Re: VM_BUG_ON(!tlb->end) on munmap() with CONT hugetlb pages

[Mike Kravetz <mike.kravetz@oracle.com>]

On 3/23/22 09:34, Steve Capper wrote:
> 
> 
> On 23/03/2022 16:21, Catalin Marinas wrote:
>> On Wed, Mar 23, 2022 at 11:51:25AM +0000, Steve Capper wrote:
>>> On 22/03/2022 17:56, Catalin Marinas wrote:
>>>> At a quick look, we wouldn't have a problem with missing TLB flushing
>>>> since huge_ptep_get_and_clear() does this for contiguous PTEs. Not sure
>>>> why it needs this though, Steve added it in commit d8bdcff28764. I think
>>>> we can defer this flushing to tlb_remove_page_size().
>>>
>>> The TLB flush in huge_ptep_get_and_clear() was added because it was called
>>> by hugetlb_change_protection() without any flushing. The concern was that,
>>> without the flush, it would be possible to get to different views of the
>>> same contiguous huge page. (Being contiguous they were not changed en masse
>>> atomically).
>>
>> Maybe the code paths have been changed since but looking at
>> hugetlb_change_protection(), we have huge_ptep_modify_prot_start()
>> calling huge_ptep_get_and_clear() which AFAICT only needs to clear the
>> ptes. huge_ptep_modify_prot_commit() calls set_huge_pte_at() which does
>> another pte clearing + TLBI (clear_flush()) before setting the new ptes.
>> So we do the pte clearing and TLBI twice already.
>>
> 
> Thanks, yeah indeed the code has changed and the flush should be removed from the arm64 huge_ptep_get_and_clear.
> 

Thanks Catalin and Steve for the arm64 expertise.

Aneesh,
IIUC powerpc also has hugetlb sizes that are not PMD_SIZE or PUD_SIZE.  Is
that correct?  If so, then I would expect the same issue there.  Steve's
patch makes tlb_remove_huge_tlb_entry able to be arch specific, so there may
need to be a powerpc specific version.
-- 
Mike Kravetz