>>>>> "Jens" == Jens Axboe <jens.axboe@oracle.com> writes:

Jens> On Tue, Oct 23 2007, John Stoffel wrote:
quoted text
>> >>>>> "Jens" == Jens Axboe <jens.axboe@oracle.com> writes:
>> 
Jens> On Tue, Oct 23 2007, David Miller wrote:
quoted text
>> >> From: Jens Axboe <jens.axboe@oracle.com>
>> >> Date: Tue, 23 Oct 2007 09:23:59 +0200
>> >> 
>> >> > On Tue, Oct 23 2007, David Miller wrote:
>> >> > > From: Jens Axboe <jens.axboe@oracle.com>
>> >> > > Date: Tue, 23 Oct 2007 09:09:33 +0200
>> >> > > 
>> >> > > > Eh this wont work, it's the wrong entry... Here's a temporary
>> >> > > > work-around.
>> >> > > > 
>> >> > > > diff --git a/drivers/ide/ide-io.c b/drivers/ide/ide-io.c
>> >> > > > index c89f0d3..108202b 100644
>> >> > > > --- a/drivers/ide/ide-io.c
>> >> > > > +++ b/drivers/ide/ide-io.c
>> >> > > > @@ -822,6 +822,7 @@ void ide_map_sg(ide_drive_t *drive, struct request *rq)
>> >> > > >  		return;
>> >> > > >  
>> >> > > >  	if (rq->cmd_type != REQ_TYPE_ATA_TASKFILE) {
>> >> > > > +		sg_init_table(hwif->sg_table, hwif->sg_max_nents);
>> >> > > >  		hwif->sg_nents = blk_rq_map_sg(drive->queue, rq, sg);
>> >> > > >  	} else {
>> >> > > >  		sg_init_one(sg, rq->buffer, rq->nr_sectors * SECTOR_SIZE);
>> >> > > 
>> >> > > That's the exact patch I'm about to boot test :-)
>> >> > 
>> >> > That should work - once you verify that, would you mind testing this one
>> >> > as well? Thanks!
>> >> 
>> >> This one works here too, thanks.
>> 
Jens> Great, thanks for testing that as well. Thinking a bit more
Jens> about it, I think the forced clear should stay in
Jens> blk_rq_map_sg() since we don't want to advertise it to
Jens> drivers. So I'll just open-code it in there.
quoted text
>> 
>> Should there be more bounds checking here, so that if you try to do a
>> _force() you at least check to make sure that there's something beyond
>> there and useable on the list?  
>> 
>> We're saving the time from not reallocating from scratch, but let's
>> make it robust by doing at least some more checks here.

Jens> We have to rely on the caller passing in an sgtable that is big enough
Jens> to map the request, we have always made that assumption. Anything else
Jens> would be a bug, of course. Now, catching that bug would indeed be nice.
Jens> Any suggestions?

Poor mans slab poisoning maybe?  As Alan Cox was saying about keeping
it simple with a NULL entry on the end of the SG list, having an easy
way to not fall of the end seems key. 

But from reading and re-reading the thread and some of the code, it
looks like this list is really allocated, then the driver can just
re-use the list (or a subset) as much as it likes without clearing and
then reallocating.

So if you've got a chain of 127 items, does it make sense when you
only use 16 of them to poison the next three or four in the chain to
really make sure you don't fall off the end?

Sorry I don't have a concrete example, my C-foo is quite weak and I
haven't had the time to really dive into this code.  So take my
comments as just an interested but ignorant bystander butting in.  :]

John

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