• Vladimir Davydov's avatar
    mm: introduce idle page tracking · 33c3fc71
    Vladimir Davydov authored
    Knowing the portion of memory that is not used by a certain application or
    memory cgroup (idle memory) can be useful for partitioning the system
    efficiently, e.g.  by setting memory cgroup limits appropriately.
    Currently, the only means to estimate the amount of idle memory provided
    by the kernel is /proc/PID/{clear_refs,smaps}: the user can clear the
    access bit for all pages mapped to a particular process by writing 1 to
    clear_refs, wait for some time, and then count smaps:Referenced.  However,
    this method has two serious shortcomings:
     - it does not count unmapped file pages
     - it affects the reclaimer logic
    To overcome these drawbacks, this patch introduces two new page flags,
    Idle and Young, and a new sysfs file, /sys/kernel/mm/page_idle/bitmap.
    A page's Idle flag can only be set from userspace by setting bit in
    /sys/kernel/mm/page_idle/bitmap at the offset corresponding to the page,
    and it is cleared whenever the page is accessed either through page tables
    (it is cleared in page_referenced() in this case) or using the read(2)
    system call (mark_page_accessed()). Thus by setting the Idle flag for
    pages of a particular workload, which can be found e.g.  by reading
    /proc/PID/pagemap, waiting for some time to let the workload access its
    working set, and then reading the bitmap file, one can estimate the amount
    of pages that are not used by the workload.
    The Young page flag is used to avoid interference with the memory
    reclaimer.  A page's Young flag is set whenever the Access bit of a page
    table entry pointing to the page is cleared by writing to the bitmap file.
    If page_referenced() is called on a Young page, it will add 1 to its
    return value, therefore concealing the fact that the Access bit was
    Note, since there is no room for extra page flags on 32 bit, this feature
    uses extended page flags when compiled on 32 bit.
    [akpm@linux-foundation.org: fix build]
    [akpm@linux-foundation.org: kpageidle requires an MMU]
    [akpm@linux-foundation.org: decouple from page-flags rework]
    Signed-off-by: default avatarVladimir Davydov <vdavydov@parallels.com>
    Reviewed-by: default avatarAndres Lagar-Cavilla <andreslc@google.com>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Michal Hocko <mhocko@suse.cz>
    Cc: Greg Thelen <gthelen@google.com>
    Cc: Michel Lespinasse <walken@google.com>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Pavel Emelyanov <xemul@parallels.com>
    Cc: Cyrill Gorcunov <gorcunov@openvz.org>
    Cc: Jonathan Corbet <corbet@lwn.net>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>