[PATCH] mm: make transparent hugepage size public

[Hugh Dickins <hughd@google.com>]

Test programs want to know the size of a transparent hugepage.
While it is commonly the same as the size of a hugetlbfs page
(shown as Hugepagesize in /proc/meminfo), that is not always so:
powerpc implements transparent hugepages in a different way from
hugetlbfs pages, so it's coincidence when their sizes are the same;
and x86 and others can support more than one hugetlbfs page size.

Add /sys/kernel/mm/transparent_hugepage/hpage_pmd_size to show the
THP size in bytes - it's the same for Anonymous and Shmem hugepages.
Call it hpage_pmd_size (after HPAGE_PMD_SIZE) rather than hpage_size,
in case some transparent support for pud and pgd pages is added later.

Signed-off-by: Hugh Dickins <hughd@google.com>
---

 Documentation/vm/transhuge.txt |    5 +++++
 mm/huge_memory.c               |   10 ++++++++++
 2 files changed, 15 insertions(+)

--- 4.9-rc8/Documentation/vm/transhuge.txt	2016-10-02 16:24:33.000000000 -0700
+++ linux/Documentation/vm/transhuge.txt	2016-12-05 20:55:12.142578631 -0800
@@ -136,6 +136,11 @@ or enable it back by writing 1:
 echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page
 echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page
 
+Some userspace (such as a test program, or an optimized memory allocation
+library) may want to know the size (in bytes) of a transparent hugepage:
+
+cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size
+
 khugepaged will be automatically started when
 transparent_hugepage/enabled is set to "always" or "madvise, and it'll
 be automatically shutdown if it's set to "never".
--- 4.9-rc8/mm/huge_memory.c	2016-12-04 16:42:39.881703357 -0800
+++ linux/mm/huge_memory.c	2016-12-05 20:58:19.953010005 -0800
@@ -285,6 +285,15 @@ static ssize_t use_zero_page_store(struc
 }
 static struct kobj_attribute use_zero_page_attr =
 	__ATTR(use_zero_page, 0644, use_zero_page_show, use_zero_page_store);
+
+static ssize_t hpage_pmd_size_show(struct kobject *kobj,
+		struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%lu\n", HPAGE_PMD_SIZE);
+}
+static struct kobj_attribute hpage_pmd_size_attr =
+	__ATTR_RO(hpage_pmd_size);
+
 #ifdef CONFIG_DEBUG_VM
 static ssize_t debug_cow_show(struct kobject *kobj,
 				struct kobj_attribute *attr, char *buf)
@@ -307,6 +316,7 @@ static struct attribute *hugepage_attr[]
 	&enabled_attr.attr,
 	&defrag_attr.attr,
 	&use_zero_page_attr.attr,
+	&hpage_pmd_size_attr.attr,
 #if defined(CONFIG_SHMEM) && defined(CONFIG_TRANSPARENT_HUGE_PAGECACHE)
 	&shmem_enabled_attr.attr,
 #endif

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