xAH1zqKjWplzqdb/dO28qk0bPso8+1oPO8oDhLm1+tY+cOvufXkBTm+whm +AyNTjaCRt6aSMnA/QHVGSJ8grrTJCoACVNhnXg/R0g90g8iV8Q+IBZyDkG0tBThaDdw1B2l asInUTeb9EiVfL/Zjdg5VWiF9LL7iS+9hTeVdR09vThQ/DhVbCNxVk+DtyBHsjOKifrVsYep WpRGBIAu3bK8eXtyvrw1igWTNs2wazJ71+0z2jMzbclKAyRHKU9JdN6Hkkgr2nPb561yjcB8 sIq1pFXKyO+nKy6SZYxOvHxCcjk2fkw6UmPU6/j/nQlj2lfOAgNVKuDLothIxzi8pndB8Jju KktE5HJqUUMXePkAYIxEQ0mMc8Po7tuXdejgPMwgP7x65xtfEqI0RuzbUioFltsp1jUaRwQZ MTsCeQDdjpgHsj+P2ZDeEKCbma4m6Ez/YWs4+zDm1X8uZDkZcfQlD9NldbKDJEXLIjYWo1PH hYepSffIWPyvBMBTW2W5FRjJ4vLRrJSUoEfJuPQ3vW9Y73foyo/qFoURHO48AinGPZ7PC7TF vUaNOTjKedrqHkaOcqB185ahG2had0xnFsDPlx5y In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 1/29/25 23:44, Kent Overstreet wrote: > On Wed, Jan 29, 2025 at 10:17:49AM -0800, Dave Hansen wrote: >> tl;dr: The VFS and several filesystems have some suspect prefaulting >> code. It is unnecessarily slow for the common case where a write's >> source buffer is resident and does not need to be faulted in. >> >> Move these "prefaulting" operations to slow paths where they ensure >> forward progress but they do not slow down the fast paths. This >> optimizes the fast path to touch userspace once instead of twice. >> >> Also update somewhat dubious comments about the need for prefaulting. >> >> This has been very lightly tested. I have not tested any of the fs/ >> code explicitly. > > Q: what is preventing us from posting code to the list that's been > properly tested? > > I just got another bcachefs patch series that blew up immediately when I > threw it at my CI. > > This is getting _utterly ridiculous_. In this case, I started with a single patch for generic code that I knew I could test. In fact, I even had the 9-year-old binary sitting on my test box. Dave Chinner suggested that I take the generic pattern go look a _bit_ more widely in the tree for a similar pattern. That search paid off, I think. But I ended up touching corners of the tree I don't know well and don't have test cases for. > I built multiuser test infrastructure with a nice dashboard that anyone > can use, and the only response I've gotten from the old guard is Ted > jumping in every time I talk about it to say "no, we just don't want to > rewrite our stuff on _your_ stuff!". Real helpful, that. Sounds pretty cool! Is this something that I could have and should have used to test the bcachefs patch? I see some trees in here: https://evilpiepirate.org/~testdashboard/ci But I'm not sure how to submit patches to it. Do you need to add users manually? I wonder, though, how we could make it easier to find. I didn't see anything Documentation/filesystems/bcachefs/ about this. >> 1. Deadlock avoidance if the source and target are the same >> folios. >> 2. To check the user address that copy_folio_from_iter_atomic() >> will touch because atomic user copies do not check the address. >> 3. "Optimization" >> >> I'm not sure any of these are actually valid reasons. >> >> The "atomic" user copy functions disable page fault handling because >> page faults are not very atomic. This makes them naturally resistant >> to deadlocking in page fault handling. They take the page fault >> itself but short-circuit any handling. > > #1 is emphatically valid: the deadlock avoidance is in _both_ using > _atomic when we have locks held, and doing the actual faulting with > locks dropped... either alone would be a buggy incomplete solution. I was (badly) attempting to separate out the two different problems: 1. Doing lock_page() twice, which I was mostly calling the "deadlock" 2. Retrying the copy_folio_from_iter_atomic() forever which I was calling the "livelock" Disabling page faults fixes #1. Doing faulting outside the locks somewhere fixes #2. So when I was talking about "Deadlock avoidance" in the cover letter, I was trying to focus on the double lock_page() problem. > This needs to be reflected and fully described in the comments, since > it's subtle and a lot of people don't fully grok what's going on. Any suggestions for fully describing the situation? I tried to sprinkle comments liberally but I'm also painfully aware that I'm not doing a perfect job of talking about the fs code. > I'm fairly certain we have ioctl code where this is mishandled and thus > buggy, because it takes some fairly particular testing for lockdep to > spot it. Yeah, I wouldn't be surprised. It was having a little chuckle thinking about how many engineers have discovered and fixed this problem independently over the years in all the file system code in all the OSes. >> copy_folio_from_iter_atomic() also *does* have user address checking. >> I get a little lost in the iov_iter code, but it does know when it's >> dealing with userspace versus kernel addresses and does seem to know >> when to do things like copy_from_user_iter() (which does access_ok()) >> versus memcpy_from_iter().[1] >> >> The "optimization" is for the case where 'source' is not faulted in. >> It can avoid the cost of a "failed" page fault (it will fail to be >> handled because of the atomic copy) and then needing to drop locks and >> repeat the fault. > > I do agree on moving it to the slowpath - I think we can expect the case > where the process's immediate workingset is faulted out while it's > running to be vanishingly small. Great! I'm glad we're on the same page there. For bcachefs specifically, how should we move forward? If you're happy with the concept, would you prefer that I do some manual bcachefs testing? Or leave a branch sitting there for a week and pray the robots test it?