Re: [PATCH v3] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

[David Hildenbrand <david@redhat.com>]

On 22.08.23 01:48, Zach O'Keefe wrote:
> The 6.0 commits:
> 
> commit 9fec51689ff6 ("mm: thp: kill transparent_hugepage_active()")
> commit 7da4e2cb8b1f ("mm: thp: kill __transhuge_page_enabled()")
> 
> merged "can we have THPs in this VMA?" logic that was previously done
> separately by fault-path, khugepaged, and smaps "THPeligible" checks.
> 
> During the process, the semantics of the fault path check changed in two
> ways:
> 
> 1) A VM_NO_KHUGEPAGED check was introduced (also added to smaps path).
> 2) We no longer checked if non-anonymous memory had a vm_ops->huge_fault
>     handler that could satisfy the fault.  Previously, this check had been
>     done in create_huge_pud() and create_huge_pmd() routines, but after
>     the changes, we never reach those routines.
> 
> During the review of the above commits, it was determined that in-tree
> users weren't affected by the change; most notably, since the only relevant
> user (in terms of THP) of VM_MIXEDMAP or ->huge_fault is DAX, which is
> explicitly approved early in approval logic.  However, there is at least
> one occurrence where an out-of-tree driver that used
> VM_HUGEPAGE|VM_MIXEDMAP with a vm_ops->huge_fault handler, was broken.

... so all we did is break an arbitrary out-of-tree driver? Sorry to 
say, but why should we care?

Is there any in-tree code affected and needs a "Fixes:" ?

-- 
Cheers,

David / dhildenb