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=-13.8 required=3.0 tests=BAYES_00, 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 3AA68C433EF for ; Tue, 14 Sep 2021 02:08:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 23DDF610A6 for ; Tue, 14 Sep 2021 02:08:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236138AbhINCKA (ORCPT ); Mon, 13 Sep 2021 22:10:00 -0400 Received: from mail109.syd.optusnet.com.au ([211.29.132.80]:50947 "EHLO mail109.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233111AbhINCKA (ORCPT ); Mon, 13 Sep 2021 22:10:00 -0400 Received: from dread.disaster.area (pa49-195-238-16.pa.nsw.optusnet.com.au [49.195.238.16]) by mail109.syd.optusnet.com.au (Postfix) with ESMTPS id 82C30ECDEAD; Tue, 14 Sep 2021 12:08:40 +1000 (AEST) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1mPxsD-00CD9j-TB; Tue, 14 Sep 2021 12:08:37 +1000 Date: Tue, 14 Sep 2021 12:08:37 +1000 From: Dave Chinner To: NeilBrown 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() Message-ID: <20210914020837.GH2361455@dread.disaster.area> References: <163157808321.13293.486682642188075090.stgit@noble.brown> <163157838440.13293.12568710689057349786.stgit@noble.brown> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <163157838440.13293.12568710689057349786.stgit@noble.brown> X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.3 cv=F8MpiZpN c=1 sm=1 tr=0 a=DzKKRZjfViQTE5W6EVc0VA==:117 a=DzKKRZjfViQTE5W6EVc0VA==:17 a=kj9zAlcOel0A:10 a=7QKq2e-ADPsA:10 a=7-415B0cAAAA:8 a=W8B-ML30YZIRN3gm7fgA:9 a=CjuIK1q_8ugA:10 a=biEYGPWJfzWAr4FL6Ov7:22 Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org 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. 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