Resurrect 'try_to_free_buffers()' VM hackery

It's not pretty, but it appears that ext3 with data=journal will clean
pages without ever actually telling the VM that they are clean.  This,
in turn, will result in the VM (and balance_dirty_pages() in particular)
to never realize that the pages got cleaned, and wait forever for an
event that already happened.

Technically, this seems to be a problem with ext3 itself, but it used to
be hidden by 'try_to_free_buffers()' noticing this situation on its own,
and just working around the filesystem problem.

This commit re-instates that hack, in order to avoid a regression for
the 2.6.20 release. This fixes bugzilla 7844:

	http://bugzilla.kernel.org/show_bug.cgi?id=7844

Peter Zijlstra points out that we should probably retain the debugging
code that this removes from cancel_dirty_page(), and I agree, but for
the imminent release we might as well just silence the warning too
(since it's not a new bug: anything that triggers that warning has been
around forever).

Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 28 insertions, 8 deletions

diff --git a/fs/buffer.c b/fs/buffer.c
index 3b116078b4c3..460f1c43238e 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2834,7 +2834,7 @@ int try_to_free_buffers(struct page *page)
         int ret = 0;
 
         BUG_ON(!PageLocked(page));
-        if (PageDirty(page) || PageWriteback(page))
+        if (PageWriteback(page))
                 return 0;
 
         if (mapping == NULL) {          /* can this still happen? */
@@ -2845,6 +2845,19 @@ int try_to_free_buffers(struct page *page)
         spin_lock(&mapping->private_lock);
         ret = drop_buffers(page, &buffers_to_free);
         spin_unlock(&mapping->private_lock);
+
+        /*
+         * If the filesystem writes its buffers by hand (eg ext3)
+         * then we can have clean buffers against a dirty page.  We
+         * clean the page here; otherwise the VM will never notice
+         * that the filesystem did any IO at all.
+         *
+         * Also, during truncate, discard_buffer will have marked all
+         * the page's buffers clean.  We discover that here and clean
+         * the page also.
+         */
+        if (ret)
+                cancel_dirty_page(page, PAGE_CACHE_SIZE);
 out:
         if (buffers_to_free) {
                 struct buffer_head *bh = buffers_to_free;
diff --git a/mm/truncate.c b/mm/truncate.c
index 6c79ca4a1ca7..3262740aa059 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -51,15 +51,22 @@ static inline void truncate_partial_page(struct page *page, unsigned partial)
                 do_invalidatepage(page, partial);
 }
 
+/*
+ * This cancels just the dirty bit on the kernel page itself, it
+ * does NOT actually remove dirty bits on any mmap's that may be
+ * around. It also leaves the page tagged dirty, so any sync
+ * activity will still find it on the dirty lists, and in particular,
+ * clear_page_dirty_for_io() will still look at the dirty bits in
+ * the VM.
+ *
+ * Doing this should *normally* only ever be done when a page
+ * is truncated, and is not actually mapped anywhere at all. However,
+ * fs/buffer.c does this when it notices that somebody has cleaned
+ * out all the buffers on a page without actually doing it through
+ * the VM. Can you say "ext3 is horribly ugly"? Tought you could.
+ */
 void cancel_dirty_page(struct page *page, unsigned int account_size)
 {
-        /* If we're cancelling the page, it had better not be mapped any more */
-        if (page_mapped(page)) {
-                static unsigned int warncount;
-
-                WARN_ON(++warncount < 5);
-        }
-                
         if (TestClearPageDirty(page)) {
                 struct address_space *mapping = page->mapping;
                 if (mapping && mapping_cap_account_dirty(mapping)) {