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    mm: keep page cache radix tree nodes in check · 449dd698
    Johannes Weiner 提交于
    Previously, page cache radix tree nodes were freed after reclaim emptied
    out their page pointers.  But now reclaim stores shadow entries in their
    place, which are only reclaimed when the inodes themselves are
    reclaimed.  This is problematic for bigger files that are still in use
    after they have a significant amount of their cache reclaimed, without
    any of those pages actually refaulting.  The shadow entries will just
    sit there and waste memory.  In the worst case, the shadow entries will
    accumulate until the machine runs out of memory.
    
    To get this under control, the VM will track radix tree nodes
    exclusively containing shadow entries on a per-NUMA node list.  Per-NUMA
    rather than global because we expect the radix tree nodes themselves to
    be allocated node-locally and we want to reduce cross-node references of
    otherwise independent cache workloads.  A simple shrinker will then
    reclaim these nodes on memory pressure.
    
    A few things need to be stored in the radix tree node to implement the
    shadow node LRU and allow tree deletions coming from the list:
    
    1. There is no index available that would describe the reverse path
       from the node up to the tree root, which is needed to perform a
       deletion.  To solve this, encode in each node its offset inside the
       parent.  This can be stored in the unused upper bits of the same
       member that stores the node's height at no extra space cost.
    
    2. The number of shadow entries needs to be counted in addition to the
       regular entries, to quickly detect when the node is ready to go to
       the shadow node LRU list.  The current entry count is an unsigned
       int but the maximum number of entries is 64, so a shadow counter
       can easily be stored in the unused upper bits.
    
    3. Tree modification needs tree lock and tree root, which are located
       in the address space, so store an address_space backpointer in the
       node.  The parent pointer of the node is in a union with the 2-word
       rcu_head, so the backpointer comes at no extra cost as well.
    
    4. The node needs to be linked to an LRU list, which requires a list
       head inside the node.  This does increase the size of the node, but
       it does not change the number of objects that fit into a slab page.
    
    [akpm@linux-foundation.org: export the right function]
    Signed-off-by: NJohannes Weiner <hannes@cmpxchg.org>
    Reviewed-by: NRik van Riel <riel@redhat.com>
    Reviewed-by: NMinchan Kim <minchan@kernel.org>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Bob Liu <bob.liu@oracle.com>
    Cc: Christoph Hellwig <hch@infradead.org>
    Cc: Dave Chinner <david@fromorbit.com>
    Cc: Greg Thelen <gthelen@google.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Jan Kara <jack@suse.cz>
    Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Cc: Luigi Semenzato <semenzato@google.com>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Metin Doslu <metin@citusdata.com>
    Cc: Michel Lespinasse <walken@google.com>
    Cc: Ozgun Erdogan <ozgun@citusdata.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Roman Gushchin <klamm@yandex-team.ru>
    Cc: Ryan Mallon <rmallon@gmail.com>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
    449dd698
mmzone.h 40.0 KB