On Wed, Apr 07, 2021 at 08:36:01AM -0700, Andy Lutomirski wrote: > On 4/6/21 6:44 PM, Michel Lespinasse wrote: > > The page table tree is walked with local irqs disabled, which prevents > > page table reclamation (similarly to what fast GUP does). The logic is > > otherwise similar to the non-speculative path, but with additional > > restrictions: in the speculative path, we do not handle huge pages or > > wiring new pages tables. > > Not on most architectures. Quoting the actual comment in mm/gup.c: > > > * Before activating this code, please be aware that the following assumptions > > * are currently made: > > * > > * *) Either MMU_GATHER_RCU_TABLE_FREE is enabled, and tlb_remove_table() is used to > > * free pages containing page tables or TLB flushing requires IPI broadcast. > > On MMU_GATHER_RCU_TABLE_FREE architectures, you cannot make the > assumption that it is safe to dereference a pointer in a page table just > because irqs are off. You need RCU protection, too. > > You have the same error in the cover letter. Hi Andy, Thanks for your comment. At first I thought did not matter, because we only enable ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT on selected architectures, and I thought MMU_GATHER_RCU_TABLE_FREE is not set on these. But I was wrong - MMU_GATHER_RCU_TABLE_FREE is enabled on X86 with paravirt. So I took another look at fast GUP to make sure I actually understand it. This brings a question about lockless_pages_from_mm() - I see it disabling interrupts, which it explains is necessary for disabling THP splitting IPIs, but I do not see it taking an RCU read lock as would be necessary for preventing paga table freeing on MMU_GATHER_RCU_TABLE_FREE configs. I figure local_irq_save() indirectly takes an rcu read lock somehow ? I think this is something I should also mention in my explanation, and I have not seen a good description of this on the fast GUP side... Thanks, -- Michel "walken" Lespinasse
