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: <20251119133615.2eefb7db@pumpkin>
Date: Wed, 19 Nov 2025 13:36:15 +0000
From: David Laight <david.laight.linux@...il.com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Christoph Hellwig <hch@....de>, 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 Wed, 19 Nov 2025 11:55:16 +0200
Leon Romanovsky <leon@...nel.org> wrote:

> 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

Did you actually look?
There will normally be 4 bytes of padding at the end of the structure.

About the only place where it will be 12 bytes is a 32bit system with
64bit phyaddr that aligns 64bit items on 32bit boundaries - so x86.

	David

> 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 
>