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Wed, 02 Dec 2020 15:41:19 -0800 (PST) Date: Wed, 2 Dec 2020 18:41:17 -0500 From: Peter Xu To: Hugh Dickins Cc: Andrea Arcangeli , Matthew Wilcox , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Mike Rapoport , David Hildenbrand Subject: Re: [PATCH v2] mm: Don't fault around userfaultfd-registered regions on reads Message-ID: <20201202234117.GD108496@xz-x1> References: <20201130230603.46187-1-peterx@redhat.com> <20201201125927.GB11935@casper.infradead.org> <20201201223033.GG3277@xz-x1> MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline 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 Wed, Dec 02, 2020 at 02:37:33PM -0800, Hugh Dickins wrote: > On Tue, 1 Dec 2020, Andrea Arcangeli wrote: > > > > Any suggestions on how to have the per-vaddr per-mm _PAGE_UFFD_WP bit > > survive the pte invalidates in a way that remains associated to a > > certain vaddr in a single mm (so it can shoot itself in the foot if it > > wants, but it can't interfere with all other mm sharing the shmem > > file) would be welcome... > > I think it has to be a new variety of swap-like non_swap_entry() pte, > see include/linux/swapops.h. Anything else would be more troublesome. > > Search for non_swap_entry and for migration_entry, to find places that > might need to learn about this new variety. > > IIUC you only need a single value, no need to carve out another whole > swp_type: could probably be swp_offset 0 of any swp_type other than 0. > > Note that fork's copy_page_range() does not "copy ptes where a page > fault will fill them correctly", so would in effect put a pte_none > into the child where the parent has this uffd_wp entry. I don't know > anything about uffd versus fork, whether that would pose a problem. Thanks for the idea, Hugh! I thought about something similar today, but instead of swap entries, I was thinking about constantly filling in a pte with a value of "_PAGE_PROTNONE | _PAGE_UFFD_WP" when e.g. we'd like to zap a page with shmem+uffd-wp. I feel like the fundamental idea is similar - we can somehow keep the pte with uffd-wp information even if zapped/swapped-out, so as long as the shmem access will fruther trap into the fault handler, then we can operate on that pte and read that information out, like recover that pte into a normal pte (with swap/page cache, and vma/addr information, we'll be able to) and then we can retry the fault. Thanks, -- Peter Xu