Commit 92c99fc6 authored Jul 14, 2025 by David Hildenbrand Committed by Andrew Morton Jul 24, 2025

mm/memory: introduce is_huge_zero_pfn() and use it in vm_normal_page_pmd()

Patch series "mm: introduce snapshot_page()", v3.

This series introduces snapshot_page(), a helper function that can be used
to create a snapshot of a struct page and its associated struct folio.

This function is intended to help callers with a consistent view of a a
folio while reducing the chance of encountering partially updated or
inconsistent state, such as during folio splitting which could lead to
crashes and BUG_ON()s being triggered.


This patch (of 4):

Let's avoid working with the PMD when not required.  If
vm_normal_page_pmd() would be called on something that is not a present
pmd, it would already be a bug (pfn possibly garbage).

While at it, let's support passing in any pfn covered by the huge zero
folio by masking off PFN bits -- which should be rather cheap.

Link: https://lkml.kernel.org/r/cover.1752499009.git.luizcap@redhat.com
Link: https://lkml.kernel.org/r/4940826e99f0c709a7cf7beb94f53288320aea5a.1752499009.git.luizcap@redhat.com


Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
Reviewed-by: Shivank Garg <shivankg@amd.com>
Tested-by: Harry Yoo <harry.yoo@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent 8678d1fa

include/linux/huge_mm.h

+11 −1

Original line number	Diff line number	Diff line
		@@ -482,9 +482,14 @@ static inline bool is_huge_zero_folio(const struct folio *folio)
		return READ_ONCE(huge_zero_folio) == folio;
		}

		static inline bool is_huge_zero_pfn(unsigned long pfn)
		{
		return READ_ONCE(huge_zero_pfn) == (pfn & ~(HPAGE_PMD_NR - 1));
		}

		static inline bool is_huge_zero_pmd(pmd_t pmd)
		{
		return pmd_present(pmd) && READ_ONCE(huge_zero_pfn) == pmd_pfn(pmd);
		return pmd_present(pmd) && is_huge_zero_pfn(pmd_pfn(pmd));
		}

		struct folio mm_get_huge_zero_folio(struct mm_struct mm);
		@@ -632,6 +637,11 @@ static inline bool is_huge_zero_folio(const struct folio *folio)
		return false;
		}

		static inline bool is_huge_zero_pfn(unsigned long pfn)
		{
		return false;
		}

		static inline bool is_huge_zero_pmd(pmd_t pmd)
		{
		return false;

mm/memory.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -668,7 +668,7 @@ struct page vm_normal_page_pmd(struct vm_area_struct vma, unsigned long addr,
		}
		}

		if (is_huge_zero_pmd(pmd))
		if (is_huge_zero_pfn(pfn))
		return NULL;
		if (unlikely(pfn > highest_memmap_pfn))
		return NULL;