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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 EDF64C433DF for ; Wed, 8 Jul 2020 08:00:07 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id ACFDD20842 for ; Wed, 8 Jul 2020 08:00:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="JIha1NjJ" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org ACFDD20842 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 2424E8D0030; Wed, 8 Jul 2020 04:00:07 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 1CACB8D000D; Wed, 8 Jul 2020 04:00:07 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 0925C8D0030; Wed, 8 Jul 2020 04:00:07 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0004.hostedemail.com [216.40.44.4]) by kanga.kvack.org (Postfix) with ESMTP id E0CD68D000D for ; Wed, 8 Jul 2020 04:00:06 -0400 (EDT) Received: from smtpin29.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 7B73E8248D7C for ; Wed, 8 Jul 2020 08:00:06 +0000 (UTC) X-FDA: 77014160412.29.home54_2c080ea26ebb Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin29.hostedemail.com (Postfix) with ESMTP id 4DB7318086E25 for ; Wed, 8 Jul 2020 08:00:06 +0000 (UTC) X-HE-Tag: home54_2c080ea26ebb X-Filterd-Recvd-Size: 4647 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf01.hostedemail.com (Postfix) with ESMTP for ; Wed, 8 Jul 2020 08:00:05 +0000 (UTC) Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 1213620672; Wed, 8 Jul 2020 08:00:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594195204; bh=0r+QvI5jh8QG7OsaTwnkEK/DAUV2jZcvDC3ZHZ6VbGY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=JIha1NjJVhWDtXJkjzvcQ1HzPxx4TQJTigYw/zH2OaO0VK62aVoV2Iyx+K0s6xVWo i+uJOexYHRqEeMA1eG5p2bOHVWKMGg0s9bjT9HTTnCbSy7dyQ/tqTrXZl5D55fIqpR c8JxYpPzyPmY4f/7DIBCw5s9SDyb+TaSwnq9Blg0= Date: Wed, 8 Jul 2020 09:00:00 +0100 From: Will Deacon To: Yang Shi Cc: hannes@cmpxchg.org, catalin.marinas@arm.com, will.deacon@arm.com, akpm@linux-foundation.org, xuyu@linux.alibaba.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [RFC PATCH] mm: avoid access flag update TLB flush for retried page fault Message-ID: <20200708075959.GA25498@willie-the-truck> References: <1594148072-91273-1-git-send-email-yang.shi@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1594148072-91273-1-git-send-email-yang.shi@linux.alibaba.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Rspamd-Queue-Id: 4DB7318086E25 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam01 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 Wed, Jul 08, 2020 at 02:54:32AM +0800, Yang Shi wrote: > Recently we found regression when running will_it_scale/page_fault3 test > on ARM64. Over 70% down for the multi processes cases and over 20% down > for the multi threads cases. It turns out the regression is caused by commit > 89b15332af7c0312a41e50846819ca6613b58b4c ("mm: drop mmap_sem before > calling balance_dirty_pages() in write fault"). > > The test mmaps a memory size file then write to the mapping, this would > make all memory dirty and trigger dirty pages throttle, that upstream > commit would release mmap_sem then retry the page fault. The retried > page fault would see correct PTEs installed by the first try then update > access flags and flush TLBs. The regression is caused by the excessive > TLB flush. It is fine on x86 since x86 doesn't need flush TLB for > access flag update. > > The page fault would be retried due to: > 1. Waiting for page readahead > 2. Waiting for page swapped in > 3. Waiting for dirty pages throttling > > The first two cases don't have PTEs set up at all, so the retried page > fault would install the PTEs, so they don't reach there. But the #3 > case usually has PTEs installed, the retried page fault would reach the > access flag update. But it seems not necessary to update access flags > for #3 since retried page fault is not real "second access", so it > sounds safe to skip access flag update for retried page fault. > > With this fix the test result get back to normal. > > Reported-by: Xu Yu > Debugged-by: Xu Yu > Tested-by: Xu Yu > Signed-off-by: Yang Shi > --- > I'm not sure if this is safe for non-x86 machines, we did some tests on arm64, but > there may be still corner cases not covered. > > mm/memory.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 87ec87c..3d4e671 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -4241,8 +4241,13 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf) > if (vmf->flags & FAULT_FLAG_WRITE) { > if (!pte_write(entry)) > return do_wp_page(vmf); > - entry = pte_mkdirty(entry); > } > + > + if ((vmf->flags & FAULT_FLAG_WRITE) && !(vmf->flags & FAULT_FLAG_TRIED)) > + entry = pte_mkdirty(entry); > + else if (vmf->flags & FAULT_FLAG_TRIED) > + goto unlock; > + Can you rewrite this as: if (vmf->flags & FAULT_FLAG_TRIED) goto unlock; if (vmf->flags & FAULT_FLAG_WRITE) entry = pte_mkdirty(entry); ? (I'm half-asleep this morning and there are people screaming and shouting outside my window, so this might be rubbish) If you _can_make that change, then I don't understand why the existing pte_mkdirty() line needs to move at all. Couldn't you just add: if (vmf->flags & FAULT_FLAG_TRIED) goto unlock; after the existing "vmf->flags & FAULT_FLAG_WRITE" block? Will