Thread (62 messages) 62 messages, 10 authors, 2012-02-10

Re: [PATCH 02/15] mm: page_alloc: update migrate type of pages on pcp when isolating

From: Mel Gorman <hidden>
Date: 2012-01-30 11:15:28
Also in: linux-arm-kernel, linux-media, lkml

On Thu, Jan 26, 2012 at 10:00:44AM +0100, Marek Szyprowski wrote:
quoted hunk ↗ jump to hunk
From: Michal Nazarewicz <redacted>

This commit changes set_migratetype_isolate() so that it updates
migrate type of pages on pcp list which is saved in their
page_private.

Signed-off-by: Michal Nazarewicz <redacted>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
---
 include/linux/page-isolation.h |    6 ++++++
 mm/page_alloc.c                |    1 +
 mm/page_isolation.c            |   24 ++++++++++++++++++++++++
 3 files changed, 31 insertions(+), 0 deletions(-)
diff --git a/include/linux/page-isolation.h b/include/linux/page-isolation.h
index 051c1b1..8c02c2b 100644
--- a/include/linux/page-isolation.h
+++ b/include/linux/page-isolation.h
@@ -27,6 +27,12 @@ extern int
 test_pages_isolated(unsigned long start_pfn, unsigned long end_pfn);
 
 /*
+ * Check all pages in pageblock, find the ones on pcp list, and set
+ * their page_private to MIGRATE_ISOLATE.
+ */
+extern void update_pcp_isolate_block(unsigned long pfn);
+
+/*
  * Internal funcs.Changes pageblock's migrate type.
  * Please use make_pagetype_isolated()/make_pagetype_movable().
  */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e1c5656..70709e7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5465,6 +5465,7 @@ out:
 	if (!ret) {
 		set_pageblock_migratetype(page, MIGRATE_ISOLATE);
 		move_freepages_block(zone, page, MIGRATE_ISOLATE);
+		update_pcp_isolate_block(pfn);
 	}
 
 	spin_unlock_irqrestore(&zone->lock, flags);
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 4ae42bb..9ea2f6e 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -139,3 +139,27 @@ int test_pages_isolated(unsigned long start_pfn, unsigned long end_pfn)
 	spin_unlock_irqrestore(&zone->lock, flags);
 	return ret ? 0 : -EBUSY;
 }
+
+/* must hold zone->lock */
+void update_pcp_isolate_block(unsigned long pfn)
+{
+	unsigned long end_pfn = pfn + pageblock_nr_pages;
+	struct page *page;
+
+	while (pfn < end_pfn) {
+		if (!pfn_valid_within(pfn)) {
+			++pfn;
+			continue;
+		}
+
There is a potential problem here that you need to be aware of.
set_pageblock_migratetype() is called from start_isolate_page_range().
I do not think there is a guarantee that pfn + pageblock_nr_pages is
not in a different block of MAX_ORDER_NR_PAGES. If that is right then
your options are to add a check like this;

if ((pfn & (MAX_ORDER_NR_PAGES - 1)) == 0 && !pfn_valid(pfn))
	break;

or else ensure that end_pfn is always MAX_ORDER_NR_PAGES aligned and in
the same block as pfn and relying on the caller to have called
pfn_valid.
+		page = pfn_to_page(pfn);
+		if (PageBuddy(page)) {
+			pfn += 1 << page_order(page);
+		} else if (page_count(page) == 0) {
+			set_page_private(page, MIGRATE_ISOLATE);
+			++pfn;
This is dangerous for two reasons. If the page_count is 0, it could
be because the page is in the process of being freed and is not
necessarily on the per-cpu lists yet and you cannot be sure if the
contents of page->private are important. Second, there is nothing to
prevent another CPU allocating this page from its per-cpu list while
the private field is getting updated from here which might lead to
some interesting races.

I recognise that what you are trying to do is respond to Gilad's
request that you really check if an IPI here is necessary. I think what
you need to do is check if a page with a count of 0 is encountered
and if it is, then a draining of the per-cpu lists is necessary. To
address Gilad's concerns, be sure to only this this once per attempt at
CMA rather than for every page encountered with a count of 0 to avoid a
storm of IPIs.
+		} else {
+			++pfn;
+		}
+	}
+}
-- 
Mel Gorman
SUSE Labs

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