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    Subject[PATCH 0/5] Direct Migration V8: Overview
    Page migration allows the moving of the physical location of pages between
    nodes in a numa system while the process is running. This means that the
    virtual addresses that the process sees do not change. However, the
    system rearranges the physical location of those pages.

    The main intend of page migration patches here is to reduce the latency of
    memory access by moving pages near to the processor where the process
    accessing that memory is running.

    The migration patchsets allow a process to manually relocate the node
    on which its pages are located through the MF_MOVE and MF_MOVE_ALL options
    while setting a new memory policy. The pages of process can also be relocated
    from another process using the sys_migrate_pages() function call. The
    migrate_pages function call takes two sets of nodes and moves pages of a
    process that are located on the from nodes to the destination nodes.

    Manual migration is very useful if for example the scheduler has relocated
    a process to a processor on a distant node. A batch scheduler or an
    administrator can detect the situation and move the pages of the process
    nearer to the new processor.

    Larger installations usually partition the system using cpusets into
    sections of nodes. Paul Jackson has equipped cpusets with the ability to
    move pages when a task is moved to another cpuset. This allows automatic
    control over locality of a process. If a task is moved to a new cpuset
    then also all its pages are moved with it so that the performance of the
    process does not sink dramatically (as is the case today).

    The swap migration patchset in 2.6.15-rc5-mm3 works by simply evicting
    the page. The pages must be faulted back in. The pages are then typically
    reallocated by the system near the node where the process is executing.
    For swap migration the destination of the move is controlled by the
    allocation policy. Cpusets set the allocation policy before calling
    sys_migrate_pages() in order to move the pages as intended.

    No allocation policy changes are performed for sys_migrate_pages(). This
    means that the pages may not faulted in to the specified nodes if no
    allocation policy was set by other means. The pages will just end up
    near the node where the fault occurred. The direct migration patchset
    extends the migration functionality to avoid going through swap.
    The destination node of the relation is controllable during the actual
    moving of pages. The crutch of using the allocation policy to relocate
    is not necessary any and the pages are moved directly to the target.

    The direct migration patchset allows the preservation of the relative
    location of pages within a group of nodes for all migration techniques which
    will preserve a particular memory allocation pattern generated even after
    migrating a process. This is necessary in order to preserve the memory
    latencies. Processes will run with similar performance after migration.

    This patch makes sys_migrate_pages() finally work as intended but does not
    do any significant modifications to APIs.

    Benefits over swap migration:

    1. It makes migrates_pages() actually migrate pages instead of just
    swapping pages from a set of nodes out.

    2. Its faster because the page does not need to be written to swap space.

    3. It does not use swap space and therefore there is no danger of running
    out of swap space.

    4. The need to write back a dirty page before migration is avoided through
    a file system specific method.

    5. Direct migration allows the preservation of the relative location of a page
    within a set of nodes.

    Many of the ideas for this code were originally developed in the memory
    hotplug project and we hope that this code also will allow the hotplug
    project to build on this patch in order to get to their goals.

    The patchset consists of five patches (only the first two are necessary to
    have basic direct migration support):

    1. SwapCache patch

    SwapCache pages may have changed their type after lock_page() if the
    page was migrated. Check for this and retry lookup if the page is no
    longer a SwapCache page.

    2. migrate_pages()

    Basic direct migration with fallback to swap if all other attempts

    3. remove_from_swap()

    Page migration installs swap ptes for anonymous pages in order to
    preserve the information contained in the page tables. This patch
    removes the swap ptes after migration and replaces them with regular

    4. upgrade of MPOL_MF_MOVE and sys_migrate_pages()

    Add logic to mm/mempolicy.c to allow the policy layer to control
    direct page migration. Thanks to Paul Jackson for the interative
    logic to move between sets of nodes.

    5. buffer_migrate_pages() patch

    Allow migration without writing back dirty pages. Add filesystem dependent
    migration support for ext2/ext3 and xfs. Use swapper space to setup a
    method to migrate anonymous pages without writeback.

    Credits (also in mm/vmscan.c):

    The idea for this scheme of page migration was first developed in the context
    of the memory hotplug project. The main authors of the migration code from
    the memory hotplug project are:

    IWAMOTO Toshihiro <>
    Hirokazu Takahashi <>
    Dave Hansen <>

    Changes V7->V8:
    - Patchset against 2.6.15-rc5-mm3
    - Export more functions so that filesystems are able to implement their own
    migrate_page() function.
    - Fix remove_from_swap() to remove the page from the swap cache in addition
    to replacing swap ptes. Call with the page lock on the new page.
    - Fix copying of struct page {} field to avoid using the macros that process
    field information.

    Changes V7->V7:
    - Rediff against 2.6.14-rc5-mm2

    Changes V6->V7:
    - Patchset agsinst 2.6.15-rc5-mm1
    - Fix one occurence of page->mapping in migrate_page_remove_references()
    - Update description]

    Changes V5->V6:
    - Patchset against 2.6.15-rc3-mm1
    - Remove checks for page count increases while migrating after Andrew assured
    me that this cannot happen. Revise documentation to reflect that. If this is
    the case then we will have no need to include the unwind code from the
    hotplug project in the future.
    - Wrong reference while calling remove_from_swap to page instead of newpage

    Changes V4->V5:
    - Patchset against 2.6.15-rc2-mm1
    - Update policy layer patch to use the generic check_range in 2.6.15-rc2-mm1.
    - Remove try_to_unmap patch since VM_RESERVED vanished under us and therefore
    there is no point anymore to distinguish between permament and transitional

    Changes V3->V4:
    - Patchset against 2.6.15-rc1-mm2 + two swap migration fixes posted today.
    - Remove what is already in 2.6.14-rc1-mm2 which results in a significant
    cleanup of the code.

    Changes V2->V3:
    - Patchset against 2.6.14-mm2
    - Fix single processor build and builds without CONFIG_MIGRATION
    - export symbols for filesystems that are modules and for
    modules using migrate_pages().
    - Paul Jackson's cpuset migration support is in 2.6.14-mm2 so
    this patchset can be easily applied to -mm2 to get from swap
    based to direct page migration.

    Changes V1->V2:
    - Call node_remap with the right parameters in do_migrate_pages().
    - Take radix tree lock while examining page count to avoid races with
    find_get_page() and various *_get_pages based on it.
    - Convert direct ptes to swap ptes before radix tree update to avoid
    more races.
    - Fix problem if CONFIG_MIGRATION is off for buffer_migrate_page
    - Add documentation about page migration
    - Change migrate_pages() api so that the caller can decide what
    to do about the migrated pages (badmem handling and hotplug
    have to remove those pages for good).
    - Drop config patch (already in mm)
    - Add try_to_unmap patch
    - Patchset now against 2.6.14-mm1 without requiring additional patches.

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      Last update: 2005-12-19 20:44    [W:0.030 / U:14.908 seconds]
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