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

[Matthew Wilcox]

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.