[PATCH] mm: page_alloc: validate buddy before check the migratetype

[Xianting Tian]

Commit 787af64d05cd ("mm: page_alloc: validate buddy before check its migratetype.")
added buddy check code. But unfortunately, this fix isn't backported to
linux-5.17.y and the former stable branches. The reason is it added wrong
fixes message:
     Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable
			   pageblocks with others")
Actually, this issue is involved by commit:
     commit d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks")

For RISC-V arch, the first 2M is reserved for sbi, so the start PFN is 512,
but it got buddy PFN 0 for PFN 0x2000:
     0 = 0x2000 ^ (1 << 12)
With the illegal buddy PFN 0, it got an illegal buddy page, which caused
crash in __get_pfnblock_flags_mask().

With the patch, it can avoid the calling of get_pageblock_migratetype() if
it isn't buddy page.

Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: zjb194813@xxxxxxxxxxxxxxx
Reported-by: tianhu.hh@xxxxxxxxxxxxxxx
Signed-off-by: Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx>
---
 mm/page_alloc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b1caa1c6c887..5b423caa68fd 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1129,6 +1129,9 @@ static inline void __free_one_page(struct page *page,
 
 			buddy_pfn = __find_buddy_pfn(pfn, order);
 			buddy = page + (buddy_pfn - pfn);
+
+			if (!page_is_buddy(page, buddy, order))
+				goto done_merging;
 			buddy_mt = get_pageblock_migratetype(buddy);
 
 			if (migratetype != buddy_mt
-- 
2.17.1