Re: [PATCH v9 22/24] nvme-pci: use a better encoding for small prp pool allocations

[Leon Romanovsky <leon@kernel.org>]

On Wed, Apr 23, 2025 at 11:05:52AM +0200, Christoph Hellwig wrote:
> On Wed, Apr 23, 2025 at 11:13:13AM +0300, Leon Romanovsky wrote:
> > From: Christoph Hellwig <hch@lst.de>
> > 
> > There is plenty of unused space in the iod next to nr_descriptors.
> > Add a separate flag to encode that the transfer is using the full
> > page sized pool, and use a normal 0..n count for the number of
> > descriptors.
> > 
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > Tested-by: Jens Axboe <axboe@kernel.dk>
> > [ Leon: changed original bool variable to be flag as was proposed by Kanchan ]
> > Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
> > Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> > ---
> >  drivers/nvme/host/pci.c | 93 ++++++++++++++++++++---------------------
> >  1 file changed, 46 insertions(+), 47 deletions(-)
> > 
> > diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> > index 638e759b29ad..7e93536d01cb 100644
> > --- a/drivers/nvme/host/pci.c
> > +++ b/drivers/nvme/host/pci.c
> > @@ -44,6 +44,7 @@
> >  #define NVME_MAX_SEGS	128
> >  #define NVME_MAX_META_SEGS 15
> >  #define NVME_MAX_NR_DESCRIPTORS	5
> > +#define NVME_SMALL_DESCRIPTOR_SIZE 256
> >  
> >  static int use_threaded_interrupts;
> >  module_param(use_threaded_interrupts, int, 0444);
> > @@ -219,6 +220,10 @@ struct nvme_queue {
> >  	struct completion delete_done;
> >  };
> >  
> > +enum {
> > +	IOD_LARGE_DESCRIPTORS = 1, /* uses the full page sized descriptor pool */
> 
> This is used as a ORable flag, I'd make that explicit:
> 
> 	/* uses the full page sized descriptor pool */
> 	IOD_LARGE_DESCRIPTORS		= 1U << 0,
> 
> and similar for the next flag added in the next patch.
> 
> >  	struct nvme_request req;
> >  	struct nvme_command cmd;
> >  	bool aborted;
> > -	/* # of PRP/SGL descriptors: (0 for small pool) */
> > -	s8 nr_descriptors;
> > +	u8 nr_descriptors;	/* # of PRP/SGL descriptors */
> > +	unsigned int flags;
> 
> And this should be limited to a u16 to not bloat the structure.

I'll limit it to u8.

Thanks

>