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")

Reviewed-by: Miaohe Lin <linmia...@huawei.com>
Reviewed-by: Christoph Hellwig <h...@lst.de>
Signed-off-by: Nicholas Piggin <npig...@gmail.com>
---
 mm/vmalloc.c | 41 ++++++++++++++++++++++++++---------------
 1 file changed, 26 insertions(+), 15 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index e6f352bf0498..62372f9e0167 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -34,7 +34,7 @@
 #include <linux/bitops.h>
 #include <linux/rbtree_augmented.h>
 #include <linux/overflow.h>
-
+#include <linux/pgtable.h>
 #include <linux/uaccess.h>
 #include <asm/tlbflush.h>
 #include <asm/shmparam.h>
@@ -343,7 +343,9 @@ int is_vmalloc_or_module_addr(const void *x)
 }
 
 /*
- * Walk a vmap address to the struct page it maps.
+ * Walk a vmap address to the struct page it maps. Huge vmap mappings will
+ * return the tail page that corresponds to the base page address, which
+ * matches small vmap mappings.
  */
 struct page *vmalloc_to_page(const void *vmalloc_addr)
 {
@@ -363,25 +365,33 @@ struct page *vmalloc_to_page(const void *vmalloc_addr)
 
        if (pgd_none(*pgd))
                return NULL;
+       if (WARN_ON_ONCE(pgd_leaf(*pgd)))
+               return NULL; /* XXX: no allowance for huge pgd */
+       if (WARN_ON_ONCE(pgd_bad(*pgd)))
+               return NULL;
+
        p4d = p4d_offset(pgd, addr);
        if (p4d_none(*p4d))
                return NULL;
-       pud = pud_offset(p4d, addr);
+       if (p4d_leaf(*p4d))
+               return p4d_page(*p4d) + ((addr & ~P4D_MASK) >> PAGE_SHIFT);
+       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;
+       if (pud_leaf(*pud))
+               return pud_page(*pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
+       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;
+       if (pmd_leaf(*pmd))
+               return pmd_page(*pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
+       if (WARN_ON_ONCE(pmd_bad(*pmd)))
                return NULL;
 
        ptep = pte_offset_map(pmd, addr);
@@ -389,6 +399,7 @@ struct page *vmalloc_to_page(const void *vmalloc_addr)
        if (pte_present(pte))
                page = pte_page(pte);
        pte_unmap(ptep);
+
        return page;
 }
 EXPORT_SYMBOL(vmalloc_to_page);
-- 
2.23.0

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