[PATCH] readahead: fix sequential read cache miss detection

[Damien Ramonda]

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@xxxxxxxxx>
Reviewed-by: Fengguang Wu <fengguang.wu@xxxxxxxxx>
Acked-by: Pierre Tardy <pierre.tardy@xxxxxxxxx>
Acked-by: David Cohen <david.a.cohen@xxxxxxxxxxxxxxx>
---
 mm/readahead.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index e4ed041..5b637b5 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -401,6 +401,7 @@ ondemand_readahead(struct address_space *mapping,
 		   unsigned long req_size)
 {
 	unsigned long max = max_sane_readahead(ra->ra_pages);
+	pgoff_t prev_offset;
 
 	/*
 	 * start of file
@@ -452,8 +453,11 @@ ondemand_readahead(struct address_space *mapping,
 
 	/*
 	 * sequential cache miss
+	 * trivial case: (offset - prev_offset) == 1
+	 * unaligned reads: (offset - prev_offset) == 0
 	 */
-	if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
+	prev_offset = (unsigned long long)ra->prev_pos >> PAGE_CACHE_SHIFT;
+	if (offset - prev_offset <= 1UL)
 		goto initial_readahead;
 
 	/*
-- 
1.7.0.4

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