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 CC: Li Shaohua Signed-off-by: Wu Fengguang --- 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. */ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/