Date:   Wed, 22 Jun 2022 09:37:06 +0800
From:   Xianting Tian <xianting.tian@...ux.alibaba.com>
To:     Greg KH <gregkh@...uxfoundation.org>, Zi Yan <ziy@...dia.com>
Cc:     akpm@...ux-foundation.org, stable@...r.kernel.org,
        guoren@...nel.org, huanyi.xj@...baba-inc.com, guohanjun@...wei.com,
        zjb194813@...baba-inc.com, tianhu.hh@...baba-inc.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5.15] mm: validate buddy page before using


在 2022/6/21 上午4:31, Greg KH 写道:
> On Mon, Jun 20, 2022 at 10:13:59AM -0400, Zi Yan wrote:
>> 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:
>>>>>> 在 2022/6/20 下午8:06, Greg KH 写道:
>>>>>>> On Mon, Jun 20, 2022 at 07:57:05PM +0800, Xianting Tian wrote:
>>>>>>>> 在 2022/6/20 下午7:42, Greg KH 写道:
>>>>>>>>> On Mon, Jun 20, 2022 at 06:54:44PM +0800, Xianting Tian wrote:
>>>>>>>>>> 在 2022/6/20 下午6:17, Greg KH 写道:
>>>>>>>>>>> On Fri, Jun 17, 2022 at 12:17:45AM +0800, Xianting Tian wrote:
>>>>>>>>>>>> Commit 787af64d05cd ("mm: page_alloc: validate buddy before check its migratetype.")
>>>>>>>>>>>> fixes a bug in 1dd214b8f21c and there is a similar bug in d9dddbf55667 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 reserved for opensbi,
>>>>>>>>>>>> so it would have pfn_base=512 and mem_map began with 512th PFN when
>>>>>>>>>>>> CONFIG_FLATMEM=y.
>>>>>>>>>>>> But __find_buddy_pfn algorithm thinks the start pfn 0, it could get 0 pfn or
>>>>>>>>>>>> less than the pfn_base value. We need page_is_buddy() to verify the buddy to
>>>>>>>>>>>> prevent accessing an invalid buddy.
>>>>>>>>>>>>
>>>>>>>>>>>> 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/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，
>>>>>>>>>>
>>>>>>>>>> commit 787af64d05cd528aac9ad16752d11bb1c6061bb9
>>>>>>>>>> Author: Zi Yan <ziy@...dia.com>
>>>>>>>>>> Date:   Wed Mar 30 15:45:43 2022 -0700
>>>>>>>>>>
>>>>>>>>>>        mm: page_alloc: validate buddy before check its migratetype.
>>>>>>>>>>
>>>>>>>>>>        Whenever a buddy page is found, page_is_buddy() should be called to
>>>>>>>>>>        check its validity.  Add the missing check during pageblock merge check.
>>>>>>>>>>
>>>>>>>>>>        Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable
>>>>>>>>>> pageblocks with others")
>>>>>>>>>>        Link:
>>>>>>>>>> https://lore.kernel.org/all/20220330154208.71aca532@gandalf.local.home/
>>>>>>>>>>        Reported-and-tested-by: Steven Rostedt <rostedt@...dmis.org>
>>>>>>>>>>        Signed-off-by: Zi Yan <ziy@...dia.com>
>>>>>>>>>>        Signed-off-by: Linus Torvalds <torvalds@...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 is the
>>>>>>>>> subject lines changed?
>>>>>>>> Yes, the changes of 5.15 are not same with others branches, because we need
>>>>>>>> additional fix for 5.15,
>>>>>>>>
>>>>>>>> You can check it in the thread:
>>>>>>>>
>>>>>>>> https://lore.kernel.org/linux-mm/435B45C3-E6A5-43B2-A5A2-318C748691FC@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,  you can just backport Zi Yan'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,  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 applied
>>>>> 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’s tree, since the bug is
>>>> fixed by another commit, which was not intended to fix the bug from the 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 d9dddbf55667.
>> Then the commit 787af64d05cd fixed the bug, but since the original code was
>> 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 change and
>> the latter fixes the bug revealed by the former.
> That is exactly what we want to apply, we almost never want to apply
> stuff that is not upstream.  When we do apply "custom" patches, they are
> almost always wrong.  We have a long history of this, please let's just
> take the originals please.
>
>> As a result, Xianting's patches fix the bug directly, looking more reasonable to me.
> Again, please no, let's take the originals and keep in step with what is
> in Linus's tree which makes maintance and tracking and everything so
> much easier over time.
If so, I think we only can backport 787af64d from Linus tree to all 
stable branches. Our ultimate purpose is to solve the problem, I think 
@Zi Yan will agree?
>
> thanks,
>
> greg k-h

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