linux-kernel - Re: [PATCH v2 1/2] nvme-pci: Use size_t for length fields to handle larger sizes

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Message-ID: <20251119095516.GA13783@unreal>
Date: Wed, 19 Nov 2025 11:55:16 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Jens Axboe <axboe@...nel.dk>, Keith Busch <kbusch@...nel.org>,
	Sagi Grimberg <sagi@...mberg.me>, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
	Chaitanya Kulkarni <kch@...dia.com>
Subject: Re: [PATCH v2 1/2] nvme-pci: Use size_t for length fields to handle
 larger sizes

On Tue, Nov 18, 2025 at 06:03:11AM +0100, Christoph Hellwig wrote:
> On Mon, Nov 17, 2025 at 09:22:43PM +0200, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@...dia.com>
> > 
> > This patch changes the length variables from unsigned int to size_t.
> > Using size_t ensures that we can handle larger sizes, as size_t is
> > always equal to or larger than the previously used u32 type.
> > 
> > Originally, u32 was used because blk-mq-dma code evolved from
> > scatter-gather implementation, which uses unsigned int to describe length.
> > This change will also allow us to reuse the existing struct phys_vec in places
> > that don't need scatter-gather.
> > 
> > Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> > Reviewed-by: Chaitanya Kulkarni <kch@...dia.com>
> > ---
> >  block/blk-mq-dma.c      | 8 ++++++--
> >  drivers/nvme/host/pci.c | 4 ++--
> >  2 files changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/block/blk-mq-dma.c b/block/blk-mq-dma.c
> > index e9108ccaf4b0..e7d9b54c3eed 100644
> > --- a/block/blk-mq-dma.c
> > +++ b/block/blk-mq-dma.c
> > @@ -8,7 +8,7 @@
> >  
> >  struct phys_vec {
> >  	phys_addr_t	paddr;
> > -	u32		len;
> > +	size_t		len;
> >  };
> 
> So we're now going to increase memory usage by 50% again after just
> reducing it by removing the scatterlist?

It is slightly less.

Before this change: 96 bits
After this change (on 64bits system): 128 bits.

It is 33% increase per-structure.

So what is the resolution? Should I drop this patch or not?

Thanks