Re: [PATCH 3/4] autofs - make mountpoint checks namespace aware

[Ian Kent]

On Thu, 2016-09-22 at 20:37 -0500, Eric W. Biederman wrote:
> Ian Kent <raven@xxxxxxxxxx> writes:
> 
> > On Thu, 2016-09-22 at 10:43 -0500, Eric W. Biederman wrote:
> > > Ian Kent <raven@xxxxxxxxxx> writes:
> > > 
> > > > Eric, Mateusz, I appreciate your spending time on this and particularly
> > > > pointing
> > > > out my embarrassingly stupid is_local_mountpoint() usage mistake.
> > > > 
> > > > Please accept my apology for the inconvenience.
> > > > 
> > > > If all goes well (in testing) I'll have follow up patches to correct
> > > > this
> > > > fairly
> > > > soon.
> > > 
> > > Related question.  Do you happen to know how many mounts per mount
> > > namespace tend to be used?  It looks like it is going to be wise to put
> > > a configurable limit on that number.  And I would like the default to be
> > > something high enough most people don't care.  I believe autofs is
> > > likely where people tend to use the most mounts.

Yes, I agree, I did want to try and avoid changing the parameters to
->d_mamange() but passing a struct path pointer might be better in the long run
anyway.


> > That's a good question.
> > 
> > I've been thinking that maybe I should have used a lookup_mnt() type check
> > as I
> > originally started out to, for this reason, as the mnt_namespace list looks
> > to
> > be a linear list.
> > 
> > But there can be a lot of mounts, and not only due to autofs, so maybe that
> > should be considered anyway.
> 
> There are two reasons for is_local_mountpoint being the way it is.
> 
> a) For the cases where you don't have the parent mount.
> b) For the cases where you want to stop things if something is mounted
>    on a dentry in the local mount namespace even if it isn't mounted
>    on that dentry at your current mount parent. (Something that was
>    important to not change the semantics of the single mount namespace case).
> 
> Both of those cases to apply to unlink, rmdir, and rename.  I don't think
> either of those cases apply to what autofs is trying to do.  Certainly
> not the second.
> 
> So if you have the parent mount I really think you want to be using some
> variation of lookup_mnt().  The fact it is rcu safe may help with some
> of those weird corner cases as well.
> 
> > The number of mounts for direct mount maps is usually not very large because
> > of
> > the way they are implemented, large direct mount maps can have performance
> > problems. There can be anywhere from a few (likely case a few hundred) to
> > less
> > than 10000, plus mounts that have been triggered and not yet expired.
> > 
> > Indirect mounts have one autofs mount at the root plus the number of mounts
> > that
> > have been triggered and not yet expired.
> > 
> > The number of autofs indirect map entries can range from a few to the common
> > case of several thousand and in rare cases up to between 30000 and 50000.
> > I've
> > not heard of people with maps larger than 50000 entries.
> > 
> > The larger the number of map entries the greater the possibility for a large
> > number of active mounts so it's not hard to expect cases of a 1000 or
> > somewhat
> > more active mounts.
> 
> Fair.  So at least 1000.  And there can be enough mounts that a limit of
> 100,000 might be necessary to give head room for the existings configurations.
> 
> Thank you.  Now I just need to wrap my head around fs/namespace.c again
> and see how bad a count like that will be to maintain.
> 
> Eric
>