Re: [PATCH]: Implementation of blk_rq_map_kern_sg() (aka New implementation of scsi_execute_async() v3)

[Jens Axboe <jens.axboe@oracle.com>]

On Wed, Aug 12 2009, Vladislav Bolkhovitin wrote:
> This patch implements function blk_rq_map_kern_sg(), which allows to map
> a kernel-originated SG vector to a block request. It is necessary to execute
> SCSI commands with from kernel going SG buffer. At the moment SCST is the only
> user of this functionality. It needs it, because its target drivers, which
> are, basically, SCSI drivers, can deal only with SGs, not with BIOs. But,
> according to the latest discussions, there can be other potential users for of
> this functionality, so I'm sending this patch in a hope that it will be
> also useful for them and eventually will be merged in the mainline kernel.
> 
> In the previous submissions this patch was called "New implementation of
> scsi_execute_async()", but since in this version scsi_execute_async() was
> removed from it by request of Boaz Harrosh the name was changed accordingly.

Generally this patch looks great, I just have one little thing I'd like
to point out:

> +	while (hbio != NULL) {
> +		bio = hbio;
> +		hbio = hbio->bi_next;
> +		bio->bi_next = NULL;
> +
> +		blk_queue_bounce(q, &bio);
> +
> +		res = blk_rq_append_bio(q, rq, bio);
> +		if (unlikely(res != 0)) {
> +			bio->bi_next = hbio;
> +			hbio = bio;
> +			/* We can have one or more bios bounced */
> +			goto out_unmap_bios;
> +		}
> +	}

Constructs like this are always dangerous, because of how mempools work.
__blk_queue_bounce() will internally do:

        bio = bio_alloc(GFP_NOIO, cnt);

so you could potentially enter a deadlock if a) you are the only one
allocating a bio currently, and b) the alloc fails and we wait for a bio
to be returned to the pool. This is highly unlikely and requires other
conditions to be dire, but it is a problem. This is not restricted to
the swap out path, the problem is purely lack of progress. So the golden
rule is always that you either allocate these units from a private pool
(which is hard for bouncing, since it does both page and bio allocations
from a mempool), or that you always ensure that a previously allocated
bio is in flight before attempting a new alloc.

-- 
Jens Axboe