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=-3.8 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 6255FC433E1 for ; Thu, 2 Jul 2020 17:30:35 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 2992A20870 for ; Thu, 2 Jul 2020 17:30:35 +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="nR1VN3uW" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2992A20870 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 E62B68D000F; Thu, 2 Jul 2020 13:30:33 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id E12D98D000C; Thu, 2 Jul 2020 13:30:33 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D4F7F8D000F; Thu, 2 Jul 2020 13:30:33 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0047.hostedemail.com [216.40.44.47]) by kanga.kvack.org (Postfix) with ESMTP id BF39F8D000C for ; Thu, 2 Jul 2020 13:30:33 -0400 (EDT) Received: from smtpin08.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 30185180AD806 for ; Thu, 2 Jul 2020 17:30:33 +0000 (UTC) X-FDA: 76993825146.08.straw40_4d127a426e8b Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin08.hostedemail.com (Postfix) with ESMTP id 1B1EB1819E62D for ; Thu, 2 Jul 2020 17:30:31 +0000 (UTC) X-HE-Tag: straw40_4d127a426e8b X-Filterd-Recvd-Size: 4597 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf39.hostedemail.com (Postfix) with ESMTP for ; Thu, 2 Jul 2020 17:30:30 +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=lMEXGkg9/1lKgDiiMTcrjkxDHT1PVSiTcXNRYHyvJiM=; b=nR1VN3uWw+bi70Q8kmATNOE5xc RIzIGoeaBwT8SyB4q4B6QocuAW3sKI5wdTS/6q9ENDIfzAXAp/A4MyWAYb44Tn/bAQQ2SUjR9NUG/ 1O7bDHuTp+b7UgXiYwainOhDFiy4FUrnPNclJ8Y+60riDfL6nXD+yRa4rlN3QcEnaY8tPyBxOMEuH w1/G0LU8A+lSq/adOGCPix7P/ATW+gSefXiYU75DrOIeieGJnS5svifbdXCbZgFO5izeIwSPvX8/Y G1EfSOa3KYcsHJdG+vBnQIcmbSYQ/HRkeEbQgQbpm0LE/xCzh2SOdOFIitjiRvQ70Ll+VN702Uted bYYbbIIA==; Received: from willy by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1jr32Y-0003zn-Cl; Thu, 02 Jul 2020 17:30:26 +0000 Date: Thu, 2 Jul 2020 18:30:26 +0100 From: Matthew Wilcox To: Andreas Gruenbacher Cc: Dave Chinner , linux-fsdevel , Linux-MM , LKML Subject: Re: [RFC] Bypass filesystems for reading cached pages Message-ID: <20200702173026.GA25523@casper.infradead.org> References: <20200619155036.GZ8681@bombadil.infradead.org> <20200622003215.GC2040@dread.disaster.area> <20200622181338.GA21350@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 1B1EB1819E62D X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 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 Thu, Jul 02, 2020 at 05:16:43PM +0200, Andreas Gruenbacher wrote: > On Wed, Jun 24, 2020 at 2:35 PM Andreas Gruenbacher wrote: > > On Mon, Jun 22, 2020 at 8:13 PM Matthew Wilcox wrote: > > > On Mon, Jun 22, 2020 at 04:35:05PM +0200, Andreas Gruenbacher wrote: > > > > I'm fine with not moving that functionality into the VFS. The problem > > > > I have in gfs2 is that taking glocks is really expensive. Part of that > > > > overhead is accidental, but we definitely won't be able to fix it in > > > > the short term. So something like the IOCB_CACHED flag that prevents > > > > generic_file_read_iter from issuing readahead I/O would save the day > > > > for us. Does that idea stand a chance? > > > > > > For the short-term fix, is switching to a trylock in gfs2_readahead() > > > acceptable? > > > > Well, it's the only thing we can do for now, right? > > It turns out that gfs2 can still deadlock with a trylock in > gfs2_readahead, just differently: in this instance, gfs2_glock_nq will > call inode_dio_wait. When there is pending direct I/O, we'll end up > waiting for iomap_dio_complete, which will call > invalidate_inode_pages2_range, which will try to lock the pages > already locked for gfs2_readahead. That seems like a bug in trylock. If I trylock something I'm not expecting it to wait; i'm expecting it to fail because it would have to wait. ie something like this: diff --git a/fs/gfs2/glops.c b/fs/gfs2/glops.c index c84887769b5a..97ca8f5ed22b 100644 --- a/fs/gfs2/glops.c +++ b/fs/gfs2/glops.c @@ -470,7 +470,10 @@ static int inode_go_lock(struct gfs2_holder *gh) return error; } - if (gh->gh_state != LM_ST_DEFERRED) + if (gh->gh_flags & LM_FLAG_TRY) { + if (atomic_read(&ip->i_inode.i_dio_count)) + return -EWOULDBLOCK; + } else if (gh->gh_state != LM_ST_DEFERRED) inode_dio_wait(&ip->i_inode); if ((ip->i_diskflags & GFS2_DIF_TRUNC_IN_PROG) && ... but probably not exactly that because I didn't try to figure out the calling conventions or whether I should set some state in the gfs2_holder. > This late in the 5.8 release cycle, I'd like to propose converting > gfs2 back to use mpage_readpages. This requires reinstating > mpage_readpages, but it's otherwise relatively trivial. > We can then introduce an IOCB_CACHED or equivalent flag, fix the > locking order in gfs2, convert gfs2 to mpage_readahead, and finally > remove mage_readpages in 5.9. I would rather not do that.