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Mon, 02 Nov 2020 08:42:32 -0800 (PST) MIME-Version: 1.0 References: <20201030100815.2269-1-daniel.vetter@ffwll.ch> <20201030100815.2269-9-daniel.vetter@ffwll.ch> <20201102072931.GA16419@infradead.org> <20201102130115.GC36674@ziepe.ca> <20201102155256.GG36674@ziepe.ca> In-Reply-To: <20201102155256.GG36674@ziepe.ca> From: Daniel Vetter Date: Mon, 2 Nov 2020 17:42:20 +0100 Message-ID: Subject: Re: [PATCH v5 08/15] mm: Add unsafe_follow_pfn To: Jason Gunthorpe Cc: Christoph Hellwig , DRI Development , LKML , KVM list , Linux MM , Linux ARM , linux-samsung-soc , "open list:DMA BUFFER SHARING FRAMEWORK" , Daniel Vetter , Kees Cook , Dan Williams , Andrew Morton , John Hubbard , "J??r??me Glisse" , Jan Kara Content-Type: text/plain; charset="UTF-8" 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 Mon, Nov 2, 2020 at 4:52 PM Jason Gunthorpe wrote: > > On Mon, Nov 02, 2020 at 02:23:58PM +0100, Daniel Vetter wrote: > > On Mon, Nov 2, 2020 at 2:01 PM Jason Gunthorpe wrote: > > > > > > On Mon, Nov 02, 2020 at 01:56:10PM +0100, Daniel Vetter wrote: > > > > On Mon, Nov 2, 2020 at 8:29 AM Christoph Hellwig wrote: > > > > > > > > > > On Fri, Oct 30, 2020 at 11:08:08AM +0100, Daniel Vetter wrote: > > > > > > Also mark up follow_pfn as EXPORT_SYMBOL_GPL. The only safe way to use > > > > > > that by drivers/modules is together with an mmu_notifier, and that's > > > > > > all _GPL stuff. > > > > > > > > > > I also think it also needs to be renamed to explicitly break any existing > > > > > users out of tree or int the submission queue. > > > > > > > > Ok I looked at the mmu notifier locking again and noticed that > > > > mm->subscriptions has its own spinlock. Since there usually shouldn't > > > > be a huge pile of these I think it's feasible to check for the mmu > > > > notifier in follow_pfn. And that would stuff this gap for good. I'll > > > > throw that on top as a final patch and see what people think. > > > > > > Probably the simplest is to just check mm_has_notifiers() when in > > > lockdep or something very simple like that > > > > lockdep feels wrong, was locking more at CONFIG_DEBUG_VM. And since > > generally you only have 1 mmu notifier (especially for kvm) I think we > > can also pay the 2nd cacheline miss and actually check the right mmu > > notifier is registered. > > Need to hold the lock to check that and there are two ways to register > notifiers these days, so it feels to expensive to me. Uh I mixed stuff up all along, struct mmu_notifier *subcription that all the mmu notifier users use has the ->mm pointer we want right there. That's good enough I think. Now I'm kinda lost in kvm code trying to wire it through, but it's looking ok-ish thus far :-) -Daniel > CH's 'export symbol only for kvm' really does seem the most robust way > to handle this though. > > Jason -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch