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

[Zach O'Keefe]

On Thu, Aug 24, 2023 at 12:39 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> 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:" ?

The in-tree code was taken care of during the rework .. but I didn't
know about the possibility of a driver hooking in here.

I don't know what the normal policy / stance here is, but I figured
the change was simple enough that it was worth helping out.

For both VM_MIXEDMAP and !DAX ->huge_fault, there is some argument to
be made that they are unnecessarily restrictive anyways.

> --
> Cheers,
>
> David / dhildenb
>