On Mon, 02 Nov 2015, Andrea Arcangeli wrote:
The rmap walk must reach every possible mapping of the page, so if a page is heavily shared (no matter if it's KSM, anon, pagecache) there will be tons of entries to walk through. All optimizations with prio_tree, anon_vma chains, interval tree, helps to find the right virtual mapping faster, but if there are lots of virtual mappings, all mapping must still be walked through. The biggest cost is for the IPIs, but regardless of the IPIs, it's generally safer to keep these cond_resched() in all cases, as even if we massively reduce the number of IPIs, the number of entries to walk IPI-less may still be large and no entry can be possibly skipped in the page migration case. Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
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