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Subject[PATCH mmotm] ksm: stop scan skipping pages
KSM can be slow to identify all mergeable pages.  There's an off-by-one
in how ksm_scan_start() proceeds (see how it does a scan_get_next_index
at the head of its loop, but also on leaving its loop), which causes it
to skip over a page occasionally. Fix that.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
---
mm/ksm.c | 46 +++++++++++++++++++---------------------------
1 file changed, 19 insertions(+), 27 deletions(-)
--- mmotm/mm/ksm.c 2009-06-08 13:14:36.000000000 +0100
+++ fixed/mm/ksm.c 2009-06-08 13:18:30.000000000 +0100
@@ -1331,30 +1331,26 @@ out:
/* return -EAGAIN - no slots registered, nothing to be done */
static int scan_get_next_index(struct ksm_scan *ksm_scan)
{
- struct ksm_mem_slot *slot;
+ struct list_head *next;

if (list_empty(&slots))
return -EAGAIN;

- slot = ksm_scan->slot_index;
-
/* Are there pages left in this slot to scan? */
- if ((slot->npages - ksm_scan->page_index - 1) > 0) {
- ksm_scan->page_index++;
+ ksm_scan->page_index++;
+ if (ksm_scan->page_index < ksm_scan->slot_index->npages)
return 0;
- }

- list_for_each_entry_from(slot, &slots, link) {
- if (slot == ksm_scan->slot_index)
- continue;
- ksm_scan->page_index = 0;
- ksm_scan->slot_index = slot;
+ ksm_scan->page_index = 0;
+ next = ksm_scan->slot_index->link.next;
+ if (next != &slots) {
+ ksm_scan->slot_index =
+ list_entry(next, struct ksm_mem_slot, link);
return 0;
}

/* look like we finished scanning the whole memory, starting again */
root_unstable_tree = RB_ROOT;
- ksm_scan->page_index = 0;
ksm_scan->slot_index = list_first_entry(&slots,
struct ksm_mem_slot, link);
return 0;
@@ -1366,21 +1362,22 @@ static int scan_get_next_index(struct ks
* pointed to was released so we have to call this function every time after
* taking the slots_lock
*/
-static void scan_update_old_index(struct ksm_scan *ksm_scan)
+static int scan_update_old_index(struct ksm_scan *ksm_scan)
{
struct ksm_mem_slot *slot;

if (list_empty(&slots))
- return;
+ return -EAGAIN;

list_for_each_entry(slot, &slots, link) {
if (ksm_scan->slot_index == slot)
- return;
+ return 0;
}

ksm_scan->slot_index = list_first_entry(&slots,
struct ksm_mem_slot, link);
ksm_scan->page_index = 0;
+ return 0;
}

/**
@@ -1399,20 +1396,14 @@ static int ksm_scan_start(struct ksm_sca
struct ksm_mem_slot *slot;
struct page *page[1];
int val;
- int ret = 0;
+ int ret;

down_read(&slots_lock);
+ ret = scan_update_old_index(ksm_scan);

- scan_update_old_index(ksm_scan);
-
- while (scan_npages > 0) {
- ret = scan_get_next_index(ksm_scan);
- if (ret)
- goto out;
-
- slot = ksm_scan->slot_index;
-
+ while (scan_npages && !ret) {
cond_resched();
+ slot = ksm_scan->slot_index;

/*
* If the page is swapped out or in swap cache, we don't want to
@@ -1433,10 +1424,11 @@ static int ksm_scan_start(struct ksm_sca
} else {
up_read(&slot->mm->mmap_sem);
}
+
+ ret = scan_get_next_index(ksm_scan);
scan_npages--;
}
- scan_get_next_index(ksm_scan);
-out:
+
up_read(&slots_lock);
return ret;
}

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