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

[David Hildenbrand <david@redhat.com>]

On 26.02.25 15:03, Matthew Wilcox 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()?

I recall we handle it in there correctly the last time I stared at it.

We check pte_none to decide which folio size we can allocate (including 
basing the decision on other factors like VMA etc), and after retaking 
the PTL, we recheck vmf_pte_changed / pte_range_none() to make sure 
there were no races.

-- 
Cheers,

David / dhildenb