mm: fix clear_page_dirty_for_io vs fault race

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 <npiggin@suse.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index 61d51da..50dd3d1 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1765,6 +1765,15 @@ 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 --git a/mm/page-writeback.c b/mm/page-writeback.c
index 886ea0d..e624827 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -918,6 +918,8 @@ int clear_page_dirty_for_io(struct page *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.
@@ -943,14 +945,19 @@ int clear_page_dirty_for_io(struct page *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;