On Mon, Nov 2, 2020 at 2:01 PM Jason Gunthorpe jgg@ziepe.ca 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 hch@infradead.org 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. -Daniel