On Mon, 14 Apr 2008 10:41:54 -0400
Pete Wyckoff <pw@xxxxxxx> wrote:
> fujita.tomonori@xxxxxxxxxxxxx wrote on Mon, 14 Apr 2008 19:50 +0900:
> > This patch changes rq->cmd from the static array to a pointer to
> > support large commands.
> >
> > We rarely handle large commands. So for optimization, a struct request
> > still has a static array for a command. rq_init sets rq->cmd pointer
> > to the static array.
> >
> > Signed-off-by: FUJITA Tomonori <fujita.tomonori@xxxxxxxxxxxxx>
> > Cc: Jens Axboe <jens.axboe@xxxxxxxxxx>
> [..]
> > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > index b3a58ad..5710ae4 100644
> > --- a/include/linux/blkdev.h
> > +++ b/include/linux/blkdev.h
> > @@ -215,8 +215,9 @@ struct request {
> > /*
> > * when request is used as a packet command carrier
> > */
> > - unsigned int cmd_len;
> > - unsigned char cmd[BLK_MAX_CDB];
> > + unsigned short cmd_len;
> > + unsigned char __cmd[BLK_MAX_CDB];
> > + unsigned char *cmd;
> >
> > unsigned int data_len;
> > unsigned int extra_len; /* length of alignment and padding */
> > @@ -812,6 +813,13 @@ static inline void put_dev_sector(Sector p)
> > page_cache_release(p.v);
> > }
> >
> > +static inline void rq_set_cmd(struct request *rq, unsigned char *cmd,
> > + unsigned short cmd_len)
> > +{
> > + rq->cmd = cmd;
> > + rq->cmd_len = cmd_len;
> > +}
>
> Here's one way this will be used, in a patched bsg that understands
> large commands. Complication is the need to copy and hold onto the
> big command across the duration of the request.
>
> Submit time is fairly clean:
>
> /* buf, len from user request */
> rq = blk_get_request(..);
> rq->cmd_len = len;
> if (len > BLK_MAX_CDB) {
> rq->cmd = kmalloc(len);
> if (rq->cmd == NULL)
> goto out;
> }
> copy_from_user(rq->cmd, buf, len);
>
> Completion time needs to know when to free rq->cmd:
>
> if (rq->cmd_len > BLK_MAX_CDB)
> kfree(rq->cmd);
> blk_put_request(rq);
>
> Could use (rq->cmd != rq->__cmd) instead, but nothing had better
> ever touch rq->cmd_len.
>
> I don't think the helper rq_set_cmd() will be very useful, as the
> caller (bsg) must think about allocation of the command buffer if it
> is big.
Yeah, I think so. If you need large command support, you need to know
how to handle it for now. Now only bsg supports large commands. So I
guess that nobody complains about the current interface.
> One option would be to handle allocation/freeing of the big command
> in rq_set_... functions, but I don't think you want to constrain the
> interface like that.
Or, you can change blk_get_request to take the command length
argument. But I don't think that such is the right approach.
> Boaz's concern about big rq->cmd_len still worries me, although I
> think this approach is better and worth solving bugs in drivers as
> they arise. It only matters in the case that someone adds, say, a
> bsg interface to all block devices though. The queuecommand of ub
> shows a good example of how this will break.
Yes, a bsg hook will need to handle large commands. I don't see any
problem about it. All a hook needs to do is just looking at the legnth
and dropping or executing the command. And of course, we are unlikely
to add a bsg device to all the block devices.
As I said, if we want to govern the command length in a common place,
we can have the limit of the command length in request queues. It's
clear than an implicit checking with two lengths, cmd_len and
ext_cdb_len.
I thought about adding the code to check the command length in UB. But
I thought that we were unlikely to create bsg devices for ub. Common
people use USB_STORAGE rather than UB, I guess.
> In sum, this is a cleaner approach, and a bit easier for callers
> with long commands to deal with. And you could get rid of the
> trivial helper.
Yeah, it's a cleaner design, that's main point, I think.
BTW, have you had a chance to try the bsg patches to fix the problems
that you reported? I think that Mike and I analyzed the problem
correctly and the patches works for me, but it would be nice if you
can confirm that they also work for you.
Thanks,
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