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Subject[PATCH] mm: make do_move_pages() complexity linear
Page migration is currently very slow because its overhead is quadratic
with the number of pages. This is caused by each single page migration
doing a linear lookup in the page array in new_page_node().

Since pages are stored in the array order in the pagelist and do_move_pages
process this list in order, new_page_node() can increase the "pm" pointer
to the page array so that the next iteration will find the next page in
0 or few lookup steps.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>
Signed-off-by: Nathalie Furmento <Nathalie.Furmento@labri.fr>

--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -837,14 +837,23 @@ struct page_to_node {
int status;
};

+/*
+ * Allocate a page on the node given as a page_to_node in private.
+ * Increase private to point to the next page_to_node so that the
+ * next iteration does not have to traverse the whole pm array.
+ */
static struct page *new_page_node(struct page *p, unsigned long private,
int **result)
{
- struct page_to_node *pm = (struct page_to_node *)private;
+ struct page_to_node **pmptr = (struct page_to_node **)private;
+ struct page_to_node *pm = *pmptr;

while (pm->node != MAX_NUMNODES && pm->page != p)
pm++;

+ /* prepare for the next iteration */
+ *pmptr = pm + 1;
+
if (pm->node == MAX_NUMNODES)
return NULL;

@@ -926,10 +935,12 @@ set_status:
pp->status = err;
}

- if (!list_empty(&pagelist))
+ if (!list_empty(&pagelist)) {
+ /* new_page_node() will modify tmp */
+ struct page_to_node *tmp = pm;
err = migrate_pages(&pagelist, new_page_node,
- (unsigned long)pm);
- else
+ (unsigned long)&tmp);
+ } else
err = -ENOENT;

up_read(&mm->mmap_sem);



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