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

Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx> · Tue, 14 Jun 2022 09:26:48 +0800

在 2022/6/14 上午12:08, Guo Ren 写道:
On Mon, Jun 13, 2022 at 9:11 PM Xianting Tian
<xianting.tian@xxxxxxxxxxxxxxxxx> wrote:
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)
How did we get 0? （Try it in gdb)
(gdb) p /x (0x2000 ^ (1<<12))
$3 = 0x3000
Sorry, the order is 0xd = 13, not 12, it is a typo.
I think it got buddy PFN 0 for PFN 0x1000, right?
(gdb) p /x (0x1000 ^ (1<<12))
$4 = 0x0

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))
Right, we need to check the buddy_pfn valid, because some SoCs would
start dram address with an offset that has an order smaller than
MAX_ORDER.

+                               goto done_merging;
                         buddy_mt = get_pageblock_migratetype(buddy);

                         if (migratetype != buddy_mt
--
2.17.1

Fixup the comment and

Reviewed-by: Guo Ren <guoren@xxxxxxxxxx>







