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
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>
--
Best Regards
Guo Ren
ML: https://lore.kernel.org/linux-csky/
