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SubjectRe: [PATCH Linux 2.6.12-rc2 02/04] blk: remove blk_queue_tag->real_max_depth optimization
Date
02_blk_tag_map_remove_real_max_depth.patch

blk_queue_tag->real_max_depth was used to optimize out
unnecessary allocations/frees on tag resize. However, the
whole thing was very broken - tag_map was never allocated to
real_max_depth resulting in access beyond the end of the map,
bits in [max_depth..real_max_depth] were set when initializing
a map and copied when resizing resulting in pre-occupied tags.

As the gain of the optimization is very small, well, almost
nill, remove the whole thing.

Signed-off-by: Tejun Heo <htejun@gmail.com>

drivers/block/ll_rw_blk.c | 35 ++++++++++-------------------------
include/linux/blkdev.h | 1 -
2 files changed, 10 insertions(+), 26 deletions(-)

Index: blk-fixes/drivers/block/ll_rw_blk.c
===================================================================
--- blk-fixes.orig/drivers/block/ll_rw_blk.c 2005-04-20 20:36:36.000000000 +0900
+++ blk-fixes/drivers/block/ll_rw_blk.c 2005-04-20 20:36:37.000000000 +0900
@@ -716,7 +716,7 @@ struct request *blk_queue_find_tag(reque
{
struct blk_queue_tag *bqt = q->queue_tags;

- if (unlikely(bqt == NULL || tag >= bqt->real_max_depth))
+ if (unlikely(bqt == NULL || tag >= bqt->max_depth))
return NULL;

return bqt->tag_index[tag];
@@ -774,9 +774,9 @@ EXPORT_SYMBOL(blk_queue_free_tags);
static int
init_tag_map(request_queue_t *q, struct blk_queue_tag *tags, int depth)
{
- int bits, i;
struct request **tag_index;
unsigned long *tag_map;
+ int nr_ulongs;

if (depth > q->nr_requests * 2) {
depth = q->nr_requests * 2;
@@ -788,24 +788,17 @@ init_tag_map(request_queue_t *q, struct
if (!tag_index)
goto fail;

- bits = (depth / BLK_TAGS_PER_LONG) + 1;
- tag_map = kmalloc(bits * sizeof(unsigned long), GFP_ATOMIC);
+ nr_ulongs = ALIGN(depth, BLK_TAGS_PER_LONG) / BLK_TAGS_PER_LONG;
+ tag_map = kmalloc(nr_ulongs * sizeof(unsigned long), GFP_ATOMIC);
if (!tag_map)
goto fail;

memset(tag_index, 0, depth * sizeof(struct request *));
- memset(tag_map, 0, bits * sizeof(unsigned long));
+ memset(tag_map, 0, nr_ulongs * sizeof(unsigned long));
tags->max_depth = depth;
- tags->real_max_depth = bits * BITS_PER_LONG;
tags->tag_index = tag_index;
tags->tag_map = tag_map;

- /*
- * set the upper bits if the depth isn't a multiple of the word size
- */
- for (i = depth; i < bits * BLK_TAGS_PER_LONG; i++)
- __set_bit(i, tag_map);
-
return 0;
fail:
kfree(tag_index);
@@ -870,32 +863,24 @@ int blk_queue_resize_tags(request_queue_
struct blk_queue_tag *bqt = q->queue_tags;
struct request **tag_index;
unsigned long *tag_map;
- int bits, max_depth;
+ int max_depth, nr_ulongs;

if (!bqt)
return -ENXIO;

/*
- * don't bother sizing down
- */
- if (new_depth <= bqt->real_max_depth) {
- bqt->max_depth = new_depth;
- return 0;
- }
-
- /*
* save the old state info, so we can copy it back
*/
tag_index = bqt->tag_index;
tag_map = bqt->tag_map;
- max_depth = bqt->real_max_depth;
+ max_depth = bqt->max_depth;

if (init_tag_map(q, bqt, new_depth))
return -ENOMEM;

memcpy(bqt->tag_index, tag_index, max_depth * sizeof(struct request *));
- bits = max_depth / BLK_TAGS_PER_LONG;
- memcpy(bqt->tag_map, tag_map, bits * sizeof(unsigned long));
+ nr_ulongs = ALIGN(max_depth, BLK_TAGS_PER_LONG) / BLK_TAGS_PER_LONG;
+ memcpy(bqt->tag_map, tag_map, nr_ulongs * sizeof(unsigned long));

kfree(tag_index);
kfree(tag_map);
@@ -925,7 +910,7 @@ void blk_queue_end_tag(request_queue_t *

BUG_ON(tag == -1);

- if (unlikely(tag >= bqt->real_max_depth))
+ if (unlikely(tag >= bqt->max_depth))
return;

if (unlikely(!__test_and_clear_bit(tag, bqt->tag_map))) {
Index: blk-fixes/include/linux/blkdev.h
===================================================================
--- blk-fixes.orig/include/linux/blkdev.h 2005-04-20 20:36:35.000000000 +0900
+++ blk-fixes/include/linux/blkdev.h 2005-04-20 20:36:37.000000000 +0900
@@ -294,7 +294,6 @@ struct blk_queue_tag {
struct list_head busy_list; /* fifo list of busy tags */
int busy; /* current depth */
int max_depth; /* what we will send to device */
- int real_max_depth; /* what the array can hold */
atomic_t refcnt; /* map can be shared */
};

-
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