On Wed, 7 Mar 2007 12:31:21 +0100
Nick Piggin <[EMAIL PROTECTED]> wrote:
> Index: linux-2.6/mm/memory.c
> ===================================================================
> --- linux-2.6.orig/mm/memory.c
> +++ linux-2.6/mm/memory.c
> @@ -1664,6 +1664,15 @@ gotten:
> unlock:
> pte_unmap_unlock(page_table, ptl);
> if (dirty_page) {
> + /*
> + * Yes, Virginia, this is actually required to prevent a race
> + * with clear_page_dirty_for_io() from clearing the page dirty
> + * bit after it clear all dirty ptes, but before a racing
> + * do_wp_page installs a dirty pte.
> + *
> + * do_no_page is protected similarly.
> + */
> + wait_on_page_locked(dirty_page);
> set_page_dirty_balance(dirty_page);
> put_page(dirty_page);
> }
> @@ -2316,6 +2325,7 @@ retry:
> unlock:
> pte_unmap_unlock(page_table, ptl);
> if (dirty_page) {
> + wait_on_page_locked(dirty_page);
> set_page_dirty_balance(dirty_page);
> put_page(dirty_page);
> }
> Index: linux-2.6/mm/page-writeback.c
now that's scary - applying this on top of your
lock-the-page-in-the-fault-handler patches gives:
if (dirty_page) {
/*
* Yes, Virginia, this is actually required to prevent a race
* with clear_page_dirty_for_io() from clearing the page dirty
* bit after it clear all dirty ptes, but before a racing
* do_wp_page installs a dirty pte.
*
* do_no_page is protected similarly.
*/
wait_on_page_locked(dirty_page);
wait_on_page_locked(dirty_page);
set_page_dirty_balance(dirty_page);
put_page(dirty_page);
}
One wonders how on earth patch(1) managed to do that. If it has inserted
the comment twice as well then it might be explicable..
Oh well, let's try this:
From: Nick Piggin <[EMAIL PROTECTED]>
Fix msync data loss and (less importantly) dirty page accounting
inaccuracies due to the race remaining in clear_page_dirty_for_io().
The deleted comment explains what the race was, and the added comments
explain how it is fixed.
Signed-off-by: Nick Piggin <[EMAIL PROTECTED]>
Cc: Linus Torvalds <[EMAIL PROTECTED]>
Cc: Miklos Szeredi <[EMAIL PROTECTED]>
Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
---
mm/memory.c | 9 +++++++++
mm/page-writeback.c | 17 ++++++++++++-----
2 files changed, 21 insertions(+), 5 deletions(-)
diff -puN mm/memory.c~mm-fix-cpdfio-vs-fault-race mm/memory.c
--- a/mm/memory.c~mm-fix-cpdfio-vs-fault-race
+++ a/mm/memory.c
@@ -1669,6 +1669,15 @@ gotten:
unlock:
pte_unmap_unlock(page_table, ptl);
if (dirty_page) {
+ /*
+ * Yes, Virginia, this is actually required to prevent a race
+ * with clear_page_dirty_for_io() from clearing the page dirty
+ * bit after it clear all dirty ptes, but before a racing
+ * do_wp_page installs a dirty pte.
+ *
+ * do_no_page is protected similarly.
+ */
+ wait_on_page_locked(dirty_page);
set_page_dirty_balance(dirty_page);
put_page(dirty_page);
}
diff -puN mm/page-writeback.c~mm-fix-cpdfio-vs-fault-race mm/page-writeback.c
--- a/mm/page-writeback.c~mm-fix-cpdfio-vs-fault-race
+++ a/mm/page-writeback.c
@@ -903,6 +903,8 @@ int clear_page_dirty_for_io(struct page
{
struct address_space *mapping = page_mapping(page);
+ BUG_ON(!PageLocked(page));
+
if (mapping && mapping_cap_account_dirty(mapping)) {
/*
* Yes, Virginia, this is indeed insane.
@@ -928,14 +930,19 @@ int clear_page_dirty_for_io(struct page
* We basically use the page "master dirty bit"
* as a serialization point for all the different
* threads doing their things.
- *
- * FIXME! We still have a race here: if somebody
- * adds the page back to the page tables in
- * between the "page_mkclean()" and the "TestClearPageDirty()",
- * we might have it mapped without the dirty bit set.
*/
if (page_mkclean(page))
set_page_dirty(page);
+ /*
+ * We carefully synchronise fault handlers against
+ * installing a dirty pte and marking the page dirty
+ * at this point. We do this by having them hold the
+ * page lock at some point after installing their
+ * pte, but before marking the page dirty.
+ * Pages are always locked coming in here, so we get
+ * the desired exclusion. See mm/memory.c:do_wp_page()
+ * for more comments.
+ */
if (TestClearPageDirty(page)) {
dec_zone_page_state(page, NR_FILE_DIRTY);
return 1;
_
-
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