mount -a remounts tmpfs entries: bug or feature?

Denys Vlasenko vda.linux at googlemail.com
Mon Nov 24 23:12:45 UTC 2008


On Monday 24 November 2008 14:59, Rob Landley wrote:
> On Sunday 23 November 2008 09:01:35 Denys Vlasenko wrote:
> > On Friday 07 November 2008 03:01, busybox at eehouse.org wrote:
> > > busybox's implementation of mount differs from the standalone version
> 
> Back in the 1.1 timeframe I rewrote it more or less from scratch, something 
> like 3 times, trying to get it to behave sanely.  (Mount is tricksy.)
> 
> I see it's been fairly heavily edited then.  Kind of horrible to read through 
> now, actually.

NFS code was merged into mount.c. Somebody asked me to do it
(it was before my maintainership). In hindsight, that was not
such a good idea - now it's not readable.

Other that NFS code - what places you don't like Rob?

> And kind of broken in several places.  Ooh, ick. 

Where? :(


> The tricky one is that you have to overmount / to switch_root off of rootfs, 
> so you can't simply say "this mount point is already in use".
> 
> The rule should probably be more like "If existing mount has same filesystem 
> type as the new one you're trying to do, bail out".  Except this won't work if 
> you try each block-backed type in /proc/filesystems in sequence autodetect 
> your mount point (which is the only sane way to do it; the kernel knows what 
> filesystems it supports, a userspace tool doesn't).  Luckily, for tmpfs you 
> _must_ specify the type (either via -t or saying it in fstab), and that's the 
> one we really care about here.
> 
> > Might get real weird in corner cases of stacked mounts,
> > union mounts, mounts with some options differing and so on.
> 
> Some filesystem types are per-instance, and some are shared with all instances 
> (most block backed ones, non-containerized versions of /proc and /sys...).
> 
> Did you ever read the thing I wrote about the four types of filesystems 
> (blocked back, ram backed, synthetic, and network)?

No. Do you have an URL?
--
vda



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