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

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

在 2022/6/16 下午11:04, David Hildenbrand 写道:
> 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@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.
>>>> 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 for the comments, I will check it for RISC-V arch.
>