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ksm: don't fail stable tree lookups if walking over stale stable_nodes 

The stable_nodes can become stale at any time if the underlying pages gets
freed.  The stable_node gets collected and removed from the stable rbtree
if that is detected during the rbtree lookups.

Don't fail the lookup if running into stale stable_nodes, just restart the
lookup after collecting the stale stable_nodes.  Otherwise the CPU spent
in the preparation stage is wasted and the lookup must be repeated at the
next loop potentially failing a second time in a second stale stable_node.

If we don't prune aggressively we delay the merging of the unstable node
candidates and at the same time we delay the freeing of the stale
stable_nodes.  Keeping stale stable_nodes around wastes memory and it
can't provide any benefit.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Petr Holasek <pholasek@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Andrea Arcangeli 2015-11-05 18:49:10 -08:00 committed by Linus Torvalds
parent ad12695f17
commit f2e5ff85ed
1 changed files with 27 additions and 5 deletions
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32
mm/ksm.c
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@ -1177,8 +1177,18 @@ again:
cond_resched();
stable_node = rb_entry(*new, struct stable_node, node);
tree_page = get_ksm_page(stable_node, false);
if (!tree_page)
return NULL;
if (!tree_page) {
/*
* If we walked over a stale stable_node,
* get_ksm_page() will call rb_erase() and it
* may rebalance the tree from under us. So
* restart the search from scratch. Returning
* NULL would be safe too, but we'd generate
* false negative insertions just because some
* stable_node was stale.
*/
goto again;
}
ret = memcmp_pages(page, tree_page);
put_page(tree_page);
@ -1254,12 +1264,14 @@ static struct stable_node *stable_tree_insert(struct page *kpage)
unsigned long kpfn;
struct rb_root *root;
struct rb_node **new;
struct rb_node *parent = NULL;
struct rb_node *parent;
struct stable_node *stable_node;
kpfn = page_to_pfn(kpage);
nid = get_kpfn_nid(kpfn);
root = root_stable_tree + nid;
again:
parent = NULL;
new = &root->rb_node;
while (*new) {
@ -1269,8 +1281,18 @@ static struct stable_node *stable_tree_insert(struct page *kpage)
cond_resched();
stable_node = rb_entry(*new, struct stable_node, node);
tree_page = get_ksm_page(stable_node, false);
if (!tree_page)
return NULL;
if (!tree_page) {
/*
* If we walked over a stale stable_node,
* get_ksm_page() will call rb_erase() and it
* may rebalance the tree from under us. So
* restart the search from scratch. Returning
* NULL would be safe too, but we'd generate
* false negative insertions just because some
* stable_node was stale.
*/
goto again;
}
ret = memcmp_pages(kpage, tree_page);
put_page(tree_page);