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From:   Zi Yan <ziy@nvidia.com>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Xianting Tian <xianting.tian@linux.alibaba.com>,
        akpm@linux-foundation.org, stable@vger.kernel.org,
        guoren@kernel.org, huanyi.xj@alibaba-inc.com, guohanjun@huawei.com,
        zjb194813@alibaba-inc.com, tianhu.hh@alibaba-inc.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 5.15] mm: validate buddy page before using
Date:   Mon, 20 Jun 2022 10:13:59 -0400
Message-ID: <62DC5603-F88E-40A3-A4AD-EEFA7027C399@nvidia.com>
In-Reply-To: <YrBuEvLX/ENMx6Zj@kroah.com>
References: <20220616161746.3565225-1-xianting.tian@linux.alibaba.com>
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On 20 Jun 2022, at 8:54, Greg KH wrote:

> On Mon, Jun 20, 2022 at 08:45:13AM -0400, Zi Yan wrote:
>> On 20 Jun 2022, at 8:24, Greg KH wrote:
>>
>>> On Mon, Jun 20, 2022 at 08:18:40PM +0800, Xianting Tian wrote:
>>>>
>>>> =E5=9C=A8 2022/6/20 =E4=B8=8B=E5=8D=888:06, Greg KH =E5=86=99=E9=81=93=
:
>>>>> On Mon, Jun 20, 2022 at 07:57:05PM +0800, Xianting Tian wrote:
>>>>>> =E5=9C=A8 2022/6/20 =E4=B8=8B=E5=8D=887:42, Greg KH =E5=86=99=E9=81=
=93:
>>>>>>> On Mon, Jun 20, 2022 at 06:54:44PM +0800, Xianting Tian wrote:
>>>>>>>> =E5=9C=A8 2022/6/20 =E4=B8=8B=E5=8D=886:17, Greg KH =E5=86=99=E9=
=81=93:
>>>>>>>>> On Fri, Jun 17, 2022 at 12:17:45AM +0800, Xianting Tian wrote:
>>>>>>>>>> Commit 787af64d05cd ("mm: page_alloc: validate buddy before ch=
eck its migratetype.")
>>>>>>>>>> fixes a bug in 1dd214b8f21c and there is a similar bug in d9dd=
dbf55667 that
>>>>>>>>>> can be fixed in a similar way too.
>>>>>>>>>>
>>>>>>>>>> In unset_migratetype_isolate(), we also need the fix, so move =
page_is_buddy()
>>>>>>>>>> from mm/page_alloc.c to mm/internal.h
>>>>>>>>>>
>>>>>>>>>> In addition, for RISC-V arch the first 2MB RAM could be reserv=
ed for opensbi,
>>>>>>>>>> so it would have pfn_base=3D512 and mem_map began with 512th P=
FN when
>>>>>>>>>> CONFIG_FLATMEM=3Dy.
>>>>>>>>>> But __find_buddy_pfn algorithm thinks the start pfn 0, it coul=
d get 0 pfn or
>>>>>>>>>> less than the pfn_base value. We need page_is_buddy() to verif=
y the buddy to
>>>>>>>>>> prevent accessing an invalid buddy.
>>>>>>>>>>
>>>>>>>>>> Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between i=
solated 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/internal.h       | 34 +++++++++++++++++++++++++++++++++=
+
>>>>>>>>>>     mm/page_alloc.c     | 37 +++------------------------------=
----
>>>>>>>>>>     mm/page_isolation.c |  3 ++-
>>>>>>>>>>     3 files changed, 39 insertions(+), 35 deletions(-)
>>>>>>>>> What is the commit id of this in Linus's tree?
>>>>>>>> It is also this one=EF=BC=8C
>>>>>>>>
>>>>>>>> commit 787af64d05cd528aac9ad16752d11bb1c6061bb9
>>>>>>>> Author: Zi Yan <ziy@nvidia.com>
>>>>>>>> Date:=C2=A0=C2=A0 Wed Mar 30 15:45:43 2022 -0700
>>>>>>>>
>>>>>>>>   =C2=A0=C2=A0=C2=A0 mm: page_alloc: validate buddy before check=
 its migratetype.
>>>>>>>>
>>>>>>>>   =C2=A0=C2=A0=C2=A0 Whenever a buddy page is found, page_is_bud=
dy() should be called to
>>>>>>>>   =C2=A0=C2=A0=C2=A0 check its validity.=C2=A0 Add the missing c=
heck during pageblock merge check.
>>>>>>>>
>>>>>>>>   =C2=A0=C2=A0=C2=A0 Fixes: 1dd214b8f21c ("mm: page_alloc: avoid=
 merging non-fallbackable
>>>>>>>> pageblocks with others")
>>>>>>>>   =C2=A0=C2=A0=C2=A0 Link:
>>>>>>>> https://lore.kernel.org/all/20220330154208.71aca532@gandalf.loca=
l.home/
>>>>>>>>   =C2=A0=C2=A0=C2=A0 Reported-and-tested-by: Steven Rostedt <ros=
tedt@goodmis.org>
>>>>>>>>   =C2=A0=C2=A0=C2=A0 Signed-off-by: Zi Yan <ziy@nvidia.com>
>>>>>>>>   =C2=A0=C2=A0=C2=A0 Signed-off-by: Linus Torvalds <torvalds@lin=
ux-foundation.org>
>>>>>>> This commit looks nothing like what you posted here.
>>>>>>>
>>>>>>> Why the vast difference with no explaination as to why these are =
so
>>>>>>> different from the other backports you provided here?  Also why i=
s the
>>>>>>> subject lines changed?
>>>>>> Yes, the changes of 5.15 are not same with others branches, becaus=
e we need
>>>>>> additional fix for 5.15,
>>>>>>
>>>>>> You can check it in the thread:
>>>>>>
>>>>>> https://lore.kernel.org/linux-mm/435B45C3-E6A5-43B2-A5A2-318C74869=
1FC@nvidia.com/ <https://lore.kernel.org/linux-mm/435B45C3-E6A5-43B2-A5A2=
-318C748691FC@nvidia.com/>
>>>>>>
>>>>>> Right. But pfn_valid_within() was removed since 5.15. So your fix =
is
>>>>>> required for kernels between 5.15 and 5.17 (inclusive).
>>>>> What is "your fix" here?
>>>>>
>>>>> This change differs a lot from what is in Linus's tree now, so this=
 all
>>>>> needs to be resend and fixed up as I mention above if we are going =
to be
>>>>> able to take this.  As-is, it's all not correct so are dropped.
>>>>
>>>> I think, for branches except 5.15,=C2=A0 you can just backport Zi Ya=
n's commit
>>>> 787af64d05cd in Linus tree. I won't send more patches further,
>>>
>>> So just for 5.18?  I am confused.
>>>
>>>> For 5.15, because it need additional fix except commit 787af64d05cd,=
=C2=A0 I will
>>>> send a new patch as your comments.
>>>>
>>>> Is it ok for you?
>>>
>>> No, please send fixed up patches for all branches you want them appli=
ed
>>> to as I do not understand what to do here at all, sorry.
>>
>> Hi Greg,
>>
>> The fixes sent by Xianting do not exist in Linus=E2=80=99s tree, since=
 the bug is
>> fixed by another commit, which was not intended to fix the bug from th=
e commit
>> d9dddbf55667. These fixes only target the stable branches.
>
> Then that all needs to be documented very very very well as to why we
> can't just take the commit that is in Linus's tree.
>
> Why can't we take that commit instead?

The situation is a little complicated.

The bug from commit d9dddbf55667 was not discovered back then. The commit=
 1dd214b8f21c
was trying to get migratetype merging more rigid and made the bug easy to=
 get
hit, but none of us were aware of that the bug also exists in commit d9dd=
dbf55667.
Then the commit 787af64d05cd fixed the bug, but since the original code w=
as
changed by commit 1dd214b8f21c, thus, it does not directly apply to
commit d9dddbf55667. So I do not think it makes sense to use the original=
 commits
1dd214b8f21c and 787af64d05cd, since the former makes a non bug fixing ch=
ange and
the latter fixes the bug revealed by the former.

As a result, Xianting's patches fix the bug directly, looking more reason=
able to me.

--
Best Regards,
Yan, Zi

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