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=-6.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 E6F7CC43467 for ; Fri, 18 Sep 2020 11:19:25 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 7FF1821D42 for ; Fri, 18 Sep 2020 11:19:25 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="OhWnaBzr" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7FF1821D42 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 0AA276B0062; Fri, 18 Sep 2020 07:19:25 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 031F36B0068; Fri, 18 Sep 2020 07:19:24 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E13DA6B006C; Fri, 18 Sep 2020 07:19:24 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0113.hostedemail.com [216.40.44.113]) by kanga.kvack.org (Postfix) with ESMTP id C61A66B0062 for ; Fri, 18 Sep 2020 07:19:24 -0400 (EDT) Received: from smtpin14.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 8647C180AD804 for ; Fri, 18 Sep 2020 11:19:24 +0000 (UTC) X-FDA: 77275936248.14.side10_1615b352712a Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin14.hostedemail.com (Postfix) with ESMTP id 600D218229818 for ; Fri, 18 Sep 2020 11:19:24 +0000 (UTC) X-HE-Tag: side10_1615b352712a X-Filterd-Recvd-Size: 4054 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf29.hostedemail.com (Postfix) with ESMTP for ; Fri, 18 Sep 2020 11:19:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=D2ju8jMz6rNG3JDmyLzknrnz9KKFGg0PYq+DeMuVZKY=; b=OhWnaBzrqkDRbzOwxxDZrAitsW uGCQAikASHjYa/PMPuiPlUOcliemRBZlSc/1RLXgVWxgpXKT26jnRaH2hmTM72gxSVdhOPqu6kQPd h+y2nYjcOI/MK6CMWwhrlZi9ICHnkGIztdyZsb1kUMTfkP470jFOrvRJfyuy8uYVb1UtlcOcYJKpU E5Rm9//Op9Yh9xVsMJdzShSN0jO9N6AGwLLD/O1r3P9sdUd2Ic22Cm/qIxyYzHn5gT/lwgN84usJW ACafXKejemQbxTWMfkPL3z3ucBuijE/zG8VKPfz4WQt3m2mP+hkouoscs89VtGRxh2HUUy/mInKXY PZ8Gd81Q==; Received: from willy by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kJEQ9-0002ra-1H; Fri, 18 Sep 2020 11:19:17 +0000 Date: Fri, 18 Sep 2020 12:19:16 +0100 From: Matthew Wilcox To: Dominique Martinet Cc: linux-fsdevel@vger.kernel.org, linux-cifs@vger.kernel.org, Richard Weinberger , ecryptfs@vger.kernel.org, linux-um@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-mtd@lists.infradead.org, v9fs-developer@lists.sourceforge.net, ceph-devel@vger.kernel.org, linux-afs@lists.infradead.org Subject: Re: [V9fs-developer] [PATCH 02/13] 9p: Tell the VFS that readpage was synchronous Message-ID: <20200918111916.GA32101@casper.infradead.org> References: <20200917151050.5363-1-willy@infradead.org> <20200917151050.5363-3-willy@infradead.org> <20200918055919.GA30929@nautica> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200918055919.GA30929@nautica> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Sep 18, 2020 at 07:59:19AM +0200, Dominique Martinet wrote: > Matthew Wilcox (Oracle) wrote on Thu, Sep 17, 2020: > > diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c > > index cce9ace651a2..506ca0ba2ec7 100644 > > --- a/fs/9p/vfs_addr.c > > +++ b/fs/9p/vfs_addr.c > > @@ -280,6 +280,10 @@ static int v9fs_write_begin(struct file *filp, struct address_space *mapping, > > goto out; > > > > retval = v9fs_fid_readpage(v9inode->writeback_fid, page); > > + if (retval == AOP_UPDATED_PAGE) { > > + retval = 0; > > + goto out; > > + } > > FWIW this is a change of behaviour; for some reason the code used to > loop back to grab_cache_page_write_begin() and bail out on > PageUptodate() I suppose; some sort of race check? > The whole pattern is a bit weird to me and 9p has no guarantee on > concurrent writes to a file with cache enabled (except that it will > corrupt something), so this part is fine with me. > > What I'm curious about is the page used to be both unlocked and put, but > now isn't either and the return value hasn't changed for the caller to > make a difference on write_begin / I don't see any code change in the > vfs to handle that. > What did I miss? The page cache is kind of subtle. The grab_cache_page_write_begin() will return a Locked page with an increased refcount. If it's Uptodate, that's exactly what we want, and we return it. If we have to read the page, readpage used to unlock the page before returning, and rather than re-lock it, we would drop the reference to the page and look it up again. It's possible that after dropping the lock on that page that the page was replaced in the page cache and so we'd get a different page. Anyway, now (unless fscache is involved), v9fs_fid_readpage will return the page without unlocking it. So we don't need to do the dance of dropping the lock, putting the refcount and looking the page back up again. We can just return the page. The VFS doesn't need a special return code because nothing has changed from the VFS's point of view -- it asked you to get a page and you got the page.