Re: GFP_DMA use in SCSI midlayer

Mike Christie
2006-10-02T18:43:49Z

On Sun, 2006-10-01 at 15:40 -0500, James Bottomley wrote:
> On Sun, 2006-10-01 at 22:13 +0200, Andi Kleen wrote:
> > GFP_DMA in general is deprecated and should be replaced by appropiate
> > dma_alloc_coherent()
>
> Um, no it shouldn't.
>
> All of the places where we use GFP_DMA are because we might be
> addressing an ISA card or other strange mask limited card (gated usually
> by unchecked_isa_dma). However, what is wanted in every case is
> ordinary memory, not coherent memory. They can't simply be replaced
> with dma_alloc_coherent because
>
> a) it will waste memory for platforms that only do it in page size
> multiples
> b) It will fail on platforms that can't do it at all.
>
> Coherent memory is really only for device drivers to use in mailboxes
> and shared ring buffers.
>
> > or similar. And I can't imagine any modern systems still need them, and for
> > the few still non CONFIG_BROKEN ISA drivers maybe some other way can be found?
> > And do they really require the mid layer data structures to be GFP_DMA too?
>
> Well, it's the "or similar" that's still the problem. All they need is
> ordinary memory which respects the device mask (then I can get rid of
> the unchecked_isa_dma flag as well).
>
> One could argue, I suppose, that when we get every SCSI path to go via
> sg, then the block layer will bounce this for us, and we don't need to
> worry at all.

I think we might just need the blk_map_kern users now. For the async
execute I added the bounce code already and the block SG_IO has it
atleady. I think the blk_map_kern bounce code got dropped because we
thought the correct gfp_t would be passed in. But I think all we need is
the patch below and all the paths are take care of. The patch is not
tested. Patch was made against scsi-misc.

The last place that is sending non sg commands may just be md/dm-emc.c
but that is is just waiting on alasdair to take some patches that fix
that and a bunch of junk in there including adding bounce support. If
the patch below is ok though and dm-emc finally gets converted then it
will have sg and bonce buffer support.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index 9c3a06b..e58b43a 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -2533,6 +2533,7 @@ int blk_rq_map_kern(request_queue_t *q,
rq->bio = rq->biotail = bio;
blk_rq_bio_prep(q, rq, bio);

+ blk_queue_bounce(q, &rq->bio);
rq->buffer = rq->data = NULL;
rq->data_len = len;
return 0;
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
diff --git a/fs/bio.c b/fs/bio.c