Re: Re: [PATCH] mm: fix kernel NULL pointer dereference in page_vma_mapped_walk

[zhenwei pi <pizhenwei@bytedance.com>]

On 4/7/22 22:17, Matthew Wilcox wrote:
> On Thu, Apr 07, 2022 at 02:54:42PM +0100, Matthew Wilcox wrote:
>> On Thu, Apr 07, 2022 at 02:40:08PM +0800, zhenwei pi wrote:
>>> size_to_hstate(4K) returns NULL pointer, this leads kernel BUG in
>>> function page_vma_mapped_walk.
>>
>> Yes, I think this is the right fix.  It's not immediately obvious from
>> the bug and the patch, but what's going on is:
>>
>> page_mapped_in_vma() sets nr_pages to 1.  This is correct because we
>> usually only want to know about the precise page, and not about the
>> folio containing it.  But hugetlbfs is special (... in so many ways ...)
>> and actually wants to work on the entire folio.  We could set nr_pages
>> specially for hugetlb pages, but it's better to ignore it in
>> page_vma_mapped_walk() for the hugetlb case.
>>
>> I'll fix up the changelog and add it to my pile of fixes that I'm
>> sending tomorrow.
>> https://git.infradead.org/users/willy/pagecache.git/shortlog/refs/heads/for-next
> 
> I looked over the patch in context, and realised we now don't need to
> call size_to_hstate since we're getting the hstate from the VMA.
> 
> https://git.infradead.org/users/willy/pagecache.git/commitdiff/98ea02597b9967c0817d29fee2f96d21b9e59ca5
> 
Thanks!
-- 
zhenwei pi