Re: [PATCH v2 1/3] mm: enable MADV_DONTNEED for hugetlb mappings

[Mike Kravetz]

On 2/9/22 19:21, Peter Xu wrote:
> (Sorry for the late comment)

Thanks for taking a look.

> 
> On Tue, Feb 01, 2022 at 05:40:32PM -0800, Mike Kravetz wrote:
>> MADV_DONTNEED is currently disabled for hugetlb mappings.  This
>> certainly makes sense in shared file mappings as the pagecache maintains
>> a reference to the page and it will never be freed.  However, it could
>> be useful to unmap and free pages in private mappings.
>>
>> The only thing preventing MADV_DONTNEED from working on hugetlb mappings
>> is a check in can_madv_lru_vma().  To allow support for hugetlb mappings
>> create and use a new routine madvise_dontneed_free_valid_vma() that will
>> allow hugetlb mappings.  Also, before calling zap_page_range in the
>> DONTNEED case align start and size to huge page size for hugetlb vmas.
>> madvise only requires PAGE_SIZE alignment, but the hugetlb unmap routine
>> requires huge page size alignment.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>> ---
>>  mm/madvise.c | 24 ++++++++++++++++++++++--
>>  1 file changed, 22 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/madvise.c b/mm/madvise.c
>> index 5604064df464..7ae891e030a4 100644
>> --- a/mm/madvise.c
>> +++ b/mm/madvise.c
>> @@ -796,10 +796,30 @@ static int madvise_free_single_vma(struct vm_area_struct *vma,
>>  static long madvise_dontneed_single_vma(struct vm_area_struct *vma,
>>  					unsigned long start, unsigned long end)
>>  {
>> +	/*
>> +	 * start and size (end - start) must be huge page size aligned
>> +	 * for hugetlb vmas.
>> +	 */
>> +	if (is_vm_hugetlb_page(vma)) {
>> +		struct hstate *h = hstate_vma(vma);
>> +
>> +		start = ALIGN_DOWN(start, huge_page_size(h));
>> +		end = ALIGN(end, huge_page_size(h));
>> +	}
>> +
> 
> Maybe check the alignment before userfaultfd_remove()?  Otherwise there'll be a
> fake message generated to the tracer.

Yes, we should pass the aligned addresses to userfaultfd_remove.  We will
also need to potentially align again after the call.

> 
>>  	zap_page_range(vma, start, end - start);
>>  	return 0;
>>  }
>>  
>> +static bool madvise_dontneed_free_valid_vma(struct vm_area_struct *vma,
>> +						int behavior)
>> +{
>> +	if (is_vm_hugetlb_page(vma))
>> +		return behavior == MADV_DONTNEED;
>> +	else
>> +		return can_madv_lru_vma(vma);
>> +}
> 
> can_madv_lru_vma() will check hugetlb again which looks a bit weird.  Would it
> look better to write it as:
> 
> madvise_dontneed_free_valid_vma()
> {
>     return !(vma->vm_flags & (VM_LOCKED|VM_PFNMAP));
> }
> 
> can_madv_lru_vma()
> {
>     return madvise_dontneed_free_valid_vma() && !is_vm_hugetlb_page(vma);
> }
> 
> ?

Yes, that would look better.

> 
> Another use case of DONTNEED upon hugetlbfs could be uffd-minor, because afaiu
> this is the only api that can force strip the hugetlb mapped pgtable without
> losing pagecache data.
> 

Correct.  However, I do not know if uffd-minor users would ever want to
do this.  Perhaps?
-- 
Mike Kravetz