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From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [RFC PATCH] vfs: Use 12:20 bit major:minor in stat everywhere
Date: Wed, 4 Mar 2015 01:47:32 +0000
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Cc: Daniel Ehrenberg <dehrenberg@google.com>,
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On Tue, Mar 03, 2015 at 05:37:31PM -0800, Gwendal Grignou wrote:
> At least, to base the device format on whether we are running on a 32
> bit or 64 bit architecture does not make sense.

Yes, it does.  Note that on 32bit ones stat64(2) *will* return an arbitrary
value.  On 64bit ones stat(2) will.

> If a tool calling stat(2) can not handle 12 bit major/20 bits minor,
> it would already break or about to break when running on a 64 bit
> machine.
> 
> Regarding SCSI, the 17th disk will use SCSI_DISK1_MAJOR (65). Only the
> 257th disk will use the first scsi major (8) again and need a  minor
> greater than 256. (see sd_major() in drivers/scsi/sd.c for details).

*nod*

It's been years since I last looked at sd.c, TBH...

Said that, with NFS it's definitely a minor per superblock, and it's not the
only set_anon_super() user.  Having a bunch of filesystems mounted over NFS
will suffice...