mm: dont mark_page_accessed in fault path

PATCH In review Nick Piggin 2 participants 2 messages Oct 21, 2008

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Memory management MM core Page cache

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Nick Piggin

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2008-10-21 07:25 UTC #0 lore raw

PATCH 1/1 mm: dont mark_page_accessed in fault path

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 <[email protected]>

---

mm/memory.c +1 -1

Index: 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);

mm/filemap.c +0 -1

Index: 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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Johannes Weiner

Acked-by on v1 1/1

2008-10-21 10:27 UTC #1 lore raw

Quoted 13 lines · 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).

Quoted 8 lines

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 <[email protected]>

Acked-by Johannes Weiner <[email protected]>