From: Wu Fengguang Disable look-ahead for loop file. Loopback files normally contain filesystems, in which case there are already proper look-aheads in the upper layer, more look-aheads on the loopback file only ruins the read-ahead hit rate. Signed-off-by: Wu Fengguang Signed-off-by: Andrew Morton --- drivers/block/loop.c | 6 ++++++ include/linux/fs.h | 1 + mm/readahead.c | 4 ++++ 3 files changed, 11 insertions(+) diff -puN drivers/block/loop.c~readahead-loop-case drivers/block/loop.c --- a/drivers/block/loop.c~readahead-loop-case +++ a/drivers/block/loop.c @@ -757,6 +757,12 @@ static int loop_set_fd(struct loop_devic mapping = file->f_mapping; inode = mapping->host; + /* Instruct the readahead code to skip look-ahead on loop file. + * The upper layer should already do proper look-ahead, + * one more look-ahead here only ruins the cache hit rate. + */ + file->f_ra.flags |= RA_FLAG_LOOP; + if (!(file->f_mode & FMODE_WRITE)) lo_flags |= LO_FLAGS_READ_ONLY; diff -puN include/linux/fs.h~readahead-loop-case include/linux/fs.h --- a/include/linux/fs.h~readahead-loop-case +++ a/include/linux/fs.h @@ -744,6 +744,7 @@ struct file_ra_state { #define RA_FLAG_MISS (1UL<<31) /* a cache miss occured against this file */ #define RA_FLAG_INCACHE (1UL<<30) /* file is already in cache */ #define RA_FLAG_MMAP (1UL<<29) /* mmap page access */ +#define RA_FLAG_LOOP (1UL<<28) /* loopback file */ struct file { /* diff -puN mm/readahead.c~readahead-loop-case mm/readahead.c --- a/mm/readahead.c~readahead-loop-case +++ a/mm/readahead.c @@ -942,6 +942,10 @@ static unsigned long ra_submit(struct fi ra->lookahead_index = eof; } + /* Disable look-ahead for loopback file. */ + if (ra->flags & RA_FLAG_LOOP) + ra->lookahead_index = ra->readahead_index; + /* Take down the current read-ahead aging value. */ ra->age = nr_scanned_pages_node(numa_node_id()); _