From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f41.google.com (mail-wm0-f41.google.com [74.125.82.41]) by kanga.kvack.org (Postfix) with ESMTP id 8FB0E6B0253 for ; Tue, 15 Dec 2015 11:59:46 -0500 (EST) Received: by mail-wm0-f41.google.com with SMTP id l126so2888881wml.1 for ; Tue, 15 Dec 2015 08:59:46 -0800 (PST) Received: from mail-wm0-f54.google.com (mail-wm0-f54.google.com. [74.125.82.54]) by mx.google.com with ESMTPS id iw7si3138483wjb.105.2015.12.15.08.59.45 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 15 Dec 2015 08:59:45 -0800 (PST) Received: by mail-wm0-f54.google.com with SMTP id l126so2888075wml.1 for ; Tue, 15 Dec 2015 08:59:45 -0800 (PST) Date: Tue, 15 Dec 2015 17:59:43 +0100 From: Michal Hocko Subject: Re: isolate_lru_page on !head pages Message-ID: <20151215165943.GB27880@dhcp22.suse.cz> References: <20151209130204.GD30907@dhcp22.suse.cz> <20151214120456.GA4201@node.shutemov.name> <20151215085232.GB14350@dhcp22.suse.cz> <20151215120318.GA11497@node.shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151215120318.GA11497@node.shutemov.name> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Andrew Morton , Minchan Kim , linux-mm@kvack.org, LKML On Tue 15-12-15 14:03:18, Kirill A. Shutemov wrote: > On Tue, Dec 15, 2015 at 09:52:33AM +0100, Michal Hocko wrote: > > On Mon 14-12-15 14:04:56, Kirill A. Shutemov wrote: > > > On Wed, Dec 09, 2015 at 02:02:05PM +0100, Michal Hocko wrote: > > > > Hi Kirill, > > > > > > [ sorry for late reply, just back from vacation. ] > > > > > > > while looking at the issue reported by Minchan [1] I have noticed that > > > > there is nothing to prevent from "isolating" a tail page from LRU because > > > > isolate_lru_page checks PageLRU which is > > > > PAGEFLAG(LRU, lru, PF_HEAD) > > > > so it is checked on the head page rather than the given page directly > > > > but the rest of the operation is done on the given (tail) page. > > > > > > Looks like most (all?) callers already exclude PTE-mapped THP already one > > > way or another. > > > > I can see e.g. do_move_page_to_node_array not doing a similar thing. It > > isolates and then migrates potentially a tail page. > > No, it doesn't. follow_page(FOLL_SPLIT) would split THP pages. Ahh, I thought it would split the pmd but this path splits the page directly. > > I haven't looked closer whether there is other hand break on the way > > though. The point I was trying to make is that this is really _subtle_. > > We are changing something else than we operate later on. > > > > > Probably, VM_BUG_ON_PAGE(PageTail(page), page) in isolate_lru_page() would > > > be appropriate. > > > > > > > This is really subtle because this expects that every caller of this > > > > function checks for the tail page otherwise we would clobber statistics > > > > and who knows what else (I haven't checked that in detail) as the page > > > > cannot be on the LRU list and the operation makes sense only on the head > > > > page. > > > > > > > > Would it make more sense to make PageLRU PF_ANY? That would return > > > > false for PageLRU on any tail page and so it would be ignored by > > > > isolate_lru_page. > > > > > > I don't think this is right way to go. What we put on LRU is compound > > > page, not 4k subpages. PageLRU() should return true if the compound page > > > is on LRU regardless if you ask for head or tail page. > > > > Hmm, but then we should operate on the head page because that is what > > PageLRU operated on, no? > > head page is what linked into LRU, but not nessesary the way we obtain the > page to check. If we check PageLRU(pte_page(*pte)) it should produce the > right result. I am not following you here. Any pfn walk could get to a tail page and if we happen to do e.g. isolate_lru_page we have to remember that we should always treat compound page differently. This is subtle. Anyway I am far from understading other parts of the refcount rework so I will spend time studying the code as soon as the time permits. In the meantime I agree that VM_BUG_ON_PAGE(PageTail(page), page) would be useful to catch all the fallouts. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org