Message-ID: <51644FB5.9050103@infradead.org>
Date: Tue, 09 Apr 2013 10:28:21 -0700
From: Randy Dunlap <rdunlap@infradead.org>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130329 Thunderbird/17.0.5
MIME-Version: 1.0
To: Rob Landley <rob@landley.net>
CC: Byron Stanoszek <bstanoszek@comtime.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC] rootmpfs
References: <1365519128.18069.55@driftwood>
In-Reply-To: <1365519128.18069.55@driftwood>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2287
Lines: 34

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/