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References: <20220613131046.3009889-1-xianting.tian@linux.alibaba.com>
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From:   Guo Ren <guoren@kernel.org>
Date:   Tue, 14 Jun 2022 00:08:50 +0800
Message-ID: <CAJF2gTQJZFfpSOx0yo3zJfECwpZR=79F5uLKVV_qqopS+F2PYA@mail.gmail.com>
Subject: Re: [RESEND PATCH] mm: page_alloc: validate buddy before check the migratetype
To:     Xianting Tian <xianting.tian@linux.alibaba.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, ziy@nvidia.com,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        stable@vger.kernel.org, huanyi.xj@alibaba-inc.com,
        zjb194813@alibaba-inc.com, tianhu.hh@alibaba-inc.com
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On Mon, Jun 13, 2022 at 9:11 PM Xianting Tian
<xianting.tian@linux.alibaba.com> wrote:
>
> Commit 787af64d05cd ("mm: page_alloc: validate buddy before check its mig=
ratetype.")
> 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 isolate=
d and other pageblocks")
>
> For RISC-V arch, the first 2M is reserved for sbi, so the start PFN is 51=
2,
> but it got buddy PFN 0 for PFN 0x2000:
>      0 =3D 0x2000 ^ (1 << 12)
How did we get 0? =EF=BC=88Try it in gdb)
(gdb) p /x (0x2000 ^ (1<<12))
$3 =3D 0x3000

I think it got buddy PFN 0 for PFN 0x1000, right?
(gdb) p /x (0x1000 ^ (1<<12))
$4 =3D 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() i=
f
> 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 *pag=
e,
>
>                         buddy_pfn =3D __find_buddy_pfn(pfn, order);
>                         buddy =3D 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 =3D get_pageblock_migratetype(buddy);
>
>                         if (migratetype !=3D buddy_mt
> --
> 2.17.1
>

Fixup the comment and

Reviewed-by: Guo Ren <guoren@kernel.org>

--=20
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/