• Eric B Munson's avatar
    mm: allow compaction of unevictable pages · 5bbe3547
    Eric B Munson authored
    Currently, pages which are marked as unevictable are protected from
    compaction, but not from other types of migration.  The POSIX real time
    extension explicitly states that mlock() will prevent a major page
    fault, but the spirit of this is that mlock() should give a process the
    ability to control sources of latency, including minor page faults.
    However, the mlock manpage only explicitly says that a locked page will
    not be written to swap and this can cause some confusion.  The
    compaction code today does not give a developer who wants to avoid swap
    but wants to have large contiguous areas available any method to achieve
    this state.  This patch introduces a sysctl for controlling compaction
    behavior with respect to the unevictable lru.  Users who demand no page
    faults after a page is present can set compact_unevictable_allowed to 0
    and users who need the large contiguous areas can enable compaction on
    locked memory by leaving the default value of 1.
    To illustrate this problem I wrote a quick test program that mmaps a
    large number of 1MB files filled with random data.  These maps are
    created locked and read only.  Then every other mmap is unmapped and I
    attempt to allocate huge pages to the static huge page pool.  When the
    compact_unevictable_allowed sysctl is 0, I cannot allocate hugepages
    after fragmenting memory.  When the value is set to 1, allocations
    Signed-off-by: default avatarEric B Munson <emunson@akamai.com>
    Acked-by: default avatarMichal Hocko <mhocko@suse.cz>
    Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
    Acked-by: default avatarChristoph Lameter <cl@linux.com>
    Acked-by: default avatarDavid Rientjes <rientjes@google.com>
    Acked-by: default avatarRik van Riel <riel@redhat.com>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Christoph Lameter <cl@linux.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Michal Hocko <mhocko@suse.cz>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
sysctl.c 63.3 KB