From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx116.postini.com [74.125.245.116]) by kanga.kvack.org (Postfix) with SMTP id 34C516B13F0 for ; Thu, 2 Feb 2012 07:47:45 -0500 (EST) Date: Thu, 2 Feb 2012 12:47:29 +0000 From: Mel Gorman Subject: Re: [PATCH 02/15] mm: page_alloc: update migrate type of pages on pcp when isolating Message-ID: <20120202124729.GA5796@csn.ul.ie> References: <1327568457-27734-1-git-send-email-m.szyprowski@samsung.com> <1327568457-27734-3-git-send-email-m.szyprowski@samsung.com> <20120130111522.GE25268@csn.ul.ie> <20120130161447.GU25268@csn.ul.ie> <022e01cce034$bc6cf440$3546dcc0$%szyprowski@samsung.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <022e01cce034$bc6cf440$3546dcc0$%szyprowski@samsung.com> Sender: owner-linux-mm@kvack.org List-ID: To: Marek Szyprowski Cc: 'Michal Nazarewicz' , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-media@vger.kernel.org, linux-mm@kvack.org, linaro-mm-sig@lists.linaro.org, 'Kyungmin Park' , 'Russell King' , 'Andrew Morton' , 'KAMEZAWA Hiroyuki' , 'Daniel Walker' , 'Arnd Bergmann' , 'Jesse Barker' , 'Jonathan Corbet' , 'Shariq Hasnain' , 'Chunsang Jeong' , 'Dave Hansen' , 'Benjamin Gaignard' On Tue, Jan 31, 2012 at 05:23:59PM +0100, Marek Szyprowski wrote: > > > >>+ page = pfn_to_page(pfn); > > > >>+ if (PageBuddy(page)) { > > > >>+ pfn += 1 << page_order(page); > > > >>+ } else if (page_count(page) == 0) { > > > >>+ set_page_private(page, MIGRATE_ISOLATE); > > > >>+ ++pfn; > > > > > > > >This is dangerous for two reasons. If the page_count is 0, it could > > > >be because the page is in the process of being freed and is not > > > >necessarily on the per-cpu lists yet and you cannot be sure if the > > > >contents of page->private are important. Second, there is nothing to > > > >prevent another CPU allocating this page from its per-cpu list while > > > >the private field is getting updated from here which might lead to > > > >some interesting races. > > > > > > > >I recognise that what you are trying to do is respond to Gilad's > > > >request that you really check if an IPI here is necessary. I think what > > > >you need to do is check if a page with a count of 0 is encountered > > > >and if it is, then a draining of the per-cpu lists is necessary. To > > > >address Gilad's concerns, be sure to only this this once per attempt at > > > >CMA rather than for every page encountered with a count of 0 to avoid a > > > >storm of IPIs. > > > > > > It's actually more then that. > > > > > > This is the same issue that I first fixed with a change to free_pcppages_bulk() > > > function[1]. At the time of positing, you said you'd like me to try and find > > > a different solution which would not involve paying the price of calling > > > get_pageblock_migratetype(). Later I also realised that this solution is > > > not enough. > > > > > > [1] http://article.gmane.org/gmane.linux.kernel.mm/70314 > > > > > > > Yes. I had forgotten the history but looking at that patch again, > > I would reach the conclusion that this was adding a new call to > > get_pageblock_migratetype() in the bulk free path. That would affect > > everybody whether they were using CMA or not. > > This will be a bit ugly, but we can also use that code and compile it conditionally > when CMA has been enabled. That would also be very unfortunate because it means enabling CMA incurs a performance cost to everyone whether they use CMA or not. For ARM, this may not be a problem but it would be for other arches if they wanted to use CMA or if it ever became part of a distro contig. > Pages, which have incorrect migrate type on free finally > causes pageblock migration type change from MIGRATE_CMA to MIGRATE_MOVABLE. I'm not quite seeing this. In free_hot_cold_page(), the pageblock type is checked so the page private should be set to MIGRATE_CMA or MIGRATE_ISOLATE for the CMA area. It's not clear how this can change a pageblock to MIGRATE_MOVABLE in error. If it turns out that you absolutely have to call get_pageblock_migratetype() from free_pcppages_bulk() and my alternative suggestion did not work out then document all these issues in a comment when putting the call under CONFIG_CMA so that it is not forgotten. -- Mel Gorman 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org