On Tue, May 02, 2023 at 09:33:45PM +0200, David Hildenbrand wrote: > On 02.05.23 21:25, Lorenzo Stoakes wrote: > > On Tue, May 02, 2023 at 04:07:50PM -0300, Jason Gunthorpe wrote: > > > On Tue, May 02, 2023 at 07:17:14PM +0100, Lorenzo Stoakes wrote: > > > > > > > On a specific point - if mapping turns out to be NULL after we confirm > > > > stable PTE, I'd be inclined to reject and let the slow path take care of > > > > it, would you agree that that's the correct approach? > > > > > > I think in general if GUP fast detects any kind of race it should bail > > > to the slow path. > > > > > > The races it tries to resolve itself should have really safe and > > > obvious solutions. > > > > > > I think this comment is misleading: > > > > > > > + /* > > > > + * GUP-fast disables IRQs - this prevents IPIs from causing page tables > > > > + * to disappear from under us, as well as preventing RCU grace periods > > > > + * from making progress (i.e. implying rcu_read_lock()). > > > > > > True, but that is not important here since we are not reading page > > > tables > > > > > > > + * This means we can rely on the folio remaining stable for all > > > > + * architectures, both those that set CONFIG_MMU_GATHER_RCU_TABLE_FREE > > > > + * and those that do not. > > > > > > Not really clear. We have a valid folio refcount here, that is all. > > > > Some of this is a product of mixed signals from different commenters and > > my being perhaps a little _too_ willing to just go with the flow. > > > > With interrupts disabled and IPI blocked, plus the assurances that > > interrupts being disabled implied the RCU version of page table > > manipulation is also blocked, my understanding was that remapping in this > > process to another page could not occur. > > > > Of course the folio is 'stable' in the sense we have a refcount on it, but > > it is unlocked so things can change. > > > > I'm guessing the RCU guarantees in the TLB logic are not as solid as IPI, > > because in the IPI case it seems to me you couldn't even clear the PTE > > entry before getting to the page table case. > > > > Otherwise, I'm a bit uncertain actually as to how we can get to the point > > where the folio->mapping is being manipulated. Is this why? > > I'll just stress again that I think there are cases where we unmap and free > a page without synchronizing against GUP-fast using an IPI or RCU. OK that explains why we need to be careful. Don't worry I am going to move the check after we confirm PTE entry hasn't changed. > > That's one of the reasons why we recheck if the PTE changed to back off, so > I've been told. > > I'm happy if someone proves me wrong and a page we just (temporarily) pinned > cannot have been freed+reused in the meantime. Let's play it safe for now :) > > -- > Thanks, > > David / dhildenb >
