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

From: Zi Yan
Date: Thu Jun 16 2022 - 10:01:20 EST

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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.

For flatmem, IIRC, the valid addresses should be aligned to MAX_ORDER_NR_PAGES.
This means first 4MB (assuming MAX_ORDER is 11 on RISC-V) should not be on
buddy allocator and hence this issue does not happen in the first place.
But correct me if I am wrong.

>>> 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@xxxxxxxxxxxxxxx
>>>>>>>>> Reported-by: zjb194813@xxxxxxxxxxxxxxx
>>>>>>>>> Reported-by: tianhu.hh@xxxxxxxxxxxxxxx
>>>>>>>>> Signed-off-by: Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx>
>>>>>>>>> ---
>>>>>>>>> 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
>> --
>> Best Regards,
>> Yan, Zi

--
Best Regards,
Yan, Zi

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