On Thu, Aug 17 2006, Matt Domsch wrote:
> Jens, I'm hoping you can clear something up for me.
>
> ll_rw_blk.c:
> void blk_rq_bio_prep(request_queue_t *q, struct request *rq, struct
> bio *bio)
> {
> /* first three bits are identical in rq->flags and bio->bi_rw
> */
> rq->flags |= (bio->bi_rw & 7);
>
> That was recently changed to be the first 2 bits not 3. But it made
> me look.
>
> /*
> * bio bi_rw flags
> *
> * bit 0 -- read (not set) or write (set)
> * bit 1 -- rw-ahead when set
> * bit 2 -- barrier
> * bit 3 -- fail fast, don't want low level driver retries
> * bit 4 -- synchronous I/O hint: the block layer will unplug immediately
> */
> #define BIO_RW 0
> #define BIO_RW_AHEAD 1
> #define BIO_RW_BARRIER 2
> #define BIO_RW_FAILFAST 3
> #define BIO_RW_SYNC 4
>
>
> /*
> * first three bits match BIO_RW* bits, important
> */
> enum rq_flag_bits {
> __REQ_RW, /* not set, read. set, write */
> __REQ_FAILFAST, /* no low level driver retries */
> __REQ_SORTED, /* elevator knows about this request
> */
>
>
> The first bit matches. The second doesn't (BIO_RW_AHEAD vs
> __REQ_FAILFAST). And obviously you just fixed the third bit.
>
> Should BIO_RW_FAILFAST == __REQ_FAILFAST instead?
It does match, a rw-ahead request implies fail fast since we don't want
to be retrying read-head type IO. Could do with a better comment though,
but it's fully on purpose.
--
Jens Axboe
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