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Date: Tue, 27 Jun 2006 17:57:47 -0700
From: Andrew Morton <akpm@osdl.org>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, hugh@veritas.com,
       dhowells@redhat.com, a.p.zijlstra@chello.nl, christoph@lameter.com,
       mbligh@google.com, npiggin@suse.de, torvalds@osdl.org
Subject: Re: [PATCH 1/5] mm: tracking shared dirty pages
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Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
>
> Tracking of dirty pages in shared writeable mmap()s.

I mangled this a bit to fit it on top of Christoph's vm counters rewrite
(mm/page-writeback.c).

I worry about the changes to __set_page_dirty_nobuffers() and
test_clear_page_dirty().

They both already require that the page be locked (or that the
address_space be otherwise pinned).  But I'm not sure we get that right at
present.  With these changes, our exposure to that gets worse, and we
additionally are exposed to the possibility of the page itself being
reclaimed, and not just the address_space.

So ho hum.  I'll stick this:

--- a/mm/page-writeback.c~mm-tracking-shared-dirty-pages-checks
+++ a/mm/page-writeback.c
@@ -625,6 +625,7 @@ EXPORT_SYMBOL(write_one_page);
  */
 int __set_page_dirty_nobuffers(struct page *page)
 {
+	WARN_ON_ONCE(!PageLocked(page));
 	if (!TestSetPageDirty(page)) {
 		struct address_space *mapping = page_mapping(page);
 		struct address_space *mapping2;
@@ -722,6 +723,7 @@ int test_clear_page_dirty(struct page *p
 	struct address_space *mapping = page_mapping(page);
 	unsigned long flags;
 
+	WARN_ON_ONCE(!PageLocked(page));
 	if (mapping) {
 		write_lock_irqsave(&mapping->tree_lock, flags);
 		if (TestClearPageDirty(page)) {
_

in there.