linux-kernel - Re: [PATCH v10 03/13] khugepaged: generalize hugepage_vma

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Message-ID: <e17a64a9-eebb-4d13-9c39-bfd04a874966@lucifer.local>
Date: Fri, 22 Aug 2025 11:25:50 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Zi Yan <ziy@...dia.com>
Cc: Wei Yang <richard.weiyang@...il.com>, Nico Pache <npache@...hat.com>,
        linux-mm@...ck.org, linux-doc@...r.kernel.org,
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Subject: Re: [PATCH v10 03/13] khugepaged: generalize hugepage_vma_revalidate
 for mTHP support

On Thu, Aug 21, 2025 at 10:09:10AM -0400, Zi Yan wrote:
> On 20 Aug 2025, at 23:41, Wei Yang wrote:
>
> > On Wed, Aug 20, 2025 at 09:40:40AM -0600, Nico Pache wrote:
> > [...]
> >>>
> >>>>       if (!thp_vma_suitable_order(vma, address, PMD_ORDER))
> >>>>               return SCAN_ADDRESS_RANGE;
> >>>> -     if (!thp_vma_allowable_order(vma, vma->vm_flags, type, PMD_ORDER))
> >>>> +     if (!thp_vma_allowable_orders(vma, vma->vm_flags, type, orders))
> >>>>               return SCAN_VMA_CHECK;
> >>>>       /*
> >>>>        * Anon VMA expected, the address may be unmapped then
> >>>> @@ -1134,7 +1135,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >>>>               goto out_nolock;
> >>>>
> >>>>       mmap_read_lock(mm);
> >>>> -     result = hugepage_vma_revalidate(mm, address, true, &vma, cc);
> >>>> +     result = hugepage_vma_revalidate(mm, address, true, &vma, cc,
> >>>> +                                      BIT(HPAGE_PMD_ORDER));
> >>>
> >>> Shouldn't this be PMD order? Seems equivalent.
> >> Yeah i'm actually not sure why we have both... they seem to be the
> >> same thing, but perhaps there is some reason for having two...
> >
> > I am confused with these two, PMD_ORDER above and HPAGE_PMD_ORDER from here.
> >
> > Do we have a guide on when to use which?
>
> Looking at the definition of HPAGE_PMD_SHIFT in huge_mm.h, it will cause a
> build bug when PMD level huge page is not supported. So I think
> HPAGE_PMD_ORDER should be used for all huge pages (both THP and hugetlb).

Thanks yeah that makes sense, so let's keep it all as HPAGE_PMD_ORDER then!

Cheers, Lorenzo