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From:   Xianting Tian <xianting.tian@linux.alibaba.com>
To:     akpm@linux-foundation.org, ziy@nvidia.com
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, huanyi.xj@alibaba-inc.com,
        guoren@kernel.org, zjb194813@alibaba-inc.com,
        tianhu.hh@alibaba-inc.com,
        Xianting Tian <xianting.tian@linux.alibaba.com>
Subject: [PATCH] mm: page_alloc: validate buddy before check the migratetype
Date:   Mon, 13 Jun 2022 21:08:58 +0800
Message-Id: <20220613130858.3009808-1-xianting.tian@linux.alibaba.com>
Precedence: bulk

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@vger.kernel.org
Reported-by: zjb194813@alibaba-inc.com
Reported-by: tianhu.hh@alibaba-inc.com
Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
---
 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