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    Subject[PATCH V2 0/4] Frontswap (was Transcendent Memory): overview
    [PATCH V2 0/4] Frontswap (was Transcendent Memory): overview

    Changes since V1:
    - Rebased to 2.6.34 (no functional changes)
    - Convert to sane types (per Al Viro comment in cleancache thread)
    - Define some raw constants (Konrad Wilk)
    - Performance analysis shows significant advantage for frontswap's
    synchronous page-at-a-time design (vs batched asynchronous speculated
    as an alternative design). See

    In previous patch postings, frontswap was part of the Transcendent
    Memory ("tmem") patchset. This patchset refocuses not on the underlying
    technology (tmem) but instead on the useful functionality provided for Linux,
    and provides a clean API so that frontswap can provide this very useful
    functionality via a Xen tmem driver OR completely independent of tmem.
    For example: an in-kernel compression "backend" for frontswap can be
    implemented and some believe frontswap will be a very nice interface
    for building RAM-like functionality for pseudo-RAM devices such as
    on-memory-bus SSD or phase-change memory; and a Pune University team
    is looking at a backend for virtio (see OLS'2010).

    A more complete description of frontswap can be found in the introductory
    comment in mm/frontswap.c (in PATCH 2/4) which is included below
    for convenience.

    Note that an earlier version of this patch is now shipping in OpenSuSE 11.2
    and will soon ship in a release of Oracle Enterprise Linux. Underlying
    tmem technology is now shipping in Oracle VM 2.2 and was just released
    in Xen 4.0 on April 15, 2010. (Search for Transcendent

    Signed-off-by: Dan Magenheimer <>
    Reviewed-by: Jeremy Fitzhardinge <>

    include/linux/frontswap.h | 98 ++++++++++++++
    include/linux/swap.h | 2
    include/linux/swapfile.h | 13 +
    mm/Kconfig | 16 ++
    mm/Makefile | 1
    mm/frontswap.c | 301 ++++++++++++++++++++++++++++++++++++++++++++++
    mm/page_io.c | 10 +
    mm/swapfile.c | 59 +++++++--
    8 files changed, 491 insertions(+), 9 deletions(-)

    Frontswap is so named because it can be thought of as the opposite of
    a "backing" store for a swap device. The storage is assumed to be
    a synchronous concurrency-safe page-oriented pseudo-RAM device (such as
    Xen's Transcendent Memory, aka "tmem", or in-kernel compressed memory,
    aka "zmem", or other RAM-like devices) which is not directly accessible
    or addressable by the kernel and is of unknown and possibly time-varying
    size. This pseudo-RAM device links itself to frontswap by setting the
    frontswap_ops pointer appropriately and the functions it provides must
    conform to certain policies as follows:

    An "init" prepares the pseudo-RAM to receive frontswap pages and returns
    a non-negative pool id, used for all swap device numbers (aka "type").
    A "put_page" will copy the page to pseudo-RAM and associate it with
    the type and offset associated with the page. A "get_page" will copy the
    page, if found, from pseudo-RAM into kernel memory, but will NOT remove
    the page from pseudo-RAM. A "flush_page" will remove the page from
    pseudo-RAM and a "flush_area" will remove ALL pages associated with the
    swap type (e.g., like swapoff) and notify the pseudo-RAM device to refuse
    further puts with that swap type.

    Once a page is successfully put, a matching get on the page will always
    succeed. So when the kernel finds itself in a situation where it needs
    to swap out a page, it first attempts to use frontswap. If the put returns
    non-zero, the data has been successfully saved to pseudo-RAM and
    a disk write and, if the data is later read back, a disk read are avoided.
    If a put returns zero, pseudo-RAM has rejected the data, and the page can
    be written to swap as usual.

    Note that if a page is put and the page already exists in pseudo-RAM
    (a "duplicate" put), either the put succeeds and the data is overwritten,
    or the put fails AND the page is flushed. This ensures stale data may
    never be obtained from pseudo-RAM.

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      Last update: 2010-05-28 19:45    [W:0.026 / U:3.888 seconds]
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