Loopback files normally contain filesystem, 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 --- I'd like to thank Tero Grundstr?m for uncovering the loopback problem. drivers/block/loop.c | 6 ++++++ include/linux/fs.h | 1 + mm/readahead.c | 8 ++++++++ 3 files changed, 15 insertions(+) --- linux.orig/include/linux/fs.h +++ linux/include/linux/fs.h @@ -623,6 +623,7 @@ struct file_ra_state { #define RA_FLAG_MISS 0x01 /* a cache miss occured against this file */ #define RA_FLAG_INCACHE 0x02 /* file is already in cache */ #define RA_FLAG_MMAP (1UL<<31) /* mmaped page access */ +#define RA_FLAG_NO_LOOKAHEAD (1UL<<30) /* disable look-ahead */ struct file { /* --- linux.orig/drivers/block/loop.c +++ linux/drivers/block/loop.c @@ -782,6 +782,12 @@ static int loop_set_fd(struct loop_devic mapping = file->f_mapping; inode = mapping->host; + /* + * 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_NO_LOOKAHEAD; + if (!(file->f_mode & FMODE_WRITE)) lo_flags |= LO_FLAGS_READ_ONLY; --- linux.orig/mm/readahead.c +++ linux/mm/readahead.c @@ -1186,6 +1186,11 @@ static inline void ra_state_update(struc if (ra_size < old_ra && ra_cache_hit(ra, 0)) ra_account(ra, RA_EVENT_READAHEAD_SHRINK, old_ra - ra_size); #endif + + /* Disable look-ahead for loopback file. */ + if (unlikely(ra->flags & RA_FLAG_NO_LOOKAHEAD)) + la_size = 0; + ra_addup_cache_hit(ra); ra->ra_index = ra->readahead_index; ra->la_index = ra->lookahead_index; @@ -1546,6 +1551,9 @@ static int query_page_cache(struct addre if (count < ra_max) goto out; + if (unlikely(ra->flags & RA_FLAG_NO_LOOKAHEAD)) + goto out; + /* * Check the far pages coarsely. * The big count here helps increase la_size. --