linux-kernel - Re: [RFC] rootmpfs

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Message-ID: <51644FB5.9050103@infradead.org>
Date:	Tue, 09 Apr 2013 10:28:21 -0700
From:	Randy Dunlap <rdunlap@...radead.org>
To:	Rob Landley <rob@...dley.net>
CC:	Byron Stanoszek <bstanoszek@...time.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] rootmpfs

On 04/09/13 07:52, Rob Landley wrote:
> On 04/05/2013 02:53:12 PM, Byron Stanoszek wrote:
>> Rob,
>>
>> FWIW I have a patch to do something like this. It even gives you a rdsize=xxx
>> tunable kernel parameter that lets you specify the size of the tmpfs, which
>> acts like the -osize= mount flag (so phrases like 100M or 20% works). So doing
>> things like 'cat /dev/zero > filename' will not run you out of all available
>> memory. (Note: If you don't specify rdsize= on the kernel command line, it will
>> not convert rootfs to tmpfs).
> 
> In init/do_mounts.c the boot infrastructure already has kernel command line options "rootflags=" and "rootfstype=", so the logical thing to do is probably to hook those up to rootfs. (That way instead of special casing a new option we use the existing tmpfs option parsing.)
> 
> The default tmpfs size is 50%, which solves the "trivial to exhaust memory and panic a kernel running under rootfs" problem. Having one tmpfs also fixes the case that multiple tmpfs mounts (for /home and /var, for example,) have separate memory limits that don't coordinate with each other, so if /home can use 30% and /var can use 30%, that's 60% plus whatever rootfs is already using, so you can easily squeeze the kernel against the wall without meaning to. (Yes, you can make one tmpfs mount and --bind mount from there to elsewhere, I've seen that done. Having rootfs just _be_ tmpfs makes this much easier to track.)
> 
>> See attached.
> 
> You're not actually changing the type of rootfs, you're overmounting it with a second filesystem instance. (Mine hasn't got a "change", it just mounts it correctly the first time, and there's just one rootfs instance.)
> 
> What _is_ wrong with my version is that if you select tmpfs as a module bad things happen; it tries to use code that's not there. I dunno of an #ifdef that distinguishes between module and builtin, so I think I have to add another kconfig symbol...

See include/linux/kconfig.h:  IS_MODULE() and IS_BUILTIN().

> 
> I'll poke at it.


-- 
~Randy
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