linux-kernel - Re: [RFC PATCH 02/26] hugetlb: sort hstates in hugetlb_init

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Message-ID: <CAHS8izP0+ThdqEmp_v4s+n=Yw15w-JR2tudwVA=rX5WQL-KwMg@mail.gmail.com>
Date:   Fri, 24 Jun 2022 11:51:38 -0700
From:   Mina Almasry <almasrymina@...gle.com>
To:     James Houghton <jthoughton@...gle.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Jue Wang <juew@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 02/26] hugetlb: sort hstates in hugetlb_init_hstates

On Fri, Jun 24, 2022 at 10:37 AM James Houghton <jthoughton@...gle.com> wrote:
>
> 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.
>

Mostly nits:

Reviewed-by: Mina Almasry <almasrymina@...gle.com>

> Signed-off-by: James Houghton <jthoughton@...gle.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)

Maybe sort_hstates_descending(void) for extra clarity.

> +{
> +       unsigned long default_hstate_sz = huge_page_size(&default_hstate);
> +
> +       /* Sort from largest to smallest. */

I'd remove this redundant comment; it's somewhat obvious what the next
line does.

> +       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. */

Same, given that above is sort_hstates().

> +       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
>