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ksm: break cow once unshared

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We kept agreeing not to bother about the unswappable shared KSM pages
which later become unshared by others: observation suggests they're not
a significant proportion.  But they are disadvantageous, and it is easier
to break COW to replace them by swappable pages, than offer statistics
to show that they don't matter; then we can stop worrying about them.

Doing this in ksm_do_scan, they don't go through cmp_and_merge_page on
this pass: give them a good chance of getting into the unstable tree
on the next pass, or back into the stable, by computing checksum now.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Acked-by: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@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:
Hugh Dickins 2009-09-21 17:02:12 -07:00 committed by Linus Torvalds
parent 473b0ce4d1
commit 26465d3ea5
1 changed files with 8 additions and 0 deletions
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8
mm/ksm.c
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@ -1268,6 +1268,14 @@ static void ksm_do_scan(unsigned int scan_npages)
return;
if (!PageKsm(page) || !in_stable_tree(rmap_item))
cmp_and_merge_page(page, rmap_item);
else if (page_mapcount(page) == 1) {
/*
* Replace now-unshared ksm page by ordinary page.
*/
break_cow(rmap_item->mm, rmap_item->address);
remove_rmap_item_from_tree(rmap_item);
rmap_item->oldchecksum = calc_checksum(page);
}
put_page(page);
}
}