• Hugh Dickins's avatar
    ksm: memory hotremove migration only · 62b61f61
    Hugh Dickins authored
    The previous patch enables page migration of ksm pages, but that soon gets
    into trouble: not surprising, since we're using the ksm page lock to lock
    operations on its stable_node, but page migration switches the page whose
    lock is to be used for that.  Another layer of locking would fix it, but
    do we need that yet?
    
    Do we actually need page migration of ksm pages?  Yes, memory hotremove
    needs to offline sections of memory: and since we stopped allocating ksm
    pages with GFP_HIGHUSER, they will tend to be GFP_HIGHUSER_MOVABLE
    candidates for migration.
    
    But KSM is currently unconscious of NUMA issues, happily merging pages
    from different NUMA nodes: at present the rule must be, not to use
    MADV_MERGEABLE where you care about NUMA.  So no, NUMA page migration of
    ksm pages does not make sense yet.
    
    So, to complete support for ksm swapping we need to make hotremove safe.
    ksm_memory_callback() take ksm_thread_mutex when MEM_GOING_OFFLINE and
    release it when MEM_OFFLINE or MEM_CANCEL_OFFLINE.  But if mapped pages
    are freed before migration reaches them, stable_nodes may be left still
    pointing to struct pages which have been removed from the system: the
    stable_node needs to identify a page by pfn rather than page pointer, then
    it can safely prune them when MEM_OFFLINE.
    
    And make NUMA migration skip PageKsm pages where it skips PageReserved.
    But it's only when we reach unmap_and_move() that the page lock is taken
    and we can be sure that raised pagecount has prevented a PageAnon from
    being upgraded: so add offlining arg to migrate_pages(), to migrate ksm
    page when offlining (has sufficient locking) but reject it otherwise.
    Signed-off-by: default avatarHugh Dickins <hugh.dickins@tiscali.co.uk>
    Cc: Izik Eidus <ieidus@redhat.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Chris Wright <chrisw@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    62b61f61
memory_hotplug.c 21.4 KB