Re: [RESEND PATCH] mm: page_alloc: validate buddy before check the migratetype

[Xianting Tian <xianting.tian@linux.alibaba.com>]

在 2022/6/14 上午8:14, Zi Yan 写道:
> On 13 Jun 2022, at 19:47, Guo Ren wrote:
>
>> On Tue, Jun 14, 2022 at 3:49 AM Zi Yan <ziy@nvidia.com> wrote:
>>> On 13 Jun 2022, at 12:32, Guo Ren wrote:
>>>
>>>> On Mon, Jun 13, 2022 at 11:23 PM Zi Yan <ziy@nvidia.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.

Zi Yan, Guo Ren,

I think we still need some imporvemnt for MASTER branch, as we discussed 
above, we will get an illegal buddy page if buddy_pfn is 0,

within page_is_buddy(), it still use the illegal buddy page to do the 
check. I think in most of cases, page_is_buddy() can return false,  but 
it still may return true with very low probablity.

I think we need to add some code to verify buddy_pfn in the first place.

Could you give some suggestions for this idea?

>
>>>>>> 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@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
>>>>
>>>>
>>>> --
>>>> 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