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Subject[PATCH v2] mm: incorporate zero pages into transparent huge pages
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This patch improves THP collapse rates, by allowing zero pages.

Currently THP can collapse 4kB pages into a THP when there
are up to khugepaged_max_ptes_none pte_none ptes in a 2MB
range. This patch counts pte none and mapped zero pages
with the same variable.

The patch was tested with a program that allocates 800MB of
memory, and performs interleaved reads and writes, in a pattern
that causes some 2MB areas to first see read accesses, resulting
in the zero pfn being mapped there.

To simulate memory fragmentation at allocation time, I modified
do_huge_pmd_anonymous_page to return VM_FAULT_FALLBACK for read
faults.

Without the patch, only %50 of the program was collapsed into
THP and the percentage did not increase over time.

With this patch after 10 minutes of waiting khugepaged had
collapsed %99 of the program's memory.

Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
---
Changes in v2:
- Check zero pfn in release_pte_pages() (Andrea Arcangeli)

mm/huge_memory.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e08e37a..a87a691 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2139,7 +2139,7 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte)
{
while (--_pte >= pte) {
pte_t pteval = *_pte;
- if (!pte_none(pteval))
+ if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)))
release_pte_page(pte_page(pteval));
}
}
@@ -2150,13 +2150,13 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
{
struct page *page;
pte_t *_pte;
- int none = 0;
+ int none_or_zero = 0;
bool referenced = false, writable = false;
for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
_pte++, address += PAGE_SIZE) {
pte_t pteval = *_pte;
- if (pte_none(pteval)) {
- if (++none <= khugepaged_max_ptes_none)
+ if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
+ if (++none_or_zero <= khugepaged_max_ptes_none)
continue;
else
goto out;
@@ -2237,7 +2237,7 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
pte_t pteval = *_pte;
struct page *src_page;

- if (pte_none(pteval)) {
+ if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
clear_user_highpage(page, address);
add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
} else {
@@ -2573,7 +2573,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
{
pmd_t *pmd;
pte_t *pte, *_pte;
- int ret = 0, none = 0;
+ int ret = 0, none_or_zero = 0;
struct page *page;
unsigned long _address;
spinlock_t *ptl;
@@ -2591,8 +2591,8 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
for (_address = address, _pte = pte; _pte < pte+HPAGE_PMD_NR;
_pte++, _address += PAGE_SIZE) {
pte_t pteval = *_pte;
- if (pte_none(pteval)) {
- if (++none <= khugepaged_max_ptes_none)
+ if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
+ if (++none_or_zero <= khugepaged_max_ptes_none)
continue;
else
goto out_unmap;
--
1.9.1


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