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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 D21BBC46475 for ; Thu, 25 Oct 2018 13:22:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7DE3D2075D for ; Thu, 25 Oct 2018 13:22:34 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7DE3D2075D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.cz Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727578AbeJYVzR (ORCPT ); Thu, 25 Oct 2018 17:55:17 -0400 Received: from mx2.suse.de ([195.135.220.15]:32878 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727234AbeJYVzQ (ORCPT ); Thu, 25 Oct 2018 17:55:16 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 97CEDAFC3; Thu, 25 Oct 2018 13:22:31 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 7FB581E06C6; Thu, 25 Oct 2018 15:22:30 +0200 (CEST) Date: Thu, 25 Oct 2018 15:22:30 +0200 From: Jan Kara To: Josef Bacik Cc: kernel-team@fb.com, hannes@cmpxchg.org, linux-kernel@vger.kernel.org, tj@kernel.org, david@fromorbit.com, akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-btrfs@vger.kernel.org, riel@fb.com, linux-mm@kvack.org Subject: Re: [PATCH 7/7] btrfs: drop mmap_sem in mkwrite for btrfs Message-ID: <20181025132230.GD7711@quack2.suse.cz> References: <20181018202318.9131-1-josef@toxicpanda.com> <20181018202318.9131-8-josef@toxicpanda.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181018202318.9131-8-josef@toxicpanda.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 18-10-18 16:23:18, Josef Bacik wrote: > ->page_mkwrite is extremely expensive in btrfs. We have to reserve > space, which can take 6 lifetimes, and we could possibly have to wait on > writeback on the page, another several lifetimes. To avoid this simply > drop the mmap_sem if we didn't have the cached page and do all of our > work and return the appropriate retry error. If we have the cached page > we know we did all the right things to set this page up and we can just > carry on. > > Signed-off-by: Josef Bacik ... > @@ -8828,6 +8830,29 @@ vm_fault_t btrfs_page_mkwrite(struct vm_fault *vmf) > > reserved_space = PAGE_SIZE; > > + /* > + * We have our cached page from a previous mkwrite, check it to make > + * sure it's still dirty and our file size matches when we ran mkwrite > + * the last time. If everything is OK then return VM_FAULT_LOCKED, > + * otherwise do the mkwrite again. > + */ > + if (vmf->flags & FAULT_FLAG_USED_CACHED) { > + lock_page(page); > + if (vmf->cached_size == i_size_read(inode) && > + PageDirty(page)) > + return VM_FAULT_LOCKED; > + unlock_page(page); > + } I guess this is similar to Dave's comment: Why is i_size so special? What makes sure that file didn't get modified between time you've prepared cached_page and now such that you need to do the preparation again? And if indeed metadata prepared for a page cannot change, what's so special about it being that particular cached_page? Maybe to phrase my objections differently: Your preparations in btrfs_page_mkwrite() are obviously related to your filesystem metadata. So why cannot you infer from that metadata (extent tree, whatever - I'd use extent status tree in ext4) whether that particular file+offset is already prepared for writing and just bail out with success in that case? Honza -- Jan Kara SUSE Labs, CR