Re: [PATCH v2] sr: don't use GFP_DMA

[Christoph Hellwig <hch@lst.de>]

On Wed, Dec 22, 2021 at 05:37:07PM +0800, Baoquan He wrote:
> >  	/* allocate transfer buffer */
> > -	buffer = kmalloc(512, GFP_KERNEL | GFP_DMA);
> > +	buffer = kmalloc(512, GFP_KERNEL);
> 
> Thanks a lot for doing this. When I browsed the code path, I come to
> blk_rq_map_kern() but I am not sure if blk_queue_may_bounce() is true in
> the sr_probe() case, then it may enter into bio_map_kern().

blk_queue_may_bounce will be entered for the few drives that set
BLK_BOUNCE_HIGH because they can't handle highmem (which is a subset
of the non-ZONE_DMA memory).  The only driver that actually requires
ZONE_DMA based bouncing is ps3rom, and that driver does a manual
and ubconditional bounce buffering.

> Next I will post my original patchset to mute the allocation failure if
> it's requesting page from DMA zone and DMA zone has no managed page. And
> meanwhile, I will try to collect those places of kmalloc(GFP_DMA) into a
> RFC mail, see if we can change them one by one. Anyone can pick one
> place to fix if interested or knowing it well.

I've already sent out patches for all of drivers/scsi/ today, except
for the ps3rom bounce buffer alllocation which is fine but should
probably be changed to use the page allocator directly.

> Finally, we can remove the
> need of dma-kmalloc() as people suggested. Any comment?

Yes.