From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id 0BE0D8E0002 for ; Thu, 3 Jan 2019 09:41:10 -0500 (EST) Received: by mail-ed1-f72.google.com with SMTP id x15so33954241edd.2 for ; Thu, 03 Jan 2019 06:41:10 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id r24si2476912edp.187.2019.01.03.06.41.08 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 03 Jan 2019 06:41:08 -0800 (PST) Date: Thu, 3 Jan 2019 15:41:07 +0100 From: Michal Hocko Subject: Re: [Bug 202089] New: transparent hugepage not compatable with madvise(MADV_DONTNEED) Message-ID: <20190103144107.GQ31793@dhcp22.suse.cz> References: <20181229125316.27f7f1fedacfe4c1a5551a2d@linux-foundation.org> <20181229224843.6vsdj3xomifjocbh@kshutemo-mobl1> <20190103094422.GC31793@dhcp22.suse.cz> <20190103143502.GO6310@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190103143502.GO6310@bombadil.infradead.org> Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Wilcox Cc: "Kirill A. Shutemov" , jianpanlanyue@163.com, Andrew Morton , linux-mm@kvack.org, bugzilla-daemon@bugzilla.kernel.org On Thu 03-01-19 06:35:02, Matthew Wilcox wrote: > On Thu, Jan 03, 2019 at 10:44:22AM +0100, Michal Hocko wrote: > > On Sun 30-12-18 01:48:43, Kirill A. Shutemov wrote: > > > On Sat, Dec 29, 2018 at 12:53:16PM -0800, Andrew Morton wrote: > > > > > 1. use mmap() to allocate 4096 bytes for 1024*512 times (4096*1024*512=2G). > > > > > 2. use madvise(MADV_DONTNEED) to free most of the above pages, but reserve a > > > > > few pages(by if（i%33==0) continue;), then process's physical memory firstly > > > > > come down, but after a few seconds, it rise back to 2G again, and can't come > > > > > down forever. > > > > > 3. if i delete this condition(if（i%33==0) continue;) or disable > > > > > transparent_hugepage by setting 'enable' and 'defrag' to never, all go well and > > > > > the physical memory can come down expectly. > > > > > > > > > > It seems like transparent_hugepage has problems with non-contiguous > > > > > madvise(MADV_DONTEED). > > > > > > It's expected behaviour. > > > > > > MADV_DONTNEED doesn't guarantee that the range will not be repopulated > > > (with or without direct action on application behalf). It's just a hint > > > for the kernel. > > > > I agree with Kirill here but I would be interested in the underlying > > usecase that triggered this. The test case is clearly artificial but is > > any userspace actually relying on MADV_DONTNEED reducing the rss > > longterm? > > > > > For sparse mappings, consider using MADV_NOHUGEPAGE. > > Should the MADV_DONTNEED hint imply MADV_NOHUGEPAGE? It'd prevent > coalescing elsewhere in the VMA, so that might negatively affect other > programs. I really do not think this is a good idea. MADV_DONTEED doesn't really imply anything to future rss. It only wipes out the current content. In other words do we want to stop fault around/readahead or any other optimistic faulting on MADV_DONTEED? -- Michal Hocko SUSE Labs