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mm: vmscan: clarify how swappiness, highest priority, memcg interact
A swappiness of 0 has a slightly different meaning for global reclaim (may swap if file cache really low) and memory cgroup reclaim (never swap, ever). In addition, global reclaim at highest priority will scan all LRU lists equal to their size and ignore other balancing heuristics. UNLESS swappiness forbids swapping, then the lists are balanced based on recent reclaim effectiveness. UNLESS file cache is running low, then anonymous pages are force-scanned. This (total mess of a) behaviour is implicit and not obvious from the way the code is organized. At least make it apparent in the code flow and document the conditions. It will be it easier to come up with sane semantics later. Signed-off-by: Johannes Weiner <firstname.lastname@example.org> Reviewed-by: Rik van Riel <email@example.com> Reviewed-by: Satoru Moriya <firstname.lastname@example.org> Reviewed-by: Michal Hocko <email@example.com> Acked-by: Mel Gorman <firstname.lastname@example.org> Cc: Hugh Dickins <email@example.com> Cc: Simon Jeons <firstname.lastname@example.org> Signed-off-by: Andrew Morton <email@example.com> Signed-off-by: Linus Torvalds <firstname.lastname@example.org>
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