(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@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.
> 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
