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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 0D8EAC43217 for ; Tue, 14 Sep 2021 02:36:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E6BC961151 for ; Tue, 14 Sep 2021 02:36:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237372AbhINCh1 (ORCPT ); Mon, 13 Sep 2021 22:37:27 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:36288 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234374AbhINChX (ORCPT ); Mon, 13 Sep 2021 22:37:23 -0400 Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id E4EDB21DDB; Tue, 14 Sep 2021 02:36:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1631586965; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Np1b24Y1jsf4JFXb99vT1VoKvE/1uNudORwJZ2vKFkc=; b=LSFcqfcpRgFTnxIQwkZmmJe5i8WfcEz6ATENYq3zy8Mt9igIghRP4XJgl57iMpRTwu3VL7 QcwyCrixojHmBVyyzyEijkfq4TtdGURNfAaQ4If3S8HH2xdNWzlKTywqXRWrg2cLUYIC02 aI6hRzXjQLSphhYMWQUymmWazx8Sw4c= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1631586965; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Np1b24Y1jsf4JFXb99vT1VoKvE/1uNudORwJZ2vKFkc=; b=c7rLL2e4zLbiZ4/9hj+D7oO6Patx1a8luivPZ/+WDAuEi5ggAe+X5Ft+bUuKnDRjfBorV7 FDrEY8jjzyqeB7DQ== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 33AB513B56; Tue, 14 Sep 2021 02:36:01 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id S0spOJEKQGHWFgAAMHmgww (envelope-from ); Tue, 14 Sep 2021 02:36:01 +0000 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit MIME-Version: 1.0 From: "NeilBrown" To: "Dave Chinner" Cc: "Andrew Morton" , "Theodore Ts'o" , "Andreas Dilger" , "Darrick J. Wong" , "Matthew Wilcox" , "Mel Gorman" , linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 6/6] XFS: remove congestion_wait() loop from xfs_buf_alloc_pages() In-reply-to: <20210914020837.GH2361455@dread.disaster.area> References: <163157808321.13293.486682642188075090.stgit@noble.brown>, <163157838440.13293.12568710689057349786.stgit@noble.brown>, <20210914020837.GH2361455@dread.disaster.area> Date: Tue, 14 Sep 2021 12:35:59 +1000 Message-id: <163158695921.3992.9776900395549582360@noble.neil.brown.name> Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Tue, 14 Sep 2021, Dave Chinner wrote: > On Tue, Sep 14, 2021 at 10:13:04AM +1000, NeilBrown wrote: > > Documentation commment in gfp.h discourages indefinite retry loops on > > ENOMEM and says of __GFP_NOFAIL that it > > > > is definitely preferable to use the flag rather than opencode > > endless loop around allocator. > > > > congestion_wait() is indistinguishable from > > schedule_timeout_uninterruptible() in practice and it is not a good way > > to wait for memory to become available. > > > > So instead of waiting, allocate a single page using __GFP_NOFAIL, then > > loop around and try to get any more pages that might be needed with a > > bulk allocation. This single-page allocation will wait in the most > > appropriate way. > > > > Signed-off-by: NeilBrown > > --- > > fs/xfs/xfs_buf.c | 6 +++--- > > 1 file changed, 3 insertions(+), 3 deletions(-) > > > > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c > > index 5fa6cd947dd4..1ae3768f6504 100644 > > --- a/fs/xfs/xfs_buf.c > > +++ b/fs/xfs/xfs_buf.c > > @@ -372,8 +372,8 @@ xfs_buf_alloc_pages( > > > > /* > > * Bulk filling of pages can take multiple calls. Not filling the entire > > - * array is not an allocation failure, so don't back off if we get at > > - * least one extra page. > > + * array is not an allocation failure, so don't fail or fall back on > > + * __GFP_NOFAIL if we get at least one extra page. > > */ > > for (;;) { > > long last = filled; > > @@ -394,7 +394,7 @@ xfs_buf_alloc_pages( > > } > > > > XFS_STATS_INC(bp->b_mount, xb_page_retries); > > - congestion_wait(BLK_RW_ASYNC, HZ / 50); > > + bp->b_pages[filled++] = alloc_page(gfp_mask | __GFP_NOFAIL); > > This smells wrong - the whole point of using the bulk page allocator > in this loop is to avoid the costly individual calls to > alloc_page(). > > What we are implementing here fail-fast semantics for readahead and > fail-never for everything else. If the bulk allocator fails to get > a page from the fast path free lists, it already falls back to > __alloc_pages(gfp, 0, ...) to allocate a single page. So AFAICT > there's no need to add another call to alloc_page() because we can > just do this instead: > > if (flags & XBF_READ_AHEAD) > gfp_mask |= __GFP_NORETRY; > else > - gfp_mask |= GFP_NOFS; > + gfp_mask |= GFP_NOFS | __GFP_NOFAIL; > > Which should make the __alloc_pages() call in > alloc_pages_bulk_array() do a __GFP_NOFAIL allocation and hence > provide the necessary never-fail guarantee that is needed here. That is a nice simplification. Mel Gorman told me https://lore.kernel.org/linux-nfs/20210907153116.GJ3828@suse.com/ that alloc_pages_bulk ignores GFP_NOFAIL. I added that to the documentation comment in an earlier patch. I had a look at the code and cannot see how it would fail to allocate at least one page. Maybe Mel can help.... NeilBrown > > At which point, the bulk allocation loop can be simplified because > we can only fail bulk allocation for readahead, so something like: > > if (filled == bp->b_page_count) { > XFS_STATS_INC(bp->b_mount, xb_page_found); > break; > } > > - if (filled != last) > + if (filled == last) { > - continue; > - > - if (flags & XBF_READ_AHEAD) { > ASSERT(flags & XBF_READ_AHEAD); > xfs_buf_free_pages(bp); > return -ENOMEM; > } > > XFS_STATS_INC(bp->b_mount, xb_page_retries); > - congestion_wait(BLK_RW_ASYNC, HZ / 50); > } > return 0; > } > > would do the right thing and still record that we are doing > blocking allocations (via the xb_page_retries stat) in this loop. > > Cheers, > > Dave. > -- > Dave Chinner > david@fromorbit.com > >