From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2930BC3A5A4 for ; Sat, 24 Aug 2019 03:18:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F2B8221848 for ; Sat, 24 Aug 2019 03:18:46 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=mbobrowski-org.20150623.gappssmtp.com header.i=@mbobrowski-org.20150623.gappssmtp.com header.b="vVkcS0BH" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726520AbfHXDSp (ORCPT ); Fri, 23 Aug 2019 23:18:45 -0400 Received: from mail-pf1-f196.google.com ([209.85.210.196]:40030 "EHLO mail-pf1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726387AbfHXDSp (ORCPT ); Fri, 23 Aug 2019 23:18:45 -0400 Received: by mail-pf1-f196.google.com with SMTP id w16so7820384pfn.7 for ; Fri, 23 Aug 2019 20:18:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mbobrowski-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=XDGdcSXIqyLF2qZ43wb756VbO6pNmebjNMnWc+tm5r4=; b=vVkcS0BH1V3C7/yaOR0KizQZ/EGsGfBWmtOyOkj5sVrYjRJfkvjsaNZT240Pq7uTvl LTWIq57t2cfCpi/xbmO7BvVlBJn93Y/u4o842j3+0pYPYXsBoTRq134I8cdJ+cc8puqV b0LGAYW6aIYp3w0Nh5bdGHqxQu8ZJNFRowvmGorsZ9cgDYsXeIxtiMk5zLHbnRTZpZTl gXUL+8BxPNtoZqvhimksXZUaGXSpfkPMLOiRw0B2CnfX2CP8ycJV4shtnR/+xqTXgtdt h27KfT+YraZz+g+TbnRJtdJxQYFS9xsphKM6E15brPmLCpMQdCM8YunfEA04aj46i0ye Sh7g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=XDGdcSXIqyLF2qZ43wb756VbO6pNmebjNMnWc+tm5r4=; b=AkaDD+KTY+/m20IeWuW33iDazFSnuL6UC+NZCcPEm+iXJWn9N7c3ZUR8GlgDZXCQiY Z2ijZ0ABhyUuoq/y5wc7DvRghDQnd2KQU2BDHNVCbGueEq6G7b5tSBwc2bf5McswuzgC F/Q1mKElL0j32eKHkCt4XlslxBBnofS5LArTewHdUB8AItWPqAMK1TH7LtXjE8EnphdC 0e2PIepHpFMFV8g949bvuJJ4wF7etXwiuWxb/I8wfqW4kxX9mMV5Ewc+oXCopMHMbJ4d d25r4vWgmaiqoiN9H1iz7uGZFMiws0h+IXwbWd7+eJVPN8NylBFQFczJjnbzkplRJX3Z nSlg== X-Gm-Message-State: APjAAAVvSyXicBWLwR+zlEboRJzbfhrfMqVG5b7hxvahZUUfCs0KLtC+ gkpFAYqrR/GVL19tvZXUF8eO X-Google-Smtp-Source: APXvYqwhKmqG0S7uZCKEa+SXvnIdTU5R2pSMzZyYYcUHuahJVlklR9GLTUg5uhWq/Fs0FkS1KV4D+w== X-Received: by 2002:a17:90a:d146:: with SMTP id t6mr8549240pjw.76.1566616719260; Fri, 23 Aug 2019 20:18:39 -0700 (PDT) Received: from poseidon.bobrowski.net ([114.78.226.167]) by smtp.gmail.com with ESMTPSA id f63sm4695894pfa.144.2019.08.23.20.18.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Aug 2019 20:18:38 -0700 (PDT) Date: Sat, 24 Aug 2019 13:18:31 +1000 From: Matthew Bobrowski To: Ritesh Harjani Cc: tytso@mit.edu, jack@suse.cz, adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, hch@infradead.org, aneesh.kumar@linux.ibm.com, darrick.wong@oracle.com Subject: Re: [PATCH 0/5] ext4: direct IO via iomap infrastructure Message-ID: <20190824031830.GB2174@poseidon.bobrowski.net> References: <20190812173150.AF04F5204F@d06av21.portsmouth.uk.ibm.com> <20190813111004.GA12682@poseidon.bobrowski.net> <20190813122723.AE6264C040@d06av22.portsmouth.uk.ibm.com> <20190821131405.GC24417@poseidon.bobrowski.net> <20190822120015.GA3330@poseidon.bobrowski.net> <20190822141126.70A94A407B@d06av23.portsmouth.uk.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190822141126.70A94A407B@d06av23.portsmouth.uk.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Aug 22, 2019 at 07:41:25PM +0530, Ritesh Harjani wrote: > > > On 8/22/19 5:30 PM, Matthew Bobrowski wrote: > > On Wed, Aug 21, 2019 at 11:14:07PM +1000, Matthew Bobrowski wrote: > > > On Tue, Aug 13, 2019 at 05:57:22PM +0530, RITESH HARJANI wrote: > > > > But what I meant was this (I may be wrong here since I haven't > > > > really looked into it), but for my understanding I would like to > > > > discuss this - > > > > > > > > So earlier with this flag(EXT4_STATE_DIO_UNWRITTEN) we were determining on > > > > whether a newextent can be merged with ex1 in function > > > > ext4_extents_can_be_merged. But now since we have removed that flag we have > > > > no way of knowing that whether this inode has any unwritten extents or not > > > > from any DIO path. > > > > > > > > What I meant is isn't this removal of setting/unsetting of > > > > flag(EXT4_STATE_DIO_UNWRITTEN) changing the behavior of this func - > > > > ext4_extents_can_be_merged? > > > > > > OK, I'm stuck and looking for either clarity, revalidation of my > > > thought process, or any input on how to solve this problem for that > > > matter. > > > > > > In the current ext4 direct IO implementation, the dynamic state flag > > > EXT4_STATE_DIO_UNWRITTEN is set/unset for synchronous direct IO > > > writes. On the other hand, the flag EXT4_IO_END_UNWRITTEN is set/unset > > > for ext4_io_end->flag, and the value of i_unwritten is > > > incremented/decremented for asynchronous direct IO writes. All > > > mechanisms by which are used to track and determine whether the inode, > > > or an IO in flight against a particular inode have any pending > > > unwritten extents that need to be converted after the IO has > > > completed. In addition to this, we have ext4_can_extents_be_merged() > > > performing explicit checks against both EXT4_STATE_DIO_UNWRITTEN and > > > i_unwritten and using them to determine whether it can or cannot merge > > > a requested extent into an existing extent. > > > > > > This is all fine for the current direct IO implementation. However, > > > while switching the direct IO code paths over to make use of the iomap > > > infrastructure, I believe that we can no longer simply track whether > > > an inode has unwritten extents needing to be converted by simply > > > setting and checking the EXT4_STATE_DIO_UNWRITTEN flag on the > > > inode. The reason being is that there can be multiple direct IO > > > operations to unwritten extents running against the inode and we don't > > > particularly distinguish synchronous from asynchronous writes within > > > ext4_iomap_begin() as there's really no need. So, the only way to > > > accurately determine whether extent conversion is deemed necessary for > > > an IO operation whether it'd be synchronous or asynchronous is by > > > checking the IOMAP_DIO_UNWRITTEN flag within the ->end_io() > > > callback. I'm certain that this portion of the logic is correct, but > > > we're still left with some issues when it comes to the checks that I > > > previously mentioned in ext4_can_extents_be_merged(), which is the > > > part I need some input on. > > > > > > I was doing some thinking and I don't believe that making use of the > > > EXT4_STATE_DIO_UNWRITTEN flag is the solution at all here. This is not > > > only for reasons that I've briefly mentioned above, but also because > > > of the fact that it'll probably lead to a lot of inaccurate judgements > > > while taking particular code paths and some really ugly code that > > > creeps close to the definition of insanity. Rather, what if we solve > > > this problem by continuing to just use i_unwritten to keep track of > > > all the direct IOs to unwritten against running against an inode? > > > Within ext4_iomap_begin() post successful creation of unwritten > > > extents we'd call atomic_inc(&EXT4_I(inode)->i_unwritten) and > > > subsequently within the ->end_io() callback whether we take the > > > success or error path we'd call > > > atomic_dec(&EXT4_I(inode)->i_unwritten) accordingly? This way we can > > > still rely on this value to be used in the check within > > > ext4_can_extents_be_merged(). Open for alternate suggestions if anyone > > > has any... > > > > Actually, no... > > > > I've done some more thinking and what I suggested above around the use > > of i_unwritten will also not work properly. Using iomap > > infrastructure, there is the possibility of calling into the > > ->iomap_begin() more than once for a single direct IO operation. This > > means that by the time we even get to decrementing i_unwritten in the > > ->end_io() callback after converting the unwritten extents we're > > already running the possibility of i_unwritten becoming unbalanced > > really quickly and staying that way. This also means that the > > statement checking i_unwritten in ext4_can_extents_be_merged() will be > > affected and potentially result in it being evaluated incorrectly. I > > was thinking that we could just decrement i_unwritten in > > ->iomap_end(), but that seems to me like it would be racy and also > > lead to incorrect results. At this point I'm out of ideas on how to > > solve this, so any other ideas would be appreciated! > > I will let others also comment, if someone has any other better approach. > > 1. One approach is to add the infrastructure in iomap with > iomap_dio->private which is filesystem specific pointer. This can be > updated by filesystem in ->iomap_begin call into iomap->private. > And in case of iomap_dio_rw, this iomap->private will be copied to > iomap_dio->private if not already set. > > But I think this will eventually become hacky in the sense when you will > have to determine on whether the dio->private is already set or not when > iomap_apply will be called second time. It will become a problem with AIO > DIO in ext4 since we use i_unwritten which tells us whether there is any > unwritten extent but it does not tell whether this unwritten extent is due > to a DIRECT AIO DIO in progress or a buffered one. > > So we can ignore this approach - unless you or someone else have some good > design ideas to build on top of above. I'm not sure whether _this_ is the solution or not, so let's maybe wait for others to comment. One thing that I and probably the iomap maintainers would like to avoid is adding any special case code to iomap infrastructure, if possible. I mean, from what you suggest it seems to be rather generic and not overly intrusive, although I know for a fact that iomap infrastructure exists because stuff like buffer_heads and the old direct IO code ended up accommodating so many different edge cases making it almost unmodifiable and unmaintainable. > 2. Second approach which I was thinking is to track only those extents which > are marked unwritten and are under IO. This can be done in ext4_map_blocks. > This way we will not have to track a particular inode has any unwritten > extents or not, but it will be extent based. > Something similar was also done a while ago. Do you think this approach will > work in our case? > > So with this extents will be scanned in extent status tree to see if any > among those are under IO and are unwritten in func > ext4_can_extents_be_merged. > > https://patchwork.ozlabs.org/patch/1013837/ Maybe this would be a better approach and I think that it'd work. Based upon what I read within in that thread there weren't really any objections to the idea, although I can't see that it made it upstream, so I may be missing something? --M