Re: [PATCH] mm: page_alloc: validate buddy before check its migratetype.

[Zi Yan <ziy@nvidia.com>]

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On 30 Mar 2022, at 19:48, Zi Yan wrote:

> On 30 Mar 2022, at 19:03, Linus Torvalds wrote:
>
>> On Wed, Mar 30, 2022 at 3:12 PM Zi Yan <zi.yan@sent.com> wrote:
>>>
>>> Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable pageblocks with others")
>>
>> Oh, btw - should this perhaps be backported further back than that
>> alleged "fixes" commit?
>>
>> It does look like maybe the problem potentially existed before too,
>> and was just much harder to trigger.
>>
>> That said, google doesn't find any other reports that look like
>> Steven's oops, so maybe it really never happened and backporting isn't
>> called for.
>>
>> Or possibly my google-fu is just bad.
>>
>
> There might not be any issue with the original code because this bug
> could only be triggered when CONFIG_FLATMEM and CONFIG_MEMORY_ISOLATION
> are both set, which never happens, since CONFIG_MEMORY_ISOLATION
> depends on CONFIG_SPARSEMEM.
>
> By checking Steven's boot log, it should be PFN 0x21ee00 that triggers
> the bug, since the physical memory range ends at PFN 0x21edff.
> PFN 0x21ee00 is 2MB aligned instead of MAX_ORDER-1 (4MB) aligned.
> The original code assumes all physical memory ranges are at least
> MAX_ORDER-1 aligned, which is true when CONFIG_SPARSEMEM is set
> (CONFIG_MEMORY_ISOLATION depends on it), since CONFIG_SPARSEMEM
> allocates pageblock_flags array (the NULL-deferenced bitmap points
> to) at section size granularity (128MB > 4MB). However, CONFIG_FLATMEM
> does not do this. It allocates pageblock_flags array at the exact size
> of the physical memory. So checking 0x21ee00 will not cause NULL
> dereferencing when CONFIG_MEMORY_ISOLATION is set and the original
> if statement can be true.
>
> Now I am wondering if the page_is_buddy() check is correct for
> CONFIG_FLATMEM. Is mem_map allocation aligned to MAX_ORDER-1
> or just the present physical memory range? Is PageBuddy(0x21ee00)
> accessing some random memory location?

OK. mem_map seems to be MAX_ORDER-1 aligned, so there is no
problem with PageBuddy(0x21ee00).



--
Best Regards,
Yan, Zi

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