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

[Michel Lespinasse]

On Wed, Apr 07, 2021 at 03:35:27AM +0100, Matthew Wilcox 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.
> 
> I still don't understand why you want to do this.  The speculative
> fault that doesn't do I/O is already here, and it's called ->map_pages
> (which I see you also do later).  So what's the point of this patch?

I have to admit I did not give much tought about which path would be
generally most common here.

The speculative vm_ops->fault path would be used:
- for private mapping write faults,
- when fault-around is disabled (probably an uncommon case in general,
  but actually common at Google).

That said, I do think your point makes sense in general, espicially if
this could help avoid the per-filesystem enable bit.