linux-kernel - Re: [PATCH] lib/scatterlist: Fix to calculate the last

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Date:   Tue, 10 Jan 2023 09:53:35 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Yishai Hadas <yishaih@...dia.com>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-mm@...ck.org, axboe@...nel.dk,
        logang@...tatee.com, hch@....de, alex.williamson@...hat.com,
        leonro@...dia.com, maorg@...dia.com
Subject: Re: [PATCH] lib/scatterlist: Fix to calculate the last_pg properly

On Tue, Jan 10, 2023 at 10:01:53AM +0200, Yishai Hadas wrote:
> On 10/01/2023 7:08, Matthew Wilcox wrote:
> > On Mon, Jan 09, 2023 at 04:47:01PM +0200, Yishai Hadas wrote:
> > >   	if (sgt_append->prv) {
> > > +		unsigned long paddr =
> > > +			(page_to_pfn(sg_page(sgt_append->prv)) * PAGE_SIZE +
> > 1. page_to_pfn() * PAGE_SIZE is spelled page_to_phys()
> > 
> > 2. physical addresses have type phys_addr_t.  Oh, wait, paddr isn't a
> > physical address, it's a pfn because you divide by PAGE_SIZE at the end.
> > But you will get truncation on 32-bit, because page_to_pfn() has type
> > unsigned long.  Anyway, this shouldn't be called paddr.  Maybe last_pfn
> > or something?
> > 
> > > +			 sgt_append->prv->offset + sgt_append->prv->length) /
> > > +			PAGE_SIZE;
> 
> In this area of the patch I just reverted to what we had in 6.1 [1] where we
> were good.
> 
> I can send V1 and replace the 'paddr' to be called 'last_pfn' if it's really
> needed.

Please change it as Matthew describes so it is clearer

Jason