Re: [RFC PATCH 24/37] mm: implement speculative handling in __do_fault()

[Peter Zijlstra]

On Tue, Apr 06, 2021 at 06:44:49PM -0700, Michel Lespinasse wrote:
> In the speculative case, call the vm_ops->fault() method from within
> an rcu read locked section, and verify the mmap sequence lock at the
> start of the section. A match guarantees that the original vma is still
> valid at that time, and that the associated vma->vm_file stays valid
> while the vm_ops->fault() method is running.
> 
> Note that this implies that speculative faults can not sleep within
> the vm_ops->fault method. We will only attempt to fetch existing pages
> from the page cache during speculative faults; any miss (or prefetch)
> will be handled by falling back to non-speculative fault handling.
> 
> The speculative handling case also does not preallocate page tables,
> as it is always called with a pre-existing page table.

So what's wrong with SRCU ? Laurent mumbled something about frequent
SRCU kthread activity being a problem; is that still so and is that
fundamentally unfixable?

Because to me it seems a much more natural solution to the whole thing.