[RFC 3/3] mm: make swapin readahead to improve thp collapse rate

[Ebru Akagunduz <ebru.akagunduz@gmail.com>]

This patch makes swapin readahead to improve thp collapse rate.
When khugepaged scanned pages, there can be a few of the pages
in swap area.

With the patch THP can collapse 4kB pages into a THP when
there are up to max_ptes_swap swap ptes in a 2MB range.

The patch was tested with a test program that allocates
800MB of memory, writes to it, and then sleeps. I force
the system to swap out all. Afterwards, the test program
touches the area by writing, it skips a page in each
20 pages of the area.

Without the patch, system did not swap in readahead.
THP rate was %47 of the program of the memory, it
did not change 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>
---
I've written down test results:

With the patch:
After swapped out:
cat /proc/pid/smaps:
Anonymous:        470760 kB
AnonHugePages:    468992 kB
Swap:             329244 kB
Fraction:         %99

After swapped in:
In ten minutes:
cat /proc/pid/smaps:
Anonymous:        769208 kB
AnonHugePages:    765952 kB
Swap:              30796 kB
Fraction:         %99

Without the patch:
After swapped out:
cat /proc/pid/smaps:
Anonymous:        238160 kB
AnonHugePages:    235520 kB
Swap:             561844 kB
Fraction:         %98

After swapped in:
cat /proc/pid/smaps:
In ten minutes:
Anonymous:        499956 kB
AnonHugePages:    235520 kB
Swap:             300048 kB
Fraction:         %47

 include/linux/mm.h                 |  4 ++++
 include/trace/events/huge_memory.h | 24 ++++++++++++++++++++++++
 mm/huge_memory.c                   | 35 +++++++++++++++++++++++++++++++++++
 mm/memory.c                        |  2 +-
 4 files changed, 64 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7f47178..f66ff8a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -29,6 +29,10 @@ struct user_struct;
 struct writeback_control;
 struct bdi_writeback;
 
+extern int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
+			unsigned long address, pte_t *page_table, pmd_t *pmd,
+			unsigned int flags, pte_t orig_pte);
+
 #ifndef CONFIG_NEED_MULTIPLE_NODES	/* Don't use mapnrs, do it properly */
 extern unsigned long max_mapnr;
 
diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index 53c9f2e..0117ab9 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -95,5 +95,29 @@ TRACE_EVENT(mm_collapse_huge_page_isolate,
 		__entry->writable)
 );
 
+TRACE_EVENT(mm_collapse_huge_page_swapin,
+
+	TP_PROTO(struct mm_struct *mm, unsigned long vm_start, int swap_pte),
+
+	TP_ARGS(mm, vm_start, swap_pte),
+
+	TP_STRUCT__entry(
+		__field(struct mm_struct *, mm)
+		__field(unsigned long, vm_start)
+		__field(int, swap_pte)
+	),
+
+	TP_fast_assign(
+		__entry->mm = mm;
+		__entry->vm_start = vm_start;
+		__entry->swap_pte = swap_pte;
+	),
+
+	TP_printk("mm=%p, vm_start=%04lx, swap_pte=%d",
+		__entry->mm,
+		__entry->vm_start,
+		__entry->swap_pte)
+);
+
 #endif /* __HUGE_MEMORY_H */
 #include <trace/define_trace.h>
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 22bc0bf..cb3e82a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2496,6 +2496,39 @@ static bool hugepage_vma_check(struct vm_area_struct *vma)
 	return true;
 }
 
+/*
+ * Bring missing pages in from swap, to complete THP collapse.
+ * Only done if khugepaged_scan_pmd believes it is worthwhile.
+ *
+ * Called and returns without pte mapped or spinlocks held,
+ * but with mmap_sem held to protect against vma changes.
+ */
+
+static void __collapse_huge_page_swapin(struct mm_struct *mm,
+					struct vm_area_struct *vma,
+					unsigned long address, pmd_t *pmd,
+					pte_t *pte)
+{
+	unsigned long _address;
+	pte_t pteval = *pte;
+	int swap_pte = 0;
+
+	pte = pte_offset_map(pmd, address);
+	for (_address = address; _address < address + HPAGE_PMD_NR*PAGE_SIZE;
+	     pte++, _address += PAGE_SIZE) {
+		pteval = *pte;
+		if (is_swap_pte(pteval)) {
+			swap_pte++;
+			do_swap_page(mm, vma, _address, pte, pmd, 0x0, pteval);
+			/* pte is unmapped now, we need to map it */
+			pte = pte_offset_map(pmd, _address);
+		}
+	}
+	pte--;
+	pte_unmap(pte);
+	trace_mm_collapse_huge_page_swapin(mm, vma->vm_start, swap_pte);
+}
+
 static void collapse_huge_page(struct mm_struct *mm,
 				   unsigned long address,
 				   struct page **hpage,
@@ -2551,6 +2584,8 @@ static void collapse_huge_page(struct mm_struct *mm,
 	if (!pmd)
 		goto out;
 
+	__collapse_huge_page_swapin(mm, vma, address, pmd, pte);
+
 	anon_vma_lock_write(vma->anon_vma);
 
 	pte = pte_offset_map(pmd, address);
diff --git a/mm/memory.c b/mm/memory.c
index e1c45d0..d801dc5 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2443,7 +2443,7 @@ EXPORT_SYMBOL(unmap_mapping_range);
  * We return with the mmap_sem locked or unlocked in the same cases
  * as does filemap_fault().
  */
-static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
+int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		unsigned long address, pte_t *page_table, pmd_t *pmd,
 		unsigned int flags, pte_t orig_pte)
 {
-- 
1.9.1

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