linux-kernel - Re: [PATCH] ramfs: ignore tmpfs options when we emulate it

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Message-Id: <1244995214.4496.234.camel@calx>
Date:	Sun, 14 Jun 2009 11:00:14 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Mike Frysinger <vapier@...too.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ramfs: ignore tmpfs options when we emulate it

On Sun, 2009-06-14 at 11:39 +0100, Hugh Dickins wrote:
> On Sun, 14 Jun 2009, Wu Fengguang wrote:
> > On Sat, Jun 13, 2009 at 10:15:51PM +0800, Hugh Dickins wrote:
> > > On Sat, 13 Jun 2009, Mike Frysinger wrote:
> > > 
> > > > On systems where CONFIG_SHMEM is disabled, mounting tmpfs filesystems can
> > > > fail when tmpfs options are used.  This is because tmpfs creates a small
> > > > wrapper around ramfs which rejects unknown options, and ramfs itself only
> > > > supports a tiny subset of what tmpfs supports.  This makes it pretty hard
> > > > to use the same userspace systems across different configuration systems.
> > > > As such, ramfs should ignore the tmpfs options when tmpfs is merely a
> > > > wrapper around ramfs.
> > > 
> > > Yes, indeed, thanks a lot for reporting this.
> > > 
> > > But I'm uneasy with making ramfs behaviour differ with CONFIG_SHMEM
> > > (perhaps that's silly: certainly tmpfs behaviour differs with it),
> > > and uneasy with coding a list of options we need to remember to keep
> > > in synch with mm/shmem.c.  It's easier to justify ignoring all options,
> > > than rejecting some while ignoring others yet not respecting them.
> > 
> > We can avoid the burden of syncing a list of options between
> > ramfs<>tmpfs by a slightly differently patch. Hopefully this makes
> > ramfs behave like other filesystems when used standalone.
> 
> We could do; but I'm still preferring not.  How about you, Matt?
> You decide, I think Andrew has chosen a different race track from
> "The Merge Window" this weekend.

I prefer the 'silently ignore' approach.

The only other approach that I think is reasonably clean is to
'subclass' ramfs by wrapping its init function with one that discards
mount args. Unfortunately that adds code and data in the 'I don't give a
damn, just keep it tiny' case, so that's a non-starter.

-- 
http://selenic.com : development and support for Mercurial and Linux


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