Re: [PATCH 01/16] mm: delayed page activation

[Nikita Danilov <nikita@clusterfs.com>]

Wu Fengguang writes:
 > When a page is referenced the second time in inactive_list, mark it with
 > PG_activate instead of moving it into active_list immediately. The actual
 > moving work is delayed to vmscan time.
 > 
 > This implies two essential changes:
 > - keeps the adjecency of pages in lru;

But this change destroys LRU ordering: at the time when shrink_list()
inspects PG_activate bit, information about order in which
mark_page_accessed() was called against pages is lost. E.g., suppose
inactive list initially contained pages

     /* head */ (P1, P2, P3) /* tail */

all of them referenced. Then mark_page_accessed(), is called against P1,
P2, and P3 (in that order). With the old code active list would end up 

     /* head */ (P3, P2, P1) /* tail */

which corresponds to LRU. With delayed page activation, pages are moved
to head of the active list in the order they are analyzed by
shrink_list(), which gives

     /* head */ (P1, P2, P3) /* tail */

on the active list, that is _inverse_ LRU order.

Nikita.