[patch] mm: dont mark_page_accessed in fault path

[patch] mm: dont mark_page_accessed in fault path

[Nick Piggin]

Doing a mark_page_accessed at fault-time, then doing SetPageReferenced at
unmap-time if the pte is young has a number of problems.

mark_page_accessed is supposed to be roughly the equivalent of a young pte
for unmapped references. Unfortunately it doesn't come with any context:
after being called, reclaim doesn't know who or why the page was touched.

So calling mark_page_accessed not only adds extra lru or PG_referenced
manipulations for pages that are already going to have pte_young ptes anyway,
but it also adds these references which are difficult to work with from the
context of vma specific references (eg. MADV_SEQUENTIAL pte_young may not
wish to contribute to the page being referenced).

Then, simply doing SetPageReferenced when zapping a pte and finding it is
young, is not a really good solution either. SetPageReferenced does not
correctly promote the page to the active list for example. So after removing
mark_page_accessed from the fault path, several mmap()+touch+munmap() would
have a very different result from several read(2) calls for example, which
is not really desirable.

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c
+++ linux-2.6/mm/memory.c
@@ -758,7 +758,7 @@ static unsigned long zap_pte_range(struc
 				if (pte_dirty(ptent))
 					set_page_dirty(page);
 				if (pte_young(ptent))
-					SetPageReferenced(page);
+					mark_page_accessed(page);
 				file_rss--;
 			}
 			page_remove_rmap(page, vma);
Index: linux-2.6/mm/filemap.c
===================================================================
--- linux-2.6.orig/mm/filemap.c
+++ linux-2.6/mm/filemap.c
@@ -1530,7 +1530,6 @@ retry_find:
 	/*
 	 * Found the page and have a reference on it.
 	 */
-	mark_page_accessed(page);
 	ra->prev_pos = (loff_t)page->index << PAGE_CACHE_SHIFT;
 	vmf->page = page;
 	return ret | VM_FAULT_LOCKED;

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Re: [patch] mm: dont mark_page_accessed in fault path

[Johannes Weiner]

Nick Piggin <npiggin@suse.de> writes:

> Doing a mark_page_accessed at fault-time, then doing SetPageReferenced at
> unmap-time if the pte is young has a number of problems.
>
> mark_page_accessed is supposed to be roughly the equivalent of a young pte
> for unmapped references. Unfortunately it doesn't come with any context:
> after being called, reclaim doesn't know who or why the page was touched.
>
> So calling mark_page_accessed not only adds extra lru or PG_referenced
> manipulations for pages that are already going to have pte_young ptes anyway,
> but it also adds these references which are difficult to work with from the
> context of vma specific references (eg. MADV_SEQUENTIAL pte_young may not
> wish to contribute to the page being referenced).
>
> Then, simply doing SetPageReferenced when zapping a pte and finding it is
> young, is not a really good solution either. SetPageReferenced does not
> correctly promote the page to the active list for example. So after removing
> mark_page_accessed from the fault path, several mmap()+touch+munmap() would
> have a very different result from several read(2) calls for example, which
> is not really desirable.
>
> Signed-off-by: Nick Piggin <npiggin@suse.de>

  Acked-by: Johannes Weiner <hannes@saeurebad.de>

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