Messages in this thread | | | Date | Wed, 3 Feb 2021 17:30:17 -0500 | From | Peter Xu <> | Subject | Re: [PATCH 4/4] hugetlb: Do early cow when page pinned on src mm |
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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
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