On Thu, 17 Jul 2014 18:17:32 +0800 Ian Kent <raven@xxxxxxxxxx> wrote:
>
> On Thu, 2014-07-17 at 18:04 +1000, NeilBrown wrote:
> > If there some documentation about the interactions between the automountd and
> > the kernel?
>
> Not really, only the ioctl interaction described in
> Documentation/filesystems/autofs4-mount-control.txt
>
> > It looks like:
> > With V5, every name in the root directory gets something mounted on it,
> > either another autofs or the target filesystem.
> > With V4, you don't mount an autofs onto an autofs, but when a name in the
> > root is automounted, all the names beneath there are created and the
> > target filesystems are mounted before the top level automount is
> > acknowledged.
> >
> > Does that sound at all right (I suspect I haven't explained it very clearly).
>
> Mostly, but let me try and simplify it (or perhaps confuse you even more
> and bore you as a bonus, ;)) and offer a broader picture.
Thanks for going into all that detail! Having read that, and read the code
(a few times) and having tried to document it myself I think I really
understand it all now.
I wanted to be sure I understood from the perspective of what the module
actually supports rather than just how the automount daemon uses it, so I put
together a document describing the module. At 3000 words, I'm not sure
whether to call it "verbose" or "thorough". I'll post it separately.
I did discover one thing that is important for the RCU-walk work.
If you have a "root-less multimount" then the top-level directory will have
DCACHE_NEED_AUTOMOUNT set even though it is not a mount-trap.
As it contains a sub-directory (or more), d_manage will return -EISDIR so
normal REF-walking will side-step d_automount and it will be treated as a
normal directory.
However in RCU-walk mode, d_manage cannot usefully return -EISDIR so the VFS
will always drop into REF-walk mode, which negates some of the value of my
RCU-walk patches.
I see two ways to fix this:
1- clear DCACHE_NEED_AUTOMOUNT when creating a subdir or symlink, just like
we do with protocol-version 4
2- teach the VFS to accept -EISDIR from d_manage and to ignore
DCACHE_NEED_AUTOMOUNT in that case. I have a patch to do this and it is
fairly straight forward.
I think I prefer 2 (it seems easier to document:-) but thought I'd ask if you
have an opinion before I post it (I haven't even tested it properly yet).
Thanks,
NeilBrown
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