Hi Xianting,

Thanks for your patch.

On 13 Jun 2022, at 9:10, Xianting Tian 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")

No, the Fixes tag is right. The commit above does need to validate buddy.

> 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().

It seems that the RISC-V arch reveals a similar bug from d9dddbf55667.
Basically, this bug will only happen when PFN=0x2000 is merging up and
there are some isolated pageblocks.

BTW, what does first reserved 2MB imply? All 4KB pages from first 2MB are
set to PageReserved?

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

You might miss the __find_buddy_pfn() caller in unset_migratetype_isolate()
from mm/page_isolation.c, if you are talking about linux-5.17.y and former
version. There, page_is_buddy() is also not called and is_migrate_isolate_page()
is called, which calls get_pageblock_migratetype() too.

>
> 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

--
Best Regards,
Yan, Zi