Re: [PATCH 4/4] hugetlb: Do early cow when page pinned on src mm

[Peter Xu <peterx@redhat.com>]

On Wed, Feb 03, 2021 at 02:04:30PM -0800, Mike Kravetz wrote:
> > @@ -3816,6 +3832,54 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
> >  			}
> >  			set_huge_swap_pte_at(dst, addr, dst_pte, entry, sz);
> >  		} else {
> > +			entry = huge_ptep_get(src_pte);
> > +			ptepage = pte_page(entry);
> > +			get_page(ptepage);
> > +
> > +			if (unlikely(page_needs_cow_for_dma(vma, ptepage))) {
> > +				/* This is very possibly a pinned huge page */
> > +				if (!prealloc) {
> > +					/*
> > +					 * Preallocate the huge page without
> > +					 * tons of locks since we could sleep.
> > +					 * Note: we can't use any reservation
> > +					 * because the page will be exclusively
> > +					 * owned by the child later.
> > +					 */
> > +					put_page(ptepage);
> > +					spin_unlock(src_ptl);
> > +					spin_unlock(dst_ptl);
> > +					prealloc = alloc_huge_page(vma, addr, 0);
> 
> One quick question:
> 
> The comment says we can't use any reservation, and I agree.  However, the
> alloc_huge_page call has 0 as the avoid_reserve argument.  Shouldn't that
> be !0 to avoid reserves?

Good point..  so I obviously wanted to skip reservation check but successfully
got cheated by the inverted name. :)

Though I do checked the reservation, so it seems not extremely important - when
we fork and copy the vma, we have already dropped the vma resv map:

		if (is_vm_hugetlb_page(tmp))
			reset_vma_resv_huge_pages(tmp);

Then in alloc_huge_page() we checked vma_resv_map() mostly everywhere we'd
check avoid_reserve too (either in vma_needs_reservation, or calculating
deferred_reserve).  It seems to be mostly useful when vma_resv_map() existed.

But I completely agree I should pass in "1" here in v2.

Thanks,

-- 
Peter Xu