On Thu, Feb 06, 2020 at 12:15:36PM -0800, Matthew Wilcox wrote: > On Thu, Feb 06, 2020 at 02:59:20PM +0100, Peter Zijlstra wrote: > > > The proposal consists of three phases. In phase 1, we convert the > > > rbtree to the maple tree, and leave the locking alone. In phase 2, > > > we change the locking to a per-VMA refcount, looked up under RCU. > > > > > > This problem arises during phase 3 where we attempt to handle page > > > faults entirely under the RCU read lock. If we encounter problems, > > > we can fall back to acquiring the VMA refcount, but we need the > > > page allocation to fail rather than sleep (or magically drop the > > > RCU lock and return an indication that it has done so, but that > > > doesn't seem to be an approach that would find any favour). > > > > So why not use SRCU? You can do full blocking faults under SRCU and > > don't need no 'stinkin' refcounts ;-) > > I have to say, SRCU is not in my mental toolbox of "how to solve a > problem", so it simply hadn't occurred to me. Thanks. > > So, we'd DEFINE_SRCU(vma_srcu); in mm/memory.c > > 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. > 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.
