From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id CD5996B0299 for ; Thu, 25 Oct 2018 09:22:33 -0400 (EDT) Received: by mail-ed1-f72.google.com with SMTP id x44-v6so4709010edd.17 for ; Thu, 25 Oct 2018 06:22:33 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id o56-v6si4901415edc.388.2018.10.25.06.22.32 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 25 Oct 2018 06:22:32 -0700 (PDT) Date: Thu, 25 Oct 2018 15:22:30 +0200 From: Jan Kara 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> Sender: owner-linux-mm@kvack.org List-ID: 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 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