Red Hat Enterprise Linux 6 の /proc/meminfo に "MemAvailable" フィールドをバックポートする

Issue

• Red Hat Enterprise Linux 6 の/proc/meminfo に MemAvailable フィールドを追加するため、アップストリームコミット id 34e431b0ae398fc54ea69ff85ec700722c9da773 をバックポートします。

$ git show 34e431b0ae398fc54ea69ff85ec700722c9da773 | head -n 34
commit 34e431b0ae398fc54ea69ff85ec700722c9da773
Author:Rik van Riel <riel@redhat.com>
Date:   Tue Jan 21 15:49:05 2014 -0800

    /proc/meminfo: provide estimated available memory

    Many load balancing and workload placing programs check /proc/meminfo to
    estimate how much free memory is available.They generally do this by
    adding up "free" and "cached", which was fine ten years ago, but is
    pretty much guaranteed to be wrong today.

    It is wrong because Cached includes memory that is not freeable as page
    cache, for example shared memory segments, tmpfs, and ramfs, and it does
    not include reclaimable slab memory, which can take up a large fraction
    of system memory on mostly idle systems with lots of files.

    Currently, the amount of memory that is available for a new workload,
    without pushing the system into swap, can be estimated from MemFree,
    Active(file), Inactive(file), and SReclaimable, as well as the "low"
    watermarks from /proc/zoneinfo.

    However, this may change in the future, and user space really should not
    be expected to know kernel internals to come up with an estimate for the
    amount of free memory.

    It is more convenient to provide such an estimate in /proc/meminfo.If
    things change in the future, we only have to change it in one place.

    Signed-off-by:Rik van Riel <riel@redhat.com>
    Reported-by:Erik Mouw <erik.mouw_2@nxp.com>
    Acked-by:Johannes Weiner <hannes@cmpxchg.org>
    Signed-off-by:Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by:Linus Torvalds <torvalds@linux-foundation.org>