Re: [PATCH v3 1/3] mm: enable MADV_DONTNEED for hugetlb mappings

[David Hildenbrand]

On 17.02.22 09:32, Yin Fengwei wrote:
> 
> 
> On 2022/2/15 08:23, 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.  In addition,
>> userfaultfd minor fault users may be able to simplify code by using
>> MADV_DONTNEED.
>>
>> The primary 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 allows hugetlb mappings in this specific case.
>>
>> For normal mappings, madvise requires the start address be PAGE aligned
>> and rounds up length to the next multiple of PAGE_SIZE.  Do similarly
> 
> In man page of mmap, NOTE for "Huge page (Huge TLB) mappings":
> 
> "For munmap(), addr, and length must both be a multiple of the 
> underlying huge page size."
> 
> Should we apply same rule to MADV_DONTNEED? Thanks.

madvise() already has different rules than mmap() for ordinary pages, so
we'd much rather try staying consistent with madvise() rules.

-- 
Thanks,

David / dhildenb