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    SubjectRe: [PATCH v9 09/11] mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate
    On Sun, Dec 13, 2020 at 11:45:32PM +0800, Muchun Song wrote:
    > All the infrastructure is ready, so we introduce nr_free_vmemmap_pages
    > field in the hstate to indicate how many vmemmap pages associated with
    > a HugeTLB page that we can free to buddy allocator. And initialize it
    "can be freed to buddy allocator"

    > in the hugetlb_vmemmap_init(). This patch is actual enablement of the
    > feature.
    >
    > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
    > Acked-by: Mike Kravetz <mike.kravetz@oracle.com>

    With below nits addressed you can add:

    Reviewed-by: Oscar Salvador <osalvador@suse.de>

    > static int __init early_hugetlb_free_vmemmap_param(char *buf)
    > {
    > + /* We cannot optimize if a "struct page" crosses page boundaries. */
    > + if (!is_power_of_2(sizeof(struct page)))
    > + return 0;
    > +

    I wonder if we should report a warning in case someone wants to enable this
    feature and stuct page size it not power of 2.
    In case someone wonders why it does not work for him/her.

    > +void __init hugetlb_vmemmap_init(struct hstate *h)
    > +{
    > + unsigned int nr_pages = pages_per_huge_page(h);
    > + unsigned int vmemmap_pages;
    > +
    > + if (!hugetlb_free_vmemmap_enabled)
    > + return;
    > +
    > + vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
    > + /*
    > + * The head page and the first tail page are not to be freed to buddy
    > + * system, the others page will map to the first tail page. So there
    > + * are the remaining pages that can be freed.
    "the other pages will map to the first tail page, so they can be freed."
    > + *
    > + * Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? It is true
    > + * on some architectures (e.g. aarch64). See Documentation/arm64/
    > + * hugetlbpage.rst for more details.
    > + */
    > + if (likely(vmemmap_pages > RESERVE_VMEMMAP_NR))
    > + h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
    > +
    > + pr_info("can free %d vmemmap pages for %s\n", h->nr_free_vmemmap_pages,
    > + h->name);

    Maybe specify this is hugetlb code:

    pr_info("%s: blabla", __func__, ...)
    or
    pr_info("hugetlb: blalala", ...);

    although I am not sure whether we need that at all, or maybe just use
    pr_debug().

    --
    Oscar Salvador
    SUSE L3

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