Building under memory pressure, with KSM on 2.6.36-rc5, collapsed with
an internal compiler error: typically indicating an error in swapping.
Perhaps there's a timing issue which makes it now more likely, perhaps
it's just a long time since I tried for so long: this bug goes back to
KSM swapping in 2.6.33.
Notice how reuse_swap_page() allows an exclusive page to be reused, but
only does SetPageDirty if it can delete it from swap cache right then -
if it's currently under Writeback, it has to be left in cache and we
don't SetPageDirty, but the page can be reused. Fine, the dirty bit
will get set in the pte; but notice how zap_pte_range() does not bother
to transfer pte_dirty to page_dirty when unmapping a PageAnon.
If KSM chooses to share such a page, it will look like a clean copy of
swapcache, and not be written out to swap when its memory is needed;
then stale data read back from swap when it's needed again.
We could fix this in reuse_swap_page() (or even refuse to reuse a
page under writeback), but it's more honest to fix my oversight in
KSM's write_protect_page(). Several days of testing on three machines
confirms that this fixes the issue they showed.
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: stable@xxxxxxxxxx
---
mm/ksm.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
--- 2.6.36-rc6/mm/ksm.c 2010-09-12 17:34:03.000000000 -0700
+++ linux/mm/ksm.c 2010-09-28 23:27:05.000000000 -0700
@@ -712,7 +712,7 @@ static int write_protect_page(struct vm_
if (!ptep)
goto out;
- if (pte_write(*ptep)) {
+ if (pte_write(*ptep) || pte_dirty(*ptep)) {
pte_t entry;
swapped = PageSwapCache(page);
@@ -735,7 +735,9 @@ static int write_protect_page(struct vm_
set_pte_at(mm, addr, ptep, entry);
goto out_unlock;
}
- entry = pte_wrprotect(entry);
+ if (pte_dirty(entry))
+ set_page_dirty(page);
+ entry = pte_mkclean(pte_wrprotect(entry));
set_pte_at_notify(mm, addr, ptep, entry);
}
*orig_pte = *ptep;
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxxx For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>