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block: add warning to blk_make_request()

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Add a note about how one needs to be careful when setting up these bio
chains.

Extracted from Boaz's updated patch.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
This commit is contained in:
Jens Axboe 2009-05-19 19:52:35 +02:00
parent a411f4bbb8
commit 53674ac5a9
1 changed files with 9 additions and 0 deletions
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9
block/blk-core.c
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@ -910,6 +910,15 @@ EXPORT_SYMBOL(blk_get_request);
* need bouncing, by calling the appropriate masked or flagged allocator,
* suitable for the target device. Otherwise the call to blk_queue_bounce will
* BUG.
*
* WARNING: When allocating/cloning a bio-chain, careful consideration should be
* given to how you allocate bios. In particular, you cannot use __GFP_WAIT for
* anything but the first bio in the chain. Otherwise you risk waiting for IO
* completion of a bio that hasn't been submitted yet, thus resulting in a
* deadlock. Alternatively bios should be allocated using bio_kmalloc() instead
* of bio_alloc(), as that avoids the mempool deadlock.
* If possible a big IO should be split into smaller parts when allocation
* fails. Partial allocation should not be an error, or you risk a live-lock.
*/
struct request *blk_make_request(struct request_queue *q, struct bio *bio,
gfp_t gfp_mask)