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: <20251119141300.2209ea72@pumpkin>
Date: Wed, 19 Nov 2025 14:13:00 +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 15:58:21 +0200
Leon Romanovsky <leon@...nel.org> wrote:

> On Wed, Nov 19, 2025 at 01:36:15PM +0000, David Laight wrote:
> > 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?  
> 
> No, I simply performed sizeof(phys_addr_t) + sizeof(size_t).
> 
> > 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.  
> 
> So does it mean that Christoph's comment about size increase is not correct?

Correct - ie there is no size increase.

> 
> Thanks
> 
> > 
> > 	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 
> > >   
> >