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([2001:a61:2482:101:3351:6160:8173:cc31]) by smtp.gmail.com with ESMTPSA id b22sm7451421wmj.1.2020.04.22.05.08.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 22 Apr 2020 05:08:03 -0700 (PDT) Cc: mtk.manpages@gmail.com, linux-man@vger.kernel.org, Sonny Rao , Jesse Barnes , Vlastimil Babka , Minchan Kim , "Kirill A. Shutemov" Subject: Re: [PATCH v2] mremap.2: Add information for MREMAP_DONTUNMAP. To: Lokesh Gidra , Brian Geffon References: <20200415164949.44562-1-bgeffon@google.com> <092fc948-4adb-cda8-f525-7d16a85674e7@gmail.com> From: "Michael Kerrisk (man-pages)" Message-ID: Date: Wed, 22 Apr 2020 14:08:02 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-man-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org Hello Brian and Loresh, >>>> +Possible applications for this behavior might be garbage collection or >>> >>> Can you elaborate the garbage collection use case a little, please? >> >> Lokesh, who is CCed, can probably expand better than I can, Lokesh >> would you mind elaborating on how the JVM plans to use this. >> > There are many GC algorithms in literature which use PROT_NONE+SIGSEGV > trick to implement concurrent compaction of java heap. In Android > Runtime we plan to use userfaultfd instead. But this requires a > stop-the-world, wherein Java threads are paused, right before starting > the compaction phase. Within this pause, the physical pages in the > Java heap will be moved to another area, so that the Java heap, which > is already registered with userfaultfd, can start 'userfaulting' (as > Java heap pages are missing) once application threads are resumed. > > In the absence of MREMAP_DONTUNMAP, I'd have to do it by first doing > mremap, and then mmaping Java heap, as its virtual mapping would be > removed by the preceding mremap. This not only causes performance > issues as two system calls need to be made instead of one, but it also > leaves a window open for a native thread, which is not paused, to > create a virtual mapping for its own usage right where Java heap is > supposed to be. Thank you both for your explanations. I added some text to the page. Does the following look okay? MREMAP_DONTUNMAP use cases Possible applications for MREMAP_DONTUNMAP include: * Non-cooperative userfaultfd(2): an application can yank out a virtual address range using MREMAP_DONTUNMAP and then employ a userfaultfd(2) handler to handle the page faults that subse‐ quently occur as other threads in the process touch pages in the yanked range. * Garbage collection: MREMAP_DONTUNMAP can be used in conjunction with userfaultfd(2) to implement garbage collection algorithms (e.g., in a Java virtual machine). Such an implementation can be cheaper (and simpler) than conventional garbage collection techniques that involve marking pages with protection PROT_NONE in conjunction with the of a SIGSEGV handler to catch accesses to those pages. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/