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Subject[PATCH 5/7] khugepaged: Allow to collapse PTE-mapped compound pages
Date
We can collapse PTE-mapped compound pages. We only need to avoid
handling them more than once: lock/unlock page only once if it's present
in the PMD range multiple times as it handled on compound level. The
same goes for LRU isolation and putpack.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
mm/khugepaged.c | 41 +++++++++++++++++++++++++++++++----------
1 file changed, 31 insertions(+), 10 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b47edfe57f7b..c8c2c463095c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -515,6 +515,17 @@ void __khugepaged_exit(struct mm_struct *mm)

static void release_pte_page(struct page *page)
{
+ /*
+ * We need to unlock and put compound page on LRU only once.
+ * The rest of the pages have to be locked and not on LRU here.
+ */
+ VM_BUG_ON_PAGE(!PageCompound(page) &&
+ (!PageLocked(page) && PageLRU(page)), page);
+
+ if (!PageLocked(page))
+ return;
+
+ page = compound_head(page);
dec_node_page_state(page, NR_ISOLATED_ANON + page_is_file_cache(page));
unlock_page(page);
putback_lru_page(page);
@@ -537,6 +548,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
pte_t *_pte;
int none_or_zero = 0, result = 0, referenced = 0;
bool writable = false;
+ LIST_HEAD(compound_pagelist);

for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
_pte++, address += PAGE_SIZE) {
@@ -561,13 +573,23 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
goto out;
}

- /* TODO: teach khugepaged to collapse THP mapped with pte */
+ VM_BUG_ON_PAGE(!PageAnon(page), page);
+
if (PageCompound(page)) {
- result = SCAN_PAGE_COMPOUND;
- goto out;
- }
+ struct page *p;
+ page = compound_head(page);

- VM_BUG_ON_PAGE(!PageAnon(page), page);
+ /*
+ * Check if we have dealt with the compount page
+ * already
+ */
+ list_for_each_entry(p, &compound_pagelist, lru) {
+ if (page == p)
+ break;
+ }
+ if (page == p)
+ continue;
+ }

/*
* We can do it before isolate_lru_page because the
@@ -640,6 +662,9 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
page_is_young(page) || PageReferenced(page) ||
mmu_notifier_test_young(vma->vm_mm, address))
referenced++;
+
+ if (PageCompound(page))
+ list_add_tail(&page->lru, &compound_pagelist);
}
if (likely(writable)) {
if (likely(referenced)) {
@@ -1185,11 +1210,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
goto out_unmap;
}

- /* TODO: teach khugepaged to collapse THP mapped with pte */
- if (PageCompound(page)) {
- result = SCAN_PAGE_COMPOUND;
- goto out_unmap;
- }
+ page = compound_head(page);

/*
* Record which node the original page is from and save this
--
2.26.0
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