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

[Johannes Weiner <hannes@saeurebad.de>]

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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