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SubjectRe: [RFC] rootmpfs
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...

I'll poke at it.

Rob

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