Re: [PATCH] mm: fix finish_fault() handling for large folios

[Brian Geffon <bgeffon@google.com>]

On Wed, Feb 26, 2025 at 9:04 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Feb 26, 2025 at 06:48:15AM -0500, Brian Geffon wrote:
> > When handling faults for anon shmem finish_fault() will attempt to install
> > ptes for the entire folio. Unfortunately if it encounters a single
> > non-pte_none entry in that range it will bail, even if the pte that
> > triggered the fault is still pte_none. When this situation happens the
> > fault will be retried endlessly never making forward progress.
> >
> > This patch fixes this behavior and if it detects that a pte in the range
> > is not pte_none it will fall back to setting just the pte for the
> > address that triggered the fault.
>
> Surely there's a similar problem in do_anonymous_page()?
>
> At any rate, what a horrid function finish_fault() has become.
> Special cases all over the place.  What we should be doing is
> deciding the range of PTEs to insert, bounded by the folio, the VMA
> and any non-none entries.  Maybe I'll get a chance to fix this up.

I agree, I wasn't thrilled that the fix looked like this but I was
trying to keep the change minimal to aid in backporting to stable
kernels where this behavior is broken. With that being said, do you
have a preference on a minimal way we can fix this before
finish_fault() gets a proper cleanup?