linux-kernel - Re: [PATCH] fs: ufs: Remove switch statement from ufs_set_de

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Date:   Thu, 18 Oct 2018 00:33:05 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Phillip Potter' <phil@...lpotter.co.uk>,
        "dushistov@...l.ru" <dushistov@...l.ru>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] fs: ufs: Remove switch statement from ufs_set_de_type
 function

On Wed, Oct 17, 2018 at 10:11:47AM +0000, David Laight wrote:
> From: Phillip Potter
> > Sent: 17 October 2018 11:08
> > 
> > Remove switch statement from ufs_set_de_type function in fs/ufs/util.h
> > header and replace with simple assignment. For each case, S_IFx >> 12
> > is equal to DT_x, so in valid cases (mode & S_IFMT) >> 12 should give
> > us the correct file type. For invalid cases, upper layer validation
> > catches this anyway, so this improves readability and arguably
> > performance by assigning (mode & S_IFMT) >> 12 directly.
> > 
> ...
> > -	case S_IFIFO:
> > -		de->d_u.d_44.d_type = DT_FIFO;
> > -		break;
> > -	default:
> > -		de->d_u.d_44.d_type = DT_UNKNOWN;
> > -	}
> > +	de->d_u.d_44.d_type = (mode & S_IFMT) >> S_SHIFT;
> 
> This requires that the two sets of constants are correctly aligned.

They are.  BSD folks had (sanely, IMO) put the 'type' bits of st_mode
into directory entry verbatim.  Again, "symbolic constant" != "can be
expected to change"...  If, e.g., some port decides to change S_IFIFO,
they'll have no end of fun accessing ext*, xfs, ufs, etc. since that
value is stored in the on-disk inode and in effect pinned down by
that.

All S_... constants are universal and going to remain unchanged on
any Unices.