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When cfq I/O scheduler is selected, get_request() in __make_request() calls 

__cfq_get_queue().  __cfq_get_queue() finds an existing queue (struct
cfq_queue) of the current process for the device and returns it.  If it's not
found, __cfq_get_queue() creates and returns a new one if __cfq_get_queue() is
called with __GFP_WAIT flag, or __cfq_get_queue() returns NULL (this means that
get_request() fails) if no __GFP_WAIT flag.

On the other hand, in __make_request(), get_request() is called without
__GFP_WAIT flag at the first time.  Thus, the get_request() fails when there is
no existing queue, typically when it's called for the first I/O request of the
process to the device.

Though it will be followed by get_request_wait() for general case,
__make_request() will just end the I/O with an error (EWOULDBLOCK) when the
request was for read-ahead.

Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
This commit is contained in:
Kiyoshi Ueda 2005-06-17 16:15:10 +02:00 committed by Jens Axboe
parent c374f127e4
commit db3b5848ea
1 changed files with 6 additions and 3 deletions
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9
drivers/block/cfq-iosched.c
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@ -1202,13 +1202,16 @@ retry:
if (new_cfqq) {
cfqq = new_cfqq;
new_cfqq = NULL;
} else if (gfp_mask & __GFP_WAIT) {
} else {
spin_unlock_irq(cfqd->queue->queue_lock);
new_cfqq = kmem_cache_alloc(cfq_pool, gfp_mask);
spin_lock_irq(cfqd->queue->queue_lock);
if (!new_cfqq && !(gfp_mask & __GFP_WAIT))
goto out;
goto retry;
} else
goto out;
}
memset(cfqq, 0, sizeof(*cfqq));