Message-ID: <435B45C3-E6A5-43B2-A5A2-318C748691FC@nvidia.com>
Date:   Mon, 13 Jun 2022 20:14:36 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Guo Ren <guoren@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Xianting Tian <xianting.tian@...ux.alibaba.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org, huanyi.xj@...baba-inc.com,
        zjb194813@...baba-inc.com, tianhu.hh@...baba-inc.com,
        Hanjun Guo <guohanjun@...wei.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Laura Abbott <labbott@...hat.com>
Subject: Re: [RESEND PATCH] mm: page_alloc: validate buddy before check the
 migratetype

On 13 Jun 2022, at 19:47, Guo Ren wrote:

> On Tue, Jun 14, 2022 at 3:49 AM Zi Yan <ziy@...dia.com> wrote:
>>
>> On 13 Jun 2022, at 12:32, Guo Ren wrote:
>>
>>> On Mon, Jun 13, 2022 at 11:23 PM Zi Yan <ziy@...dia.com> wrote:
>>>>
>>>> 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.
>>> I think Xianting is right. The “Fixes:" tag is not accurate and the
>>> page_is_buddy() is necessary here.
>>>
>>> This patch could be applied to the early version of the stable tree
>>> (eg: Linux-5.10.y, not the master tree)
>>
>> This is quite misleading. Commit 787af64d05cd applies does not mean it is
>> intended to fix the preexisting bug. Also it does not apply cleanly
>> to commit d9dddbf55667, there is a clear indentation mismatch. At best,
>> you can say the way of 787af64d05cd fixing 1dd214b8f21c also fixes d9dddbf55667.
>> There is no way you can apply 787af64d05cd to earlier trees and call it a day.
>>
>> You can mention 787af64d05cd that it fixes a bug in 1dd214b8f21c and there is
>> a similar bug in d9dddbf55667 that can be fixed in a similar way too. Saying
>> the fixes message is wrong just misleads people, making them think there is
>> no bug in 1dd214b8f21c. We need to be clear about this.
> First, d9dddbf55667 is earlier than 1dd214b8f21c in Linus tree. The
> origin fixes could cover the Linux-5.0.y tree if they give the
> accurate commit number and that is the cause we want to point out.

Yes, I got that d9dddbf55667 is earlier and commit 787af64d05cd fixes
the issue introduced by d9dddbf55667. But my point is that 787af64d05cd
is not intended to fix d9dddbf55667 and saying it has a wrong fixes
message is misleading. This is the point I want to make.

>
> Second, if the patch is for d9dddbf55667 then it could cover any tree
> in the stable repo. Actually, we only know Linux-5.10.y has the
> problem.

But it is not and does not apply to d9dddbf55667 cleanly.

>
> Maybe, Gregkh could help to direct us on how to deal with the issue:
> (Fixup a bug which only belongs to the former stable branch.)
>

I think you just need to send this patch without saying “commit
787af64d05cd fixes message is wrong” would be a good start. You also
need extra fix to mm/page_isolation.c for kernels between 5.15 and 5.17
(inclusive). So there will need to be two patches:

1) your patch to stable tree prior to 5.15 and

2) your patch with an additional mm/page_isolation.c fix to stable tree
between 5.15 and 5.17.

>>
>> Also, you will need to fix the mm/page_isolation.c code too to make this patch
>> complete, unless you can show that PFN=0x1000 is never going to be encountered
>> in the mm/page_isolation.c code I mentioned below.
> No, we needn't fix mm/page_isolation.c in linux-5.10.y, because it had
> pfn_valid_within(buddy_pfn) check after __find_buddy_pfn() to prevent
> buddy_pfn=0.
> The root cause comes from __find_buddy_pfn():
> return page_pfn ^ (1 << order);

Right. But pfn_valid_within() was removed since 5.15. So your fix is
required for kernels between 5.15 and 5.17 (inclusive).

>
> When page_pfn is the same as the order size, it will return the
> previous buddy not the next. That is the only exception for this
> algorithm, right?
>
>
>
>
> In fact, the bug is a very long time to reproduce and is not easy to
> debug, so we want to contribute it to the community to prevent other
> guys from wasting time. Although there is no new patch at all.

Thanks for your reporting and sending out the patch. I really
appreciate it. We definitely need your inputs. Throughout the email
thread, I am trying to help you clarify the bug and how to fix it
properly:

1. The commit 787af64d05cd does not apply cleanly to commits
d9dddbf55667, meaning you cannot just cherry-pick that commit to
fix the issue. That is why we need your patch to fix the issue.
And saying it has a wrong fixes message in this patch’s git log is
misleading.

2. For kernels between 5.15 and 5.17 (inclusive), an additional fix
to mm/page_isolation.c is also needed, since pfn_valid_within() was
removed since 5.15 and the issue can appear during page isolation.

3. For kernels before 5.15, this patch will apply.

>
>>
>>>
>>>>
>>>>> 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.
>>> Not PFN=0x2000, it's PFN=0x1000, I guess.
>>>
>>> RISC-V's first 2MB RAM could reserve for opensbi, so it would have
>>> riscv_pfn_base=512 and mem_map began with 512th PFN when
>>> CONFIG_FLATMEM=y.
>>> (Also, csky has the same issue: a non-zero pfn_base in some scenarios.)
>>>
>>> But __find_buddy_pfn algorithm thinks the start address is 0, it could
>>> get 0 pfn or less than the pfn_base value. We need another check to
>>> prevent that.
>>>
>>>>
>>>> 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@...r.kernel.org
>>>>> Reported-by: zjb194813@...baba-inc.com
>>>>> Reported-by: tianhu.hh@...baba-inc.com
>>>>> Signed-off-by: Xianting Tian <xianting.tian@...ux.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
>>>
>>>
>>>
>>> --
>>> Best Regards
>>>  Guo Ren
>>>
>>> ML: https://lore.kernel.org/linux-csky/
>>
>> --
>> Best Regards,
>> Yan, Zi
>
>
>
> -- 
> Best Regards
>  Guo Ren
>
> ML: https://lore.kernel.org/linux-csky/

--
Best Regards,
Yan, Zi

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