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Subject[RFC PATCH 02/26] hugetlb: sort hstates in hugetlb_init_hstates
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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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