Re: [RFC v5 3/3] mm: make swapin readahead to improve thp collapse rate

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, Sep 14, 2015 at 10:31:45PM +0300, Ebru Akagunduz wrote:
> 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
> 400B 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 %65 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>
> Acked-by: Rik van Riel <riel@redhat.com>

[ resend with correct TO/CC lists]

>From 9b28cf3159ff200d4a2543d73a06ae84d050aea2 Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Date: Thu, 17 Sep 2015 16:35:57 +0300
Subject: [PATCH] khugepaged: avoid usage of uninitialized variable 'isolated'
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

In file included from include/trace/events/huge_memory.h:7:0,
                 from mm/huge_memory.c:62:
include/linux/tracepoint.h:141:5: warning: ‘isolated’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     ((void(*)(proto))(it_func))(args); \
     ^
mm/huge_memory.c:2327:6: note: ‘isolated’ was declared here
  int isolated, result = 0;

We make use of isolated in error path before it get initialized.

The patch also makes trivial cleanup of exit path of the function.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/huge_memory.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 5ad5c443cc94..857f92e85c38 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2656,8 +2656,6 @@ static void __collapse_huge_page_swapin(struct mm_struct *mm,
 	trace_mm_collapse_huge_page_swapin(mm, swapped_in, 1);
 }
 
-
-
 static void collapse_huge_page(struct mm_struct *mm,
 				   unsigned long address,
 				   struct page **hpage,
@@ -2669,7 +2667,7 @@ static void collapse_huge_page(struct mm_struct *mm,
 	pgtable_t pgtable;
 	struct page *new_page;
 	spinlock_t *pmd_ptl, *pte_ptl;
-	int isolated, result = 0;
+	int isolated = 0, result = 0;
 	unsigned long hstart, hend;
 	struct mem_cgroup *memcg;
 	unsigned long mmun_start;	/* For mmu_notifiers */
@@ -2805,9 +2803,6 @@ static void collapse_huge_page(struct mm_struct *mm,
 	result = SCAN_SUCCEED;
 out_up_write:
 	up_write(&mm->mmap_sem);
-	trace_mm_collapse_huge_page(mm, isolated, result);
-	return;
-
 out_nolock:
 	trace_mm_collapse_huge_page(mm, isolated, result);
 	return;
-- 
 Kirill A. Shutemov