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    Subject[PATCH 8/9] readahead: basic support for backwards prefetching
    Add the backwards prefetching feature. It's pretty simple if we don't
    support async prefetching and interleaved reads.

    Here is the behavior with an 8-page read sequence from 10000 down to 0.
    (The readahead size is a bit large since it's an NFS mount.)

    readahead-random(dev=0:16, ino=3948605, req=10000+8, ra=10000+8-0, async=0) = 8
    readahead-backwards(dev=0:16, ino=3948605, req=9992+8, ra=9968+32-0, async=0) = 32
    readahead-backwards(dev=0:16, ino=3948605, req=9960+8, ra=9840+128-0, async=0) = 128
    readahead-backwards(dev=0:16, ino=3948605, req=9832+8, ra=9584+256-0, async=0) = 256
    readahead-backwards(dev=0:16, ino=3948605, req=9576+8, ra=9072+512-0, async=0) = 512
    readahead-backwards(dev=0:16, ino=3948605, req=9064+8, ra=8048+1024-0, async=0) = 1024
    readahead-backwards(dev=0:16, ino=3948605, req=8040+8, ra=6128+1920-0, async=0) = 1920
    readahead-backwards(dev=0:16, ino=3948605, req=6120+8, ra=4208+1920-0, async=0) = 1920
    readahead-backwards(dev=0:16, ino=3948605, req=4200+8, ra=2288+1920-0, async=0) = 1920
    readahead-backwards(dev=0:16, ino=3948605, req=2280+8, ra=368+1920-0, async=0) = 1920
    readahead-backwards(dev=0:16, ino=3948605, req=360+8, ra=0+368-0, async=0) = 368

    And a simple 1-page read sequence from 10000 down to 0.

    readahead-random(dev=0:16, ino=3948605, req=10000+1, ra=10000+1-0, async=0) = 1
    readahead-backwards(dev=0:16, ino=3948605, req=9999+1, ra=9996+4-0, async=0) = 4
    readahead-backwards(dev=0:16, ino=3948605, req=9995+1, ra=9980+16-0, async=0) = 16
    readahead-backwards(dev=0:16, ino=3948605, req=9979+1, ra=9916+64-0, async=0) = 64
    readahead-backwards(dev=0:16, ino=3948605, req=9915+1, ra=9660+256-0, async=0) = 256
    readahead-backwards(dev=0:16, ino=3948605, req=9659+1, ra=9148+512-0, async=0) = 512
    readahead-backwards(dev=0:16, ino=3948605, req=9147+1, ra=8124+1024-0, async=0) = 1024
    readahead-backwards(dev=0:16, ino=3948605, req=8123+1, ra=6204+1920-0, async=0) = 1920
    readahead-backwards(dev=0:16, ino=3948605, req=6203+1, ra=4284+1920-0, async=0) = 1920
    readahead-backwards(dev=0:16, ino=3948605, req=4283+1, ra=2364+1920-0, async=0) = 1920
    readahead-backwards(dev=0:16, ino=3948605, req=2363+1, ra=444+1920-0, async=0) = 1920
    readahead-backwards(dev=0:16, ino=3948605, req=443+1, ra=0+444-0, async=0) = 444

    CC: Andi Kleen <andi@firstfloor.org>
    CC: Li Shaohua <shaohua.li@intel.com>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    include/linux/fs.h | 2 ++
    mm/readahead.c | 15 +++++++++++++++
    2 files changed, 17 insertions(+)

    --- linux-next.orig/include/linux/fs.h 2011-11-29 20:55:27.000000000 +0800
    +++ linux-next/include/linux/fs.h 2011-11-29 20:57:07.000000000 +0800
    @@ -968,6 +968,7 @@ struct file_ra_state {
    * streams.
    * RA_PATTERN_MMAP_AROUND read-around on mmap page faults
    * (w/o any sequential/random hints)
    + * RA_PATTERN_BACKWARDS reverse reading detected
    * RA_PATTERN_FADVISE triggered by POSIX_FADV_WILLNEED or FMODE_RANDOM
    * RA_PATTERN_OVERSIZE a random read larger than max readahead size,
    * do max readahead to break down the read size
    @@ -978,6 +979,7 @@ enum readahead_pattern {
    RA_PATTERN_SUBSEQUENT,
    RA_PATTERN_CONTEXT,
    RA_PATTERN_MMAP_AROUND,
    + RA_PATTERN_BACKWARDS,
    RA_PATTERN_FADVISE,
    RA_PATTERN_OVERSIZE,
    RA_PATTERN_RANDOM,
    --- linux-next.orig/mm/readahead.c 2011-11-29 20:57:03.000000000 +0800
    +++ linux-next/mm/readahead.c 2011-11-29 20:57:07.000000000 +0800
    @@ -23,6 +23,7 @@ static const char * const ra_pattern_nam
    [RA_PATTERN_SUBSEQUENT] = "subsequent",
    [RA_PATTERN_CONTEXT] = "context",
    [RA_PATTERN_MMAP_AROUND] = "around",
    + [RA_PATTERN_BACKWARDS] = "backwards",
    [RA_PATTERN_FADVISE] = "fadvise",
    [RA_PATTERN_OVERSIZE] = "oversize",
    [RA_PATTERN_RANDOM] = "random",
    @@ -676,6 +677,20 @@ ondemand_readahead(struct address_space
    }

    /*
    + * backwards reading
    + */
    + if (offset < ra->start && offset + req_size >= ra->start) {
    + ra->pattern = RA_PATTERN_BACKWARDS;
    + ra->size = get_next_ra_size(ra, max);
    + max = ra->start;
    + if (ra->size > max)
    + ra->size = max;
    + ra->async_size = 0;
    + ra->start -= ra->size;
    + goto readit;
    + }
    +
    + /*
    * Query the page cache and look for the traces(cached history pages)
    * that a sequential stream would leave behind.
    */



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