[patch] not to disturb page LRU state when unmapping memory range

["Ken Chen" <kenchen@google.com>]

I stomped on another piece of code in zap_pte_range() that is a bit
questionable: when kernel unmaps an address range, it needs to transfer
PTE state into page struct. Currently, kernel transfer both dirty bit
and access bit via set_page_dirty and mark_page_accessed.

set_page_dirty is necessary and required.  However, transfering access
bit doesn't look logical.  Kernel usually mark the page accessed at the
time of fault, for example shmem_nopage() does so.  At unmap, another
call to mark_page_accessed is called and this causes page LRU state to
be bumped up one step closer to more recently used state. It is causing
quite a bit headache in a scenario when a process creates a shmem segment,
touch a whole bunch of pages, then unmaps it. The unmapping takes a long
time because mark_page_accessed() will start moving pages from inactive
to active list.

I'm not too much concerned with moving the page from one list to another
in LRU. Sooner or later it might be moved because of multiple mappings
from various processes.  But it just doesn't look logical that when user
asks a range to be unmapped, it's his intention that the process is no
longer interested in these pages. Moving those pages to active list (or
bumping up a state towards more active) seems to be an over reaction. It
also prolongs unmapping latency which is the core issue I'm trying to solve.

Given that the LRU state is maintained properly at fault time, I think we
should remove it in the unmap path.

Signed-off-by: Ken Chen <kenchen@google.com>

---
Hugh, would you please review?

diff -Nurp linux-2.6.20-rc6/mm/memory.c linux-2.6.20-rc6.unmap/mm/memory.c
--- linux-2.6.20-rc6/mm/memory.c	2007-01-30 19:23:45.000000000 -0800
+++ linux-2.6.20-rc6.unmap/mm/memory.c	2007-01-30 19:25:38.000000000 -0800
@@ -677,8 +677,6 @@ static unsigned long zap_pte_range(struc
 			else {
 				if (pte_dirty(ptent))
 					set_page_dirty(page);
-				if (pte_young(ptent))
-					mark_page_accessed(page);
 				file_rss--;
 			}
 			page_remove_rmap(page, vma);

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