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 <[email protected]>
---
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.
--