[PATCH] mm: fixup validation of buddy pfn

[Xianting Tian <xianting.tian@linux.alibaba.com>]

For RISC-V arch the first 2MB RAM could be reserved for opensbi,
and the arch code may don't create pages for the first 2MB RAM,
so it would have pfn_base=512 and mem_map began with 512th PFN when
CONFIG_FLATMEM=y.

But __find_buddy_pfn algorithm thinks the start PFN 0, it could get
0 PFN or less than the pfn_base value, so page_is_buddy() can't
verify the page whose PFN is 0 ~ 511, actually we don't have valid
pages for PFN 0 ~ 511.

Actually, buddy system should not assume Arch cretaed pages for
reserved memory, Arch may don't know the implied limitation.
With this patch, we can gurantee a valid buddy no matter what we
have pages for reserved memory or not.

Fixes: 8170ac4700d26f65 ("mm: wrap __find_buddy_pfn() with a necessary buddy page validation")
Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
---
 mm/internal.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/internal.h b/mm/internal.h
index c0f8fbe0445b..0ec446caeb2e 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -322,7 +322,8 @@ __find_buddy_pfn(unsigned long page_pfn, unsigned int order)
  * The found buddy can be a non PageBuddy, out of @page's zone, or its order is
  * not the same as @page. The validation is necessary before use it.
  *
- * Return: the found buddy page or NULL if not found.
+ * Return: the found buddy page or NULL if not found or NULL if buddy pfn is
+ *         not valid.
  */
 static inline struct page *find_buddy_page_pfn(struct page *page,
 			unsigned long pfn, unsigned int order, unsigned long *buddy_pfn)
@@ -330,6 +331,9 @@ static inline struct page *find_buddy_page_pfn(struct page *page,
 	unsigned long __buddy_pfn = __find_buddy_pfn(pfn, order);
 	struct page *buddy;
 
+	if (!pfn_valid(__buddy_pfn))
+		return NULL;
+
 	buddy = page + (__buddy_pfn - pfn);
 	if (buddy_pfn)
 		*buddy_pfn = __buddy_pfn;
-- 
2.17.1