tree 35ca58b3831270a3015e64297eb2cbf3332439ca
parent c78e93630d15b5f5774213aad9bdc9f52473a89b
author Damien Ramonda <damien.ramonda@intel.com> 1384297696 -0800
committer Linus Torvalds <torvalds@linux-foundation.org> 1384312149 +0900

readahead: fix sequential read cache miss detection

The kernel's readahead algorithm sometimes interprets random read
accesses as sequential and triggers unnecessary data prefecthing from
storage device (impacting random read average latency).

In order to identify sequential cache read misses, the readahead
algorithm intends to check whether offset - previous offset == 1
(trivial sequential reads) or offset - previous offset == 0 (sequential
reads not aligned on page boundary):

  if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)

The current offset is stored in the "offset" variable of type "pgoff_t"
(unsigned long), while previous offset is stored in "ra->prev_pos" of
type "loff_t" (long long).  Therefore, operands of the if statement are
implicitly converted to type long long.  Consequently, when previous
offset > current offset (which happens on random pattern), the if
condition is true and access is wrongly interpeted as sequential.  An
unnecessary data prefetching is triggered, impacting the average random
read latency.

Storing the previous offset value in a "pgoff_t" variable (unsigned
long) fixes the sequential read detection logic.

Signed-off-by: Damien Ramonda <damien.ramonda@intel.com>
Reviewed-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Pierre Tardy <pierre.tardy@intel.com>
Acked-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>