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    Subject[PATCH] (1/2) reverse mapping VM for 2.5.23 (rmap-13b)


    This patch implements Rik van Riel's patches for a reverse mapping VM
    atop the 2.5.23 kernel infrastructure. The principal sticky bits in
    the port are correct interoperability with Andrew Morton's patches to
    cleanup and extend the writeback and readahead code, among other things.
    This patch reinstates Rik's (active, inactive dirty, inactive clean)
    LRU list logic with the rmap information used for proper selection of pages
    for eviction and better page aging. It seems to do a pretty good job even
    for a first porting attempt. A simple, indicative test suite on a 192 MB
    PII machine (loading a large image in GIMP, loading other applications,
    heightening memory load to moderate swapout, then going back and
    manipulating the original Gimp image to test page aging, then closing all
    apps to the starting configuration) shows the following:

    2.5.22 vanilla:
    Total kernel swapouts during test = 29068 kB
    Total kernel swapins during test = 16480 kB
    Elapsed time for test: 141 seconds

    Total kernel swapouts during test = 40696 kB
    Total kernel swapins during test = 380 kB
    Elapsed time for test: 133 seconds

    Although rmap's page_launder evicts a ton of pages under load, it seems to
    swap the 'right' pages, as it doesn't need to swap them back in again.
    This is a good sign. [recent 2.4-aa work pretty nicely too]

    Various details for the curious or bored:

    - Tested: UP, 16 MB < mem < 256 MB, x86 arch.
    Untested: SMP, highmem, other archs.

    In particular, I didn't even attempt to port rmap-related
    changes to 2.5's arch/arm/mm/mm-armv.c.

    - page_launder() is coarse and tends to clean/flush too
    many pages at once. This is known behavior, but seems slightly
    worse in 2.5 for some reason.

    - pf_gfp_mask() doesn't exist in 2.5, nor does PF_NOIO. I have
    simply dropped the call in try_to_free_pages() in vmscan.c, but
    there is probably a way to reinstate its logic
    (i.e. avoid memory balancing I/O if the current task
    can't block on I/O). I didn't even attempt it.

    - Writeback: instead of forcing reinstating a page on the
    inactive list when !PageActive, page->mapping, !Pagedirty, and
    !PageWriteback (see mm/page-writeback.c, fs/mpage.c), I just
    let it go without any LRU list changes. If the page is
    inactive and needs attention, it'll end up on the inactive
    dirty list soon anyway, AFAICT. Seems okay so far, but that
    may be flawed/sloppy reasoning... We could always look at the
    page flags and reinstate the page to the appropriate LRU list
    (i.e. inactive clean or dirty) if this turns out to be a

    - Make shrink_[i,d,dq]cache_memory return the result of
    kmem_cache_shrink(), not simply 0. Seems pointless to waste
    that information, since we're getting it for free. Rik's patch
    wants that info anyway...

    - Readahead and drop_behind: With the new readahead code, we have
    some choices regarding under what circumstances we choose to
    drop_behind (i.e. only drop_behind if the reads look really
    sequential, etc...). This patch blindly calls drop_behind at
    the conclusion of page_cache_readahead(). Hopefully the
    drop_behind code correctly interprets the new readahead indices.
    It *seems* to behave correctly, but a quick look by another
    pair of eyes would be reassuring.

    - A couple of trivial rmap cleanups for Rik:
    a) Semicolon day! System fails to boot if rmap debugging
    is enabled in rmap.c. Fix is to remove the extraneous
    semicolon in page_add_rmap():

    if (!ptep_to_mm(ptep)); <--

    b) The pte_chain_unlock/lock() pair between the tests for
    "The page is in active use" and "Anonymous process
    memory without backing store" in vmscan.c seems

    c) Drop PG_launder page flag, ala current 2.5 tree.

    d) if(page_count(page)) == 0) ---> if(!page_count(page))
    and things like that...

    - To be consistent with 2.4-rmap, this patch includes a
    minimal BIO-ified port of Andrew Morton's read-latency2 patch
    (i.e. minus the elvtune ioctl stuff) to 2.5, from his patch
    sets. This adds about 7 kB to the patch.

    - The patch also includes compilation fixes:
    drivers/scsi/constants.c (undeclared integer variable)
    drivers/pci/pci-driver.c (unresolved symbol in pcmcia_core)
    include/linux/smp.h (define cpu_online_map for UP)
    kernel/ksyms.c (export default_wake_function for modules)
    arch/i386/i386_syms.c (export ioremap_nocache for modules)

    Hope this is of use to someone! It's certainly been a fun and
    instructive exercise for me so far. ;)

    I'll attempt to keep up with the 2.5 and rmap changes, fix inevitable
    bugs in porting, and will upload regular patches to the above URL, at
    least until the usual VM suspects start paying more attention to 2.5.
    I'll post a quick changelog to the list occasionally if and when any
    changes are significant, i.e. other then boring hand patching and

    Comments, feedback & patches always appreciated!

    Craig Kulesa
    Steward Observatory, Univ. of Arizona

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