Grouping Pages By Mobility

“‘The objective of this patchset is to keep the system in a
state where actions such as page reclaim or memory compaction will
reduce external fragmentation in the system,’ Mel Gorman described
his set of thirteen patches labeled ‘reduce external fragmentation
by grouping pages by mobility v30.’ He explained, ‘it works by
grouping pages of similar mobility together in PAGEBLOCK_NR_PAGES
areas.’ He defined four mobility types as: ‘UNMOVABLE–Pages that
cannot be trivially reclaimed or moved; MOVABLE–Pages that can be
moved using the page migration mechanism; RECLAIMABLE–Pages that
the kernel can often directly reclaim such as those used for inode
caches; RESERVE–The areas where min_free_kbyte-related pages
should be stored…'”