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

[Michel Lespinasse]

On Wed, Apr 07, 2021 at 04:40:34PM +0200, Peter Zijlstra wrote:
> 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.

The short answer is that I did not try SRCU. My thought process was,
page cache already uses an RCU read lock, I just need to expand its
scope a little.

Using SRCU might allow us to hit disk during speculative faults; OTOH
we may need to switch to a more robust validation mechanism than the
global counter to reap any benefits.