[PATCH v2 3/3] mm/vmalloc: fix vmalloc_to_page for huge vmap mappings

[Nicholas Piggin <npiggin@gmail.com>]

vmalloc_to_page returns NULL for addresses mapped by larger pages[*].
Whether or not a vmap is huge depends on the architecture details,
alignments, boot options, etc., which the caller can not be expected
to know. Therefore HUGE_VMAP is a regression for vmalloc_to_page.

This change teaches vmalloc_to_page about larger pages, and returns
the struct page that corresponds to the offset within the large page.
This makes the API agnostic to mapping implementation details.

[*] As explained by commit 029c54b095995 ("mm/vmalloc.c: huge-vmap:
    fail gracefully on unexpected huge vmap mappings")

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 include/asm-generic/4level-fixup.h |  1 +
 include/asm-generic/5level-fixup.h |  1 +
 mm/vmalloc.c                       | 37 +++++++++++++++++++-----------
 3 files changed, 26 insertions(+), 13 deletions(-)

diff --git a/include/asm-generic/4level-fixup.h b/include/asm-generic/4level-fixup.h
index e3667c9a33a5..3cc65a4dd093 100644
--- a/include/asm-generic/4level-fixup.h
+++ b/include/asm-generic/4level-fixup.h
@@ -20,6 +20,7 @@
 #define pud_none(pud)			0
 #define pud_bad(pud)			0
 #define pud_present(pud)		1
+#define pud_large(pud)			0
 #define pud_ERROR(pud)			do { } while (0)
 #define pud_clear(pud)			pgd_clear(pud)
 #define pud_val(pud)			pgd_val(pud)
diff --git a/include/asm-generic/5level-fixup.h b/include/asm-generic/5level-fixup.h
index bb6cb347018c..c4377db09a4f 100644
--- a/include/asm-generic/5level-fixup.h
+++ b/include/asm-generic/5level-fixup.h
@@ -22,6 +22,7 @@
 #define p4d_none(p4d)			0
 #define p4d_bad(p4d)			0
 #define p4d_present(p4d)		1
+#define p4d_large(p4d)			0
 #define p4d_ERROR(p4d)			do { } while (0)
 #define p4d_clear(p4d)			pgd_clear(p4d)
 #define p4d_val(p4d)			pgd_val(p4d)
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 0f76cca32a1c..09a283866368 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -36,6 +36,7 @@
 #include <linux/rbtree_augmented.h>
 
 #include <linux/uaccess.h>
+#include <asm/pgtable.h>
 #include <asm/tlbflush.h>
 #include <asm/shmparam.h>
 
@@ -284,25 +285,35 @@ struct page *vmalloc_to_page(const void *vmalloc_addr)
 
 	if (pgd_none(*pgd))
 		return NULL;
+
 	p4d = p4d_offset(pgd, addr);
 	if (p4d_none(*p4d))
 		return NULL;
-	pud = pud_offset(p4d, addr);
+#ifdef CONFIG_HAVE_ARCH_HUGE_VMAP
+	if (p4d_large(*p4d))
+		return p4d_page(*p4d) + ((addr & ~P4D_MASK) >> PAGE_SHIFT);
+#endif
+	if (WARN_ON_ONCE(p4d_bad(*p4d)))
+		return NULL;
 
-	/*
-	 * Don't dereference bad PUD or PMD (below) entries. This will also
-	 * identify huge mappings, which we may encounter on architectures
-	 * that define CONFIG_HAVE_ARCH_HUGE_VMAP=y. Such regions will be
-	 * identified as vmalloc addresses by is_vmalloc_addr(), but are
-	 * not [unambiguously] associated with a struct page, so there is
-	 * no correct value to return for them.
-	 */
-	WARN_ON_ONCE(pud_bad(*pud));
-	if (pud_none(*pud) || pud_bad(*pud))
+	pud = pud_offset(p4d, addr);
+	if (pud_none(*pud))
+		return NULL;
+#ifdef CONFIG_HAVE_ARCH_HUGE_VMAP
+	if (pud_large(*pud))
+		return pud_page(*pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
+#endif
+	if (WARN_ON_ONCE(pud_bad(*pud)))
 		return NULL;
+
 	pmd = pmd_offset(pud, addr);
-	WARN_ON_ONCE(pmd_bad(*pmd));
-	if (pmd_none(*pmd) || pmd_bad(*pmd))
+	if (pmd_none(*pmd))
+		return NULL;
+#ifdef CONFIG_HAVE_ARCH_HUGE_VMAP
+	if (pmd_large(*pmd))
+		return pmd_page(*pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
+#endif
+	if (WARN_ON_ONCE(pmd_bad(*pmd)))
 		return NULL;
 
 	ptep = pte_offset_map(pmd, addr);
-- 
2.20.1