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Shutemov" To: Catalin Marinas Cc: Peter Collingbourne , Andrew Morton , Evgenii Stepanov , Linux ARM , linux-mm@kvack.org Subject: Re: [PATCH] mm: introduce reference pages Message-ID: <20200804154804.hm5bglb7nrmzoxky@box> References: <20200731203241.50427-1-pcc@google.com> <20200803093259.ookknl4y7ee5hun7@box> <20200803120134.GD6132@gaia> <20200804152749.GE31076@gaia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200804152749.GE31076@gaia> X-Rspamd-Queue-Id: 8027A18140B7C X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam01 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Aug 04, 2020 at 04:27:50PM +0100, Catalin Marinas wrote: > On Mon, Aug 03, 2020 at 05:50:32PM -0700, Peter Collingbourne wrote: > > On Mon, Aug 3, 2020 at 5:01 AM Catalin Marinas wrote: > > > I think this would work even for the arm64 MTE (though I haven't tried): > > > use memfd_create() to get such file descriptor, mmap() it as MAP_SHARED > > > to populate the initial pattern, mmap() it as MAP_PRIVATE for any > > > subsequent mapping that needs to be copied-on-write. > > > > That would require a separate mmap() (i.e. separate VMA) for each > > page, no? That sounds like it could be expensive both in terms of VMAs > > and the number of mmap syscalls required (i.e. N/PAGE_SIZE). You could > > decrease these costs by increasing the size of the memfd files to more > > than a page, but that would also increase the amount of memory > > required for the reference pages. > > I think I get it now. You'd like a multiple page mmap() to be covered by > a single reference page. The memfd trick wouldn't give you this without > multiple mmap() calls, one for each page. That's why I suggested a special file descriptor that would give the same page on any access. We can piggy back on memfd infrastrucure or create a new interface. -- Kirill A. Shutemov