On Thu, Jan 07, 2021 at 01:05:19PM -0800, Linus Torvalds wrote: > Side note, and not really related to UFFD, but the mmap_sem in > general: I was at one point actually hoping that we could make the > mmap_sem a spinlock, or at least make the rule be that we never do any > IO under it. At which point a write lock hopefully really shouldn't be > such a huge deal.
There's a (small) group of us working towards that. It has some prerequisites, but where we're hoping to go currently: - Replace the vma rbtree with a b-tree protected with a spinlock - Page faults walk the b-tree under RCU, like peterz/laurent's SPF patchset - If we need to do I/O, take a refcount on the VMA After that, we can gradually move things out from mmap_sem protection to just the vma tree spinlock, or whatever makes sense for them. In a very real way the mmap_sem is the MM layer's BKL.
