On Fri, Feb 5, 2021 at 5:09 PM Oscar Salvador [off-list ref] wrote:

On Thu, Feb 04, 2021 at 11:50:43AM +0800, Muchun Song wrote:

quoted

We cannot optimize if a "struct page" crosses page boundaries. If
it is true, we can optimize the code with the help of a compiler.
When free_vmemmap_pages_per_hpage() returns zero, most functions are
optimized by the compiler.

"When the "struct page size" crosses page boundaries we cannot
 make use of this feature.
 Let free_vmemmap_pages_per_hpage() return zero if that is the case,
 most of the functions can be optimized away."

I think the above is more clear, but just a suggestion.

OK. I will use it. Thanks for your suggestions.

quoted

Signed-off-by: Muchun Song <redacted>

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

Thanks.

quoted

---
 include/linux/hugetlb.h |  3 ++-
 mm/hugetlb_vmemmap.c    | 13 +++++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 822ab2f5542a..7bfb06e16298 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h

@@ -878,7 +878,8 @@ extern bool hugetlb_free_vmemmap_enabled;

 static inline bool is_hugetlb_free_vmemmap_enabled(void)
 {
-     return hugetlb_free_vmemmap_enabled;
+     return hugetlb_free_vmemmap_enabled &&
+            is_power_of_2(sizeof(struct page));
 }
 #else
 static inline bool is_hugetlb_free_vmemmap_enabled(void)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 8efad9978821..068d0e0cebc8 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c

@@ -211,6 +211,12 @@ early_param("hugetlb_free_vmemmap", early_hugetlb_free_vmemmap_param);
  */
 static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
 {
+     /*
+      * This check aims to let the compiler help us optimize the code as
+      * much as possible.
+      */
+     if (!is_power_of_2(sizeof(struct page)))
+             return 0;
      return h->nr_free_vmemmap_pages;
 }

@@ -280,6 +286,13 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
      BUILD_BUG_ON(NR_USED_SUBPAGE >=
                   RESERVE_VMEMMAP_SIZE / sizeof(struct page));

+     /*
+      * The compiler can help us to optimize this function to null
+      * when the size of the struct page is not power of 2.
+      */
+     if (!is_power_of_2(sizeof(struct page)))
+             return;
+
      if (!hugetlb_free_vmemmap_enabled)
              return;

--

2.11.0

--
Oscar Salvador
SUSE L3