Re: [PATCH v1 2/2] mm/huge_memory: don't mark refcounted pages special in vmf_insert_folio_pud()

[David Hildenbrand]

On 03.06.25 23:16, David Hildenbrand wrote:
> Marking PUDs that map a "normal" refcounted folios as special is
> against our rules documented for vm_normal_page().
>
> Fortunately, there are not that many pud_special() check that can be
> mislead and are right now rather harmless: e.g., none so far
> bases decisions whether to grab a folio reference on that decision.
>
> Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big
> implications as it seems.
>
> Getting this right will get more important as we introduce
> folio_normal_page_pud() and start using it in more place where we
> currently special-case based on other VMA flags.
>
> Fix it by just inlining the relevant code, making the whole
> pud_none() handling cleaner.
>
> Add folio_mk_pud() to mimic what we do with folio_mk_pmd().
>
> While at it, make sure that the pud that is non-none is actually present
> before comparing PFNs.
>
> Fixes: dbe54153296d ("mm/huge_memory: add vmf_insert_folio_pud()")
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>   include/linux/mm.h | 15 +++++++++++++++
>   mm/huge_memory.c   | 33 +++++++++++++++++++++++----------
>   2 files changed, 38 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 0ef2ba0c667af..047c8261d4002 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1816,6 +1816,21 @@ static inline pmd_t folio_mk_pmd(struct folio *folio, pgprot_t pgprot)
>   {
>   	return pmd_mkhuge(pfn_pmd(folio_pfn(folio), pgprot));
>   }
> +
> +/**
> + * folio_mk_pud - Create a PUD for this folio
> + * @folio: The folio to create a PUD for
> + * @pgprot: The page protection bits to use
> + *
> + * Create a page table entry for the first page of this folio.
> + * This is suitable for passing to set_pud_at().
> + *
> + * Return: A page table entry suitable for mapping this folio.
> + */
> +static inline pud_t folio_mk_pud(struct folio *folio, pgprot_t pgprot)
> +{
> +	return pud_mkhuge(pfn_pud(folio_pfn(folio), pgprot));
> +}
>   #endif
>   #endif /* CONFIG_MMU */

The following on top should make cross-compiles happy (git diff output because it's late
here, probably whitespace messed up):


diff --git a/include/linux/mm.h b/include/linux/mm.h
index 047c8261d4002..b7e2abd8ce0df 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1817,6 +1817,7 @@ static inline pmd_t folio_mk_pmd(struct folio *folio, pgprot_t pgprot)
        return pmd_mkhuge(pfn_pmd(folio_pfn(folio), pgprot));
 }
 
+#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 /**
  * folio_mk_pud - Create a PUD for this folio
  * @folio: The folio to create a PUD for
@@ -1831,7 +1832,8 @@ static inline pud_t folio_mk_pud(struct folio *folio, pgprot_t pgprot)
 {
        return pud_mkhuge(pfn_pud(folio_pfn(folio), pgprot));
 }
-#endif
+#endif /* CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD */
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 #endif /* CONFIG_MMU */
 
 static inline bool folio_has_pincount(const struct folio *folio

--
Cheers,

David / dhildenb