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Shutemov" , Johannes Weiner , "Dr . David Alan Gilbert" , Bobby Powers , Maya Gokhale , Jerome Glisse , Mike Kravetz , Matthew Wilcox , Marty McFadden , Mel Gorman , Hugh Dickins , Brian Geffon , Denis Plotnikov , Pavel Emelyanov Subject: Re: [PATCH RESEND v6 00/16] mm: Page fault enhancements Message-ID: <20200309195100.GD4206@xz-x1> References: <20200220155353.8676-1-peterx@redhat.com> <1eb7bdd4-348f-da87-47a1-0b022b70e918@redhat.com> <20200307214743.GA4206@xz-x1> <6d8ed084-0740-cee1-663e-a78a2faee432@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <6d8ed084-0740-cee1-663e-a78a2faee432@redhat.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Mar 08, 2020 at 01:12:34PM +0100, David Hildenbrand wrote: > [...] > > > Yes, IIUC the race can happen like this in your below test: > > > > main thread uffd thread disgard thread > > =========== =========== ============== > > access page > > uffd page fault > > wait for page > > UFFDIO_ZEROCOPY > > put a page P there > > MADV_DONTNEED on P > > wakeup main thread > > return from fault > > page still missing > > uffd page fault again > > (without ALLOW_RETRY) > > --> SIGBUS. > > Exactly! > > >> Can we please have a way to identify that this "feature" is available? > >> I'd appreciate a new read-only UFFD_FEAT_ , so we can detect this from > >> user space easily and use concurrent discards without crashing our applications. > > > > I'm not sure how others think about it, but to me this still fells > > into the bucket of "solving an existing problem" rather than a > > feature. Also note that this should change the behavior for the page > > fault logic in general, rather than an uffd-only change. So I'm also > > not sure whether UFFD_FEAT_* suites here even if we want it. > > So, are we planning on backporting this to stable kernels? I don't have a plan so far. I'm still at the phase to only worry about whether it can be at least merged in master.. :) I would think it won't worth it to backport this to stables though, considering that it could potentially change quite a bit for faulting procedures, and after all the issues we're fixing shouldn't be common to general users. > > Imagine using this in QEMU/KVM to allow discards (e.g., balloon > inflation) while postcopy is active . You certainly don't want random > guest crashes. So either, we treat this as a fix (and backport) or as a > change in behavior/feature. I think we don't need to worry on that - QEMU will prohibit ballooning during postcopy starting from the first day. Feel free to see QEMU commit 371ff5a3f04cd7 ("Inhibit ballooning during postcopy"). > > [...] > > >> > >> 2. What will happen if I don't place a page on a pagefault, but only do a UFFDIO_WAKE? > >> For now we were able to trigger a signal this way. > > > > If I'm not mistaken the UFFDIO_WAKE will directly trigger the sigbus > > even without the help of the MADV_DONTNEED race. > > Yes, that's the current way of injecting a SIGBUS instead of resolving > the pagefault. And AFAIKs, you're changing that behavior. (I am not > aware of a user, there could be use cases, but somehow it's strange to > get a signal when accessing memory that is mapped READ|WRITE and also > represented like this in e.g., /proc/$PID/maps). So IMHO, only the new > behavior makes really sense. I agree, I'm not sure how other people think on ABI stability, but... for my own preference I don't worry much on ABI breakage for a problem like this. Thanks, -- Peter Xu