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    Subject[RFC PATCH 02/26] hugetlb: sort hstates in hugetlb_init_hstates
    From
    When using HugeTLB high-granularity mapping, we need to go through the
    supported hugepage sizes in decreasing order so that we pick the largest
    size that works. Consider the case where we're faulting in a 1G hugepage
    for the first time: we want hugetlb_fault/hugetlb_no_page to map it with
    a PUD. By going through the sizes in decreasing order, we will find that
    PUD_SIZE works before finding out that PMD_SIZE or PAGE_SIZE work too.

    Signed-off-by: James Houghton <jthoughton@google.com>
    ---
    mm/hugetlb.c | 40 +++++++++++++++++++++++++++++++++++++---
    1 file changed, 37 insertions(+), 3 deletions(-)

    diff --git a/mm/hugetlb.c b/mm/hugetlb.c
    index a57e1be41401..5df838d86f32 100644
    --- a/mm/hugetlb.c
    +++ b/mm/hugetlb.c
    @@ -33,6 +33,7 @@
    #include <linux/migrate.h>
    #include <linux/nospec.h>
    #include <linux/delayacct.h>
    +#include <linux/sort.h>

    #include <asm/page.h>
    #include <asm/pgalloc.h>
    @@ -48,6 +49,10 @@

    int hugetlb_max_hstate __read_mostly;
    unsigned int default_hstate_idx;
    +/*
    + * After hugetlb_init_hstates is called, hstates will be sorted from largest
    + * to smallest.
    + */
    struct hstate hstates[HUGE_MAX_HSTATE];

    #ifdef CONFIG_CMA
    @@ -3144,14 +3149,43 @@ static void __init hugetlb_hstate_alloc_pages(struct hstate *h)
    kfree(node_alloc_noretry);
    }

    +static int compare_hstates_decreasing(const void *a, const void *b)
    +{
    + const int shift_a = huge_page_shift((const struct hstate *)a);
    + const int shift_b = huge_page_shift((const struct hstate *)b);
    +
    + if (shift_a < shift_b)
    + return 1;
    + if (shift_a > shift_b)
    + return -1;
    + return 0;
    +}
    +
    +static void sort_hstates(void)
    +{
    + unsigned long default_hstate_sz = huge_page_size(&default_hstate);
    +
    + /* Sort from largest to smallest. */
    + sort(hstates, hugetlb_max_hstate, sizeof(*hstates),
    + compare_hstates_decreasing, NULL);
    +
    + /*
    + * We may have changed the location of the default hstate, so we need to
    + * update it.
    + */
    + default_hstate_idx = hstate_index(size_to_hstate(default_hstate_sz));
    +}
    +
    static void __init hugetlb_init_hstates(void)
    {
    struct hstate *h, *h2;

    - for_each_hstate(h) {
    - if (minimum_order > huge_page_order(h))
    - minimum_order = huge_page_order(h);
    + sort_hstates();

    + /* The last hstate is now the smallest. */
    + minimum_order = huge_page_order(&hstates[hugetlb_max_hstate - 1]);
    +
    + for_each_hstate(h) {
    /* oversize hugepages were init'ed in early boot */
    if (!hstate_is_gigantic(h))
    hugetlb_hstate_alloc_pages(h);
    --
    2.37.0.rc0.161.g10f37bed90-goog
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