Re: [PATCH v8 mm-new 06/12] mm: thp: enable THP allocation exclusively through khugepaged

[Yafang Shao]

On Fri, Sep 26, 2025 at 11:27 PM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
>
>
>
> On 26/09/2025 10:33, Yafang Shao wrote:
> > khugepaged_enter_vma() ultimately invokes any attached BPF function with
> > the TVA_KHUGEPAGED flag set when determining whether or not to enable
> > khugepaged THP for a freshly faulted in VMA.
> >
> > Currently, on fault, we invoke this in do_huge_pmd_anonymous_page(), as
> > invoked by create_huge_pmd() and only when we have already checked to
> > see if an allowable TVA_PAGEFAULT order is specified.
> >
> > Since we might want to disallow THP on fault-in but allow it via
> > khugepaged, we move things around so we always attempt to enter
> > khugepaged upon fault.
> >
> > This change is safe because:
> > - the checks for thp_vma_allowable_order(TVA_KHUGEPAGED) and
> >   thp_vma_allowable_order(TVA_PAGEFAULT) are functionally equivalent
>
> hmm I dont think this is the case. __thp_vma_allowable_orders
> deals with TVA_PAGEFAULT (in_pf) differently from TVA_KHUGEPAGED.

Since this change only applies when vma_is_anonymous(vma) is true, we
can safely focus the logic in __thp_vma_allowable_orders() on
anonymous VMAs. For such VMAs, the TVA_KHUGEPAGED check is strictly
more restrictive than the TVA_PAGEFAULT check. Specifically:

- If __thp_vma_allowable_orders(TVA_PAGEFAULT) returns 0 (disallowed),
then __thp_vma_allowable_orders(TVA_KHUGEPAGED) will also return 0.
- Even if the page fault check returns a set of orders, the khugepaged
check may still return 0.

Thus, this change is safe. I'll clarify this in the commit log. Please
correct me if I'm missing something.

-- 
Regards
Yafang