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=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 BFAD0C433F5 for ; Tue, 14 Sep 2021 21:14:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AC5FD61216 for ; Tue, 14 Sep 2021 21:14:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234066AbhINVPU (ORCPT ); Tue, 14 Sep 2021 17:15:20 -0400 Received: from smtp-out2.suse.de ([195.135.220.29]:46900 "EHLO smtp-out2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233567AbhINVPT (ORCPT ); Tue, 14 Sep 2021 17:15:19 -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-out2.suse.de (Postfix) with ESMTPS id B76D51FE30; Tue, 14 Sep 2021 21:14:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1631654040; 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=p7HOhMJiSvEdnqg2c8rgOboivLABb0GSRM2C1XyfMK0=; b=D1GJj9aNpzzAT0YUJGnFdmW0iot6sNFvPRFUKGthNXwX2VG+VdlQH86I3u4pFey4+I+5bk P8KNxZGnYuGQgjj84ICJDWVeJJBZe8qDO+CYJBuVpDZa4bb4Q2k2F7VGHJ0Qky/19CwcHp fORe0hxvENVJigFZ/wIHzwIxrEdDZjg= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1631654040; 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=p7HOhMJiSvEdnqg2c8rgOboivLABb0GSRM2C1XyfMK0=; b=pI1W8hgJ2Vg6NRag8uc+MK00GPMD7aCANHwz9e1iXwyYnyAcjTQtnZadcfDajSuz0jYDPt CHwO2jm8qSSDNlBg== 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 2216913B43; Tue, 14 Sep 2021 21:13:56 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id v5amNJQQQWFzMwAAMHmgww (envelope-from ); Tue, 14 Sep 2021 21:13:56 +0000 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 From: "NeilBrown" To: "Mel Gorman" Cc: "Dave Chinner" , "Andrew Morton" , "Theodore Ts'o" , "Andreas Dilger" , "Darrick J. Wong" , "Matthew Wilcox" , 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: <20210914164504.GS3828@suse.com> References: <163157808321.13293.486682642188075090.stgit@noble.brown>, <163157838440.13293.12568710689057349786.stgit@noble.brown>, <20210914020837.GH2361455@dread.disaster.area>, <163158695921.3992.9776900395549582360@noble.neil.brown.name>, <20210914164504.GS3828@suse.com> Date: Wed, 15 Sep 2021 07:13:54 +1000 Message-id: <163165403435.3992.14639160345151711607@noble.neil.brown.name> Precedence: bulk List-ID: X-Mailing-List: linux-nfs@vger.kernel.org On Wed, 15 Sep 2021, Mel Gorman wrote: > On Tue, Sep 14, 2021 at 12:35:59PM +1000, NeilBrown wrote: > > 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 > > > >=20 > > > > is definitely preferable to use the flag rather than opencode > > > > endless loop around allocator. > > > >=20 > > > > congestion_wait() is indistinguishable from > > > > schedule_timeout_uninterruptible() in practice and it is not a good w= ay > > > > to wait for memory to become available. > > > >=20 > > > > 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. > > > >=20 > > > > Signed-off-by: NeilBrown > > > > --- > > > > fs/xfs/xfs_buf.c | 6 +++--- > > > > 1 file changed, 3 insertions(+), 3 deletions(-) > > > >=20 > > > > 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( > > > > =20 > > > > /* > > > > * Bulk filling of pages can take multiple calls. Not filling the e= ntire > > > > - * 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 =3D filled; > > > > @@ -394,7 +394,7 @@ xfs_buf_alloc_pages( > > > > } > > > > =20 > > > > XFS_STATS_INC(bp->b_mount, xb_page_retries); > > > > - congestion_wait(BLK_RW_ASYNC, HZ / 50); > > > > + bp->b_pages[filled++] =3D alloc_page(gfp_mask | __GFP_NOFAIL); > > >=20 > > > 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(). > > >=20 > > > 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: > > >=20 > > > if (flags & XBF_READ_AHEAD) > > > gfp_mask |=3D __GFP_NORETRY; > > > else > > > - gfp_mask |=3D GFP_NOFS; > > > + gfp_mask |=3D GFP_NOFS | __GFP_NOFAIL; > > >=20 > > > 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. > >=20 > > 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. > >=20 > > I had a look at the code and cannot see how it would fail to allocate at > > least one page. Maybe Mel can help.... > >=20 >=20 > If there are already at least one page an the array and the first attempt > at bulk allocation fails, it'll simply return. It's an odd corner case > that may never apply but it's possible. That said, I'm of the opinion that > __GFP_NOFAIL should not be expanded and instead congestion_wait should be > deleted and replaced with something triggered by reclaim making progress. Ahh.... that was (I think) fixed by https://patchwork.kernel.org/project/linux-mm/patch/163027609524.7591.4987241= 695872857175@noble.neil.brown.name/ (which I cannot find on lore.kernel.org - strange) which you acked - and which I meant to include in this series but somehow missed. NeilBrown