linux-ext4 - Re: [PATCH 2/3] fs/ext2/inode: Fix a type cast error for fsdax

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Date:   Mon, 2 Jul 2018 09:55:43 +0200
From:   Jan Kara <jack@...e.cz>
To:     Huaisheng Ye <yehs2007@...o.com>
Cc:     "\"Al Viro\"" <viro@...IV.linux.org.uk>, dan.j.williams@...el.com,
        dave.jiang@...el.com, willy@...radead.org,
        ross.zwisler@...ux.intel.com, vishal.l.verma@...el.com,
        jack@...e.com, chengnt@...ovo.com, linux-nvdimm@...ts.01.org,
        linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
        "\"Huaisheng Ye\"" <yehs1@...ovo.com>
Subject: Re: [PATCH 2/3] fs/ext2/inode: Fix a type cast error for fsdax

On Mon 02-07-18 14:23:42, Huaisheng Ye wrote:
>  ---- On Mon, 02 Jul 2018 03:26:00 +0800 Al Viro <viro@...IV.linux.org.uk> wrote ---- 
>  > On Sun, Jul 01, 2018 at 02:18:47PM +0800, Huaisheng Ye wrote: 
>  > > From: Huaisheng Ye <yehs1@...ovo.com> 
>  > >  
>  > > The type of offset within struct iomap is loff_t, which represents 
>  > > file offset of mapping. 
>  > >  
>  > > In ext2_iomap_begin, iomap->offset shall be given a type cast as 
>  > > loff_t instead of u64. 
>  >  
>  > Why is it an error?  loff_t is uniformly typedefed to long long. 
>  > In which case the second variant is different from the first one 
>  > *and* does not step into nasal demon territory?
> 
> Sorry for my inaccuracy.
> The type of iomap->offset is loff_t, is it better to cast first_block
> to loff_t, then do the left shift operation?

I think what Al meant is that in the kernel loff_t is 64-bit anyway and
furthermore it is signed which means most people have to look up C standard
to remember how right shift is (un)defined if it would overflow ;). So this
does not really make the code any more readable. Rather on contrary...

								Honza

>  
>  >  
>  > > -    iomap->offset = (u64)first_block << blkbits; 
>  > > +    iomap->offset = (loff_t)first_block << blkbits; 
>  > 
> 
> ---
> Cheers,
> Huaisheng
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR