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Mon, 27 Jan 2020 19:06:55 +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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=joA7qCcJZmOd/St3AaxfEoZkhZv+CN/oIGc9Bx8tpPw=; b=u4xhGbg+2z92uX2qkwm1j9mOW 1w8+sa7/W77bfyn7zWcPpiZuhizAq5HO6SrT7C5x7riSggoMndCIGzh4RfUESk3yr/q6Wp6iQrzHa fpdc2foDbwWQm4U5naCZ+uCfhMNFY5dZgjn8OlPMsTD/M+XX4e6AdmStBfBgwKg92TXxbahjiR/VI K38Dj8p80QtyjGJoC/yNwqgalSp62Dnk7lf70Os0rZKsLgmrilHx+Vc/2AgDIEoITBdEms1mOuYUP td6jtYZDjiFQ/+WP5dBGnbA1QZ96fsas1Xg6bBO91jJFvfKI5fbAz5/IoBFVrsU5NPDNkJgbVY4Q+ 8U8PRWHkA==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1iw9in-0002X5-DR; Mon, 27 Jan 2020 19:06:53 +0000 Date: Mon, 27 Jan 2020 11:06:53 -0800 From: Matthew Wilcox To: Michal Hocko Cc: Cong Wang , LKML , Andrew Morton , linux-mm , Mel Gorman , Vlastimil Babka Subject: Re: [PATCH] mm: avoid blocking lock_page() in kcompactd Message-ID: <20200127190653.GA8708@bombadil.infradead.org> References: <20200109225646.22983-1-xiyou.wangcong@gmail.com> <20200110073822.GC29802@dhcp22.suse.cz> <20200121090048.GG29276@dhcp22.suse.cz> <20200126233935.GA11536@bombadil.infradead.org> <20200127150024.GN1183@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200127150024.GN1183@dhcp22.suse.cz> 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 Mon, Jan 27, 2020 at 04:00:24PM +0100, Michal Hocko wrote: > On Sun 26-01-20 15:39:35, Matthew Wilcox wrote: > > On Sun, Jan 26, 2020 at 11:53:55AM -0800, Cong Wang wrote: > > > I suspect the process gets stuck in the retry loop in try_charge(), as > > > the _shortest_ stacktrace of the perf samples indicated: > > > > > > cycles:ppp: > > > ffffffffa72963db mem_cgroup_iter > > > ffffffffa72980ca mem_cgroup_oom_unlock > > > ffffffffa7298c15 try_charge > > > ffffffffa729a886 mem_cgroup_try_charge > > > ffffffffa720ec03 __add_to_page_cache_locked > > > ffffffffa720ee3a add_to_page_cache_lru > > > ffffffffa7312ddb iomap_readpages_actor > > > ffffffffa73133f7 iomap_apply > > > ffffffffa73135da iomap_readpages > > > ffffffffa722062e read_pages > > > ffffffffa7220b3f __do_page_cache_readahead > > > ffffffffa7210554 filemap_fault > > > ffffffffc039e41f __xfs_filemap_fault > > > ffffffffa724f5e7 __do_fault > > > ffffffffa724c5f2 __handle_mm_fault > > > ffffffffa724cbc6 handle_mm_fault > > > ffffffffa70a313e __do_page_fault > > > ffffffffa7a00dfe page_fault > > > > > > But I don't see how it could be, the only possible case is when > > > mem_cgroup_oom() returns OOM_SUCCESS. However I can't > > > find any clue in dmesg pointing to OOM. These processes in the > > > same memcg are either running or sleeping (that is not exiting or > > > coredump'ing), I don't see how and why they could be selected as > > > a victim of OOM killer. I don't see any signal pending either from > > > their /proc/X/status. > > > > I think this is a situation where we might end up with a genuine deadlock > > if we're not trylocking the pages. readahead allocates a batch of > > locked pages and adds them to the pagecache. If it has allocated, > > say, 5 pages, successfully inserted the first three into i_pages, then > > needs to allocate memory to insert the fourth one into i_pages, and > > the process then attempts to migrate the pages which are still locked, > > they will never come unlocked because they haven't yet been submitted > > to the filesystem for reading. > > Just to make sure I understand. Do you mean this? > lock_page(A) > alloc_pages > try_to_compact_pages > compact_zone_order > compact_zone(MIGRATE_SYNC_LIGHT) > migrate_pages > unmap_and_move > __unmap_and_move > lock_page(A) Yes. There's a little more to it than that, eg slab is involved, but you have it in a nutshell.