From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Baron Subject: Re: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is MLOCK_ONFAULT Date: Fri, 15 Jun 2018 15:28:57 -0400 Message-ID: <44e46470-ba0f-8270-ae69-dfb87fbd5a0a@akamai.com> References: <1528484212-7199-1-git-send-email-jbaron@akamai.com> <20180611072005.GC13364@dhcp22.suse.cz> <4c4de46d-c55a-99a8-469f-e1e634fb8525@akamai.com> <20180611150330.GQ13364@dhcp22.suse.cz> <775adf2d-140c-1460-857f-2de7b24bafe7@akamai.com> <20180612074646.GS13364@dhcp22.suse.cz> <5a9398f4-453c-5cb5-6bbc-f20c3affc96a@akamai.com> <20180613091355.GI13364@dhcp22.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180613091355.GI13364@dhcp22.suse.cz> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Michal Hocko Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Vlastimil Babka , Joonsoo Kim , Mel Gorman , "Kirill A. Shutemov" , linux-api@vger.kernel.org, emunson@mgebm.net List-Id: linux-api@vger.kernel.org On 06/13/2018 05:13 AM, Michal Hocko wrote: > On Tue 12-06-18 10:11:33, Jason Baron wrote: > [...] >> Ok, I share the concern that there is a chance that userspace is relying >> on MADV_DONTNEED not free'ing locked memory. In that case, what if we >> introduce a MADV_DONTNEED_FORCE, which does everything that >> MADV_DONTNEED currently does but in addition will also free mlock areas. > > What about other types of vmas that are not allowed to MADV_DONTNEED? > _FORCE suggests that the user of the API know what he is doing so why > shouldn't we allow unmapping hugetlb pages or special PFNMAPS? Or do we > want to add MADV_DONTNEED_FORCE_FOR_REAL when somebody comes with > another usecase? > > I agree with Vlastimil that adding new madvise mode for niche case > sounds like a bad idea so we should better be sure that a new flag has > a reasonable semantic. Just allow mlocked pages is more of a tweak than > a proper semantic. So making it force for real requires to analyze what > that would mean for other vmas which are excluded now. > If its a new flag, I agree it makes sense to look at hugetlb and pfnmaps. pfnmaps might be more work, since it may require callbacks to do driver specific actions... I was able to do something very close to the original requirement of free'ing mlock'd pages, via using a tmpfs mmap that is mlock'd. And then using fallocate(FALLOC_FL_PUNCH_HOLE) to free the locked pages. I think the tmpfs is sufficient for my needs, I wonder if there is any other interest in this feature? Thanks, -Jason