Re: [PATCH v2 4/5] mm/hugetlb: use PMD page lock to protect CONT-PTE entries

[Baolin Wang]

On 8/23/2022 4:14 PM, David Hildenbrand wrote:
> On 23.08.22 09:50, Baolin Wang wrote:
> > Considering the pmd entries of a CONT-PMD hugetlb can not span on
> > multiple PMDs, we can change to use the PMD page lock, which can
> > be much finer grain that lock in the mm.
> >
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   include/linux/hugetlb.h | 12 ++++++++++--
> >   1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index 3a96f67..d4803a89 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -892,9 +892,17 @@ static inline gfp_t htlb_modify_alloc_mask(struct hstate *h, gfp_t gfp_mask)
> >   static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
> >   					   struct mm_struct *mm, pte_t *pte)
> >   {
> > -	VM_BUG_ON(huge_page_size(h) == PAGE_SIZE);
> > +	unsigned long hp_size = huge_page_size(h);
> >   
> > -	if (huge_page_size(h) == PMD_SIZE) {
> > +	VM_BUG_ON(hp_size == PAGE_SIZE);
> > +
> > +	/*
> > +	 * Considering CONT-PMD size hugetlb, since the CONT-PMD entry
> > +	 * can not span multiple PMDs, then we can use the fine grained
> > +	 * PMD page lock.
> > +	 */
> > +	if (hp_size == PMD_SIZE ||
> > +	    (hp_size > PMD_SIZE && hp_size < PUD_SIZE)) {
> >   		return pmd_lockptr(mm, (pmd_t *) pte);
> >   	} else if (huge_page_size(h) < PMD_SIZE) {
> >   		unsigned long mask = ~(PTRS_PER_PTE * sizeof(pte_t) - 1);
>
> Is there a measurable performance gain? IOW, do we really care?

IMO, It's just a theoretical analysis now:) Let me think about how to 
measure the performance gain.