linux-kernel - Re: [RFC PATCH 24/37] mm: implement speculative handling in __do

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Date:   Tue, 6 Apr 2021 19:53:20 -0700
From:   Michel Lespinasse <michel@...pinasse.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Michel Lespinasse <michel@...pinasse.org>,
        Linux-MM <linux-mm@...ck.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Rik van Riel <riel@...riel.com>,
        Paul McKenney <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Rom Lemarchand <romlem@...gle.com>,
        Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 24/37] mm: implement speculative handling in
 __do_fault()

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.