On Thu, Feb 06, 2020 at 09:55:29PM +0100, Peter Zijlstra wrote: > On Thu, Feb 06, 2020 at 12:15:36PM -0800, Matthew Wilcox wrote: > > then, at the beginning of a page fault call srcu_read_lock(&vma_srcu); > > walk the tree as we do now, allocate memory for PTEs, sleep waiting for > > pages to arrive back from disc, etc, etc, then at the end of the fault, > > call srcu_read_unlock(&vma_srcu). > > So far so good,... > > > munmap() would consist of removing the > > VMA from the tree, then calling synchronize_srcu() to wait for all faults > > to finish, then putting the backing file, etc, etc and freeing the VMA. > > call_srcu(), and the (s)rcu callback will then fput() and such things > more. > > synchronize_srcu() (like synchronize_rcu()) is stupid slow and would > make munmap()/exit()/etc.. unusable. I'll need to think about that a bit. I was convinced we needed to wait for the current pagefaults to finish before we could return from munmap(). I need to convince myself that it's OK to return to userspace while the page faults for that range are still proceeding on other CPUs. > > This seems pretty reasonable, and investigation could actually proceed > > before the Maple tree work lands. Today, that would be: > > > > srcu_read_lock(&vmas_srcu); > > down_read(&mm->mmap_sem); > > find_vma(mm, address); > > up_read(&mm->mmap_sem); > > ... rest of fault handler path ... > > srcu_read_unlock(&vmas_srcu); > > > > Kind of a pain because we still call find_vma() in the per-arch page > > fault handler, but for prototyping, we'd only have to do one or two > > architectures. > > If you look at the earlier speculative page-fault patches by Laurent, > which were based on my still earlier patches, you'll find most of this > there. > > The tricky bit was validating everything on the second page-table walk, > so see if nothing had fundamentally changed, specifically the VMA, > before installing the PTE. If you do this without mmap_sem, you need to > hold ptlock to pin stuff while validating everything you did earlier. The patches Laurent posted used regular RCU and a per-VMA refcount, not SRCU. If you use SRCU, why would you need a second page table walk?
