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

[Peter Xu <peterx@redhat.com>]

(Sorry for the late comment)

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@oracle.com>
> ---
>  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.

>  	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);
}

?

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.

Thanks,

-- 
Peter Xu