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

[David Hildenbrand]

On 16.06.22 16:01, Zi Yan wrote:
> On 15 Jun 2022, at 12:15, Xianting Tian wrote:
> 
>> 在 2022/6/15 下午9:55, Zi Yan 写道:
>>> On 15 Jun 2022, at 2:47, Xianting Tian wrote:
>>>
>>>> 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@xxxxxxxxxx> wrote:
>>>>>>> On 13 Jun 2022, at 12:32, Guo Ren wrote:
>>>>>>>
>>>>>>>> On Mon, Jun 13, 2022 at 11:23 PM Zi Yan <ziy@xxxxxxxxxx> 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.
>>> Can you elaborate more on this? What kind of page can lead to page_is_buddy()
>>> returning true? You said it is buddy_pfn is 0, but if the page is reserved,
>>> if (!page_is_guard(buddy) && !PageBuddy(buddy)) should return false.
>>> Maybe show us the dump_page() that offending page.
>>>
>>> Thanks.
>>
>> Let‘s take the issue we met on RISC-V arch for example,
>>
>> pfn_base is 512 as we reserved 2M RAM for opensbi, mem_map's value is 0xffffffe07e205000, which is the page address of PFN 512.
>>
>> __find_buddy_pfn() returned 0 for PFN 0x2000 with order 0xd.
>> We know PFN 0 is not a valid pfn for buddy system, because 512 is the first PFN for buddy system.
>>
>> Then it use below code to get buddy page with buddy_pfn 0:
>> buddy = page + (buddy_pfn - pfn);
>> So buddy page address is:
>> 0xffffffe07e1fe000 = (struct page*)0xffffffe07e26e000 + (0 - 0x2000)
>>
>> we can know this buddy page's address is less than mem_map(0xffffffe07e1fe000 < 0xffffffe07e205000),
>> actually 0xffffffe07e1fe000 is not a valid page's address. If we use 0xffffffe07e1fe000
>> as the page's address to extract the value of a member in 'struct page', we may get an uncertain value.
>> That's why I say page_is_buddy() may return true with very low probablity.
>>
>> So I think we need to add the code the verify buddy_pfn in the first place:
>> 	pfn_valid(buddy_pfn)
>>
> 
> +DavidH on how memory section works.
> 
> This 2MB RAM reservation does not sound right to me. How does it work in sparsemem?
> RISC-V has SECTION_SIZE_BITS=27, i.e., 128MB a section. All pages within
> a section should have their corresponding struct page (mem_map). So in this case,
> the first 2MB pages should have mem_map and can be marked as PageReserved. As a
> result, page_is_buddy() will return false.

Yes. Unless there is a BUG :)

init_unavailable_range() is supposed to initialize the memap of
unavailable ranges and mark it reserved.

I wonder if we're missing a case in memmap_init(), to also initialize
holes at the beginning of a section, before RAM (we do handle sections
in a special way if the end of RAM falls in the middle of a section).

If it's not initialized, it might contain garbage.

-- 
Thanks,

David / dhildenb