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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 538B1C3B187 for ; Tue, 11 Feb 2020 17:28:09 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id ED2C1206D7 for ; Tue, 11 Feb 2020 17:28:08 +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="IeCjs878" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org ED2C1206D7 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 4CD356B0300; Tue, 11 Feb 2020 12:28:08 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 47EFD6B0301; Tue, 11 Feb 2020 12:28:08 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 393C16B0302; Tue, 11 Feb 2020 12:28:08 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0036.hostedemail.com [216.40.44.36]) by kanga.kvack.org (Postfix) with ESMTP id 1E8BB6B0300 for ; Tue, 11 Feb 2020 12:28:08 -0500 (EST) Received: from smtpin02.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 95B4D181AEF0B for ; Tue, 11 Feb 2020 17:28:07 +0000 (UTC) X-FDA: 76478529414.02.shirt45_cae343dee625 X-HE-Tag: shirt45_cae343dee625 X-Filterd-Recvd-Size: 5602 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) by imf25.hostedemail.com (Postfix) with ESMTP for ; Tue, 11 Feb 2020 17:28:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.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=y/1okny0VePHyoAJzMRYVsHHPu38TK+N4jnvS6Wtx0M=; b=IeCjs878c3YJEgecyvTjbm4GMS F6W3BR+opYpShaCzFOABn4eP/1L1N2Q34a5pcCgKU92dVu5WfzhQYFJh+1PVdb9F3OKZog707eVE4 S22Tbk0Q4TcW/ajRWm2xOeYwINPE2IIbsWG5aNOt5rQq48p/MqL5rEpEDXOAjYgvCT03SioVVq4gb FHu7L36oWnolgIqnWtSoOgsAC1Hxx7aDdZvWzO90kWSig7W9au+8X4FY8J4quli/8OVHSkZrw0IfL PxWrBg2Abz3wW0CbqqQ0eT88I527MesK+oEZAz07xMNA2NyijoTn0D7Xata5N65mBjhFGyKW1UyuX 88yhC0Kg==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1j1ZKN-0008Ou-Ni; Tue, 11 Feb 2020 17:28:03 +0000 Date: Tue, 11 Feb 2020 09:28:03 -0800 From: Matthew Wilcox To: Minchan Kim Cc: Andrew Morton , linux-mm , Josef Bacik , Johannes Weiner , Jan Kara , LKML Subject: Re: [PATCH] mm: fix long time stall from mm_populate Message-ID: <20200211172803.GA7778@bombadil.infradead.org> References: <20200211001958.170261-1-minchan@kernel.org> <20200211011021.GP8731@bombadil.infradead.org> <20200211035004.GA242563@google.com> <20200211035412.GR8731@bombadil.infradead.org> <20200211042536.GB242563@google.com> <20200211122323.GS8731@bombadil.infradead.org> <20200211163404.GC242563@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200211163404.GC242563@google.com> 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 Tue, Feb 11, 2020 at 08:34:04AM -0800, Minchan Kim wrote: > On Tue, Feb 11, 2020 at 04:23:23AM -0800, Matthew Wilcox wrote: > > On Mon, Feb 10, 2020 at 08:25:36PM -0800, Minchan Kim wrote: > > > On Mon, Feb 10, 2020 at 07:54:12PM -0800, Matthew Wilcox wrote: > > > > On Mon, Feb 10, 2020 at 07:50:04PM -0800, Minchan Kim wrote: > > > > > On Mon, Feb 10, 2020 at 05:10:21PM -0800, Matthew Wilcox wrote: > > > > > > On Mon, Feb 10, 2020 at 04:19:58PM -0800, Minchan Kim wrote: > > > > > > > filemap_fault > > > > > > > find a page form page(PG_uptodate|PG_readahead|PG_writeback) > > > > > > > > > > > > Uh ... That shouldn't be possible. > > > > > > > > > > Please see shrink_page_list. Vmscan uses PG_reclaim to accelerate > > > > > page reclaim when the writeback is done so the page will have both > > > > > flags at the same time and the PG reclaim could be regarded as > > > > > PG_readahead in fault conext. > > > > > > > > What part of fault context can make that mistake? The snippet I quoted > > > > below is from page_cache_async_readahead() where it will clearly not > > > > make that mistake. There's a lot of code here; please don't presume I > > > > know all the areas you're talking about. > > > > > > Sorry about being not clear. I am saying filemap_fault -> > > > do_async_mmap_readahead > > > > > > Let's assume the page is hit in page cache and vmf->flags is !FAULT_FLAG > > > TRIED so it calls do_async_mmap_readahead. Since the page has PG_reclaim > > > and PG_writeback by shrink_page_list, it goes to > > > > > > do_async_mmap_readahead > > > if (PageReadahead(page)) > > > fpin = maybe_unlock_mmap_for_io(); > > > page_cache_async_readahead > > > if (PageWriteback(page)) > > > return; > > > ClearPageReadahead(page); <- doesn't reach here until the writeback is clear > > > > > > So, mm_populate will repeat the loop until the writeback is done. > > > It's my just theory but didn't comfirm it by the testing. > > > If I miss something clear, let me know it. > > > > Ah! Surely the right way to fix this is ... > > I'm not sure it's right fix. Actually, I wanted to remove PageWriteback check > in page_cache_async_readahead because I don't see corelation. Why couldn't we > do readahead if the marker page is PG_readahead|PG_writeback design PoV? > Only reason I can think of is it makes *a page* will be delayed for freeing > since we removed PG_reclaim bit, which would be over-optimization for me. You're confused. Because we have a shortage of bits in the page flags, we use the same bit for both PageReadahead and PageReclaim. That doesn't mean that a page marked as PageReclaim should be treated as PageReadahead. > Other concern is isn't it's racy? IOW, page was !PG_writeback at the check below > in your snippet but it was under PG_writeback in page_cache_async_readahead and > then the IO was done before refault reaching the code again. It could be repeated > *theoretically* even though it's very hard to happen in real practice. > Thus, I think it would be better to remove PageWriteback check from > page_cache_async_readahead if we really want to go the approach. PageReclaim is always cleared before PageWriteback. eg here: void end_page_writeback(struct page *page) ... if (PageReclaim(page)) { ClearPageReclaim(page); rotate_reclaimable_page(page); } if (!test_clear_page_writeback(page)) BUG(); so if PageWriteback is clear, PageReclaim must already be observable as clear.