Re: [PATCH v4] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

[David Hildenbrand]

On 06.10.23 19:58, Andrew Morton wrote:
> On Mon, 25 Sep 2023 13:01:10 -0700 "Zach O'Keefe" <zokeefe@xxxxxxxxxx> 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, this was a bad
> > assumption to make as it assumes the only reason to support ->huge_fault
> > was for DAX (which is not true in general).
> >
> > Remove the VM_NO_KHUGEPAGED check when not in collapse path and give
> > any ->huge_fault handler a chance to handle the fault.  Note that we
> > don't validate the file mode or mapping alignment, which is consistent
> > with the behavior before the aforementioned commits.
> >
> > ...
> >
> > @@ -100,11 +100,11 @@ bool hugepage_vma_check(struct vm_area_struct *vma, unsigned long vm_flags,
> >   		return in_pf;
> >   
>
> Ryan's "mm: thp: introduce anon_orders and anon_always_mask sysfs
> files" changes hugepage_vma_check() to return an unsigned int, so this
> patch will need some rework to fit in after that.
>
> However Ryan's overall series "variable-order, large folios for
> anonymous memory" is in early days and might not make it.
>
> And as I don't know what is the urgency of this patch ("mm/thp: fix
> "mm: thp: kill __transhuge_page_enabled()"), I'm unable to decide which
> patch needs to come first (thus requiring rework of the other patch).
>
> Please discuss!

IMHO clearly this one.

--
Cheers,

David / dhildenb