Re: [PATCH 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when unmapping

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 5/7/2022 2:55 AM, Mike Kravetz wrote:
> On 4/29/22 01:14, Baolin Wang wrote:
>> On some architectures (like ARM64), it can support CONT-PTE/PMD size
>> hugetlb, which means it can support not only PMD/PUD size hugetlb:
>> 2M and 1G, but also CONT-PTE/PMD size: 64K and 32M if a 4K page
>> size specified.
>>
>> When unmapping a hugetlb page, we will get the relevant page table
>> entry by huge_pte_offset() only once to nuke it. This is correct
>> for PMD or PUD size hugetlb, since they always contain only one
>> pmd entry or pud entry in the page table.
>>
>> However this is incorrect for CONT-PTE and CONT-PMD size hugetlb,
>> since they can contain several continuous pte or pmd entry with
>> same page table attributes, so we will nuke only one pte or pmd
>> entry for this CONT-PTE/PMD size hugetlb page.
>>
>> And now we only use try_to_unmap() to unmap a poisoned hugetlb page,
> 
> Since try_to_unmap can be called for non-hugetlb pages, perhaps the following
> is more accurate?
> 
> try_to_unmap is only passed a hugetlb page in the case where the
> hugetlb page is poisoned.

Yes, will update in next version.

> It does concern me that this assumption is built into the code as
> pointed out in your discussion with Gerald.  Should we perhaps add
> a VM_BUG_ON() to make sure the passed huge page is poisoned?  This
> would be in the same 'if block' where we call
> adjust_range_if_pmd_sharing_possible.
Good point. Will do in next version. Thanks.