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

[Yin Fengwei <fengwei.yin@intel.com>]

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.


Regards
Yin, Fengwei



> for hugetlb mappings: require start address be huge page size aligned and
> round up length to the next multiple of huge page size.  Use the new
> madvise_dontneed_free_valid_vma routine to check alignment and round up
> length/end.  zap_page_range requires this alignment for hugetlb vmas
> otherwise we will hit BUGs.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>   mm/madvise.c | 33 ++++++++++++++++++++++++++++++---
>   1 file changed, 30 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index bed872a2ad5f..ede6affa1350 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -554,9 +554,14 @@ static void madvise_cold_page_range(struct mmu_gather *tlb,
>   	tlb_end_vma(tlb, vma);
>   }
>   
> +static inline bool can_madv_lru_non_huge_vma(struct vm_area_struct *vma)
> +{
> +	return !(vma->vm_flags & (VM_LOCKED|VM_PFNMAP));
> +}
> +
>   static inline bool can_madv_lru_vma(struct vm_area_struct *vma)
>   {
> -	return !(vma->vm_flags & (VM_LOCKED|VM_HUGETLB|VM_PFNMAP));
> +	return can_madv_lru_non_huge_vma(vma) && !is_vm_hugetlb_page(vma);
>   }
>   
>   static long madvise_cold(struct vm_area_struct *vma,
> @@ -829,6 +834,23 @@ static long madvise_dontneed_single_vma(struct vm_area_struct *vma,
>   	return 0;
>   }
>   
> +static bool madvise_dontneed_free_valid_vma(struct vm_area_struct *vma,
> +					    unsigned long start,
> +					    unsigned long *end,
> +					    int behavior)
> +{
> +	if (!is_vm_hugetlb_page(vma))
> +		return can_madv_lru_non_huge_vma(vma);
> +
> +	if (behavior != MADV_DONTNEED)
> +		return false;
> +	if (start & ~huge_page_mask(hstate_vma(vma)))
> +		return false;
> +
> +	*end = ALIGN(*end, huge_page_size(hstate_vma(vma)));
> +	return true;
> +}
> +
>   static long madvise_dontneed_free(struct vm_area_struct *vma,
>   				  struct vm_area_struct **prev,
>   				  unsigned long start, unsigned long end,
> @@ -837,7 +859,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
>   	struct mm_struct *mm = vma->vm_mm;
>   
>   	*prev = vma;
> -	if (!can_madv_lru_vma(vma))
> +	if (!madvise_dontneed_free_valid_vma(vma, start, &end, behavior))
>   		return -EINVAL;
>   
>   	if (!userfaultfd_remove(vma, start, end)) {
> @@ -859,7 +881,12 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
>   			 */
>   			return -ENOMEM;
>   		}
> -		if (!can_madv_lru_vma(vma))
> +		/*
> +		 * Potential end adjustment for hugetlb vma is OK as
> +		 * the check below keeps end within vma.
> +		 */
> +		if (!madvise_dontneed_free_valid_vma(vma, start, &end,
> +						     behavior))
>   			return -EINVAL;
>   		if (end > vma->vm_end) {
>   			/*