Re: [PATCH 3/3] autofs - fix AT_NO_AUTOMOUNT not being honored

[Ian Kent]

Adding Al Viro to the Cc list as I believe Stephen Whitehouse and
Al have discussed something similar, please feel free to chime in
with your thoughts Al.

On 29/11/17 09:17, NeilBrown wrote:
> On Tue, Nov 28 2017, Mike Marion wrote:
> 
>> On Tue, Nov 28, 2017 at 07:43:05AM +0800, Ian Kent wrote:
>>
>>> I think the situation is going to get worse before it gets better.
>>>
>>> On recent Fedora and kernel, with a large map and heavy mount activity
>>> I see:
>>>
>>> systemd, udisksd, gvfs-udisks2-volume-monitor, gvfsd-trash,
>>> gnome-settings-daemon, packagekitd and gnome-shell
>>>
>>> all go crazy consuming large amounts of CPU.
>>
>> Yep.  I'm not even worried about the CPU usage as much (yet, I'm sure 
>> it'll be more of a problem as time goes on).  We have pretty huge
>> direct maps and our initial startup tests on a new host with the link vs
>> file took >6 hours.  That's not a typo.  We worked with Suse engineering 
>> to come up with a fix, which should've been pushed here some time ago.
>>
>> Then, there's shutdowns (and reboots). They also took a long time (on
>> the order of 20+min) because it would walk the entire /proc/mounts
>> "unmounting" things.  Also fixed now.  That one had something to do in
>> SMP code as if you used a single CPU/core, it didn't take long at all.
>>
>> Just got a fix for the suse grub2-mkconfig script to fix their parsing 
>> looking for the root dev to skip over fstype autofs
>> (probe_nfsroot_device function).
>>
>>> The symlink change was probably the start, now a number of applications
>>> now got directly to the proc file system for this information.
>>>
>>> For large mount tables and many processes accessing the mount table
>>> (probably reading the whole thing, either periodically or on change
>>> notification) the current system does not scale well at all.
>>
>> We use Clearcase in some instances as well, and that's yet another thing
>> adding mounts, and its startup is very slow, due to the size of
>> /proc/mounts.  
>>
>> It's definitely something that's more than just autofs and probably
>> going to get worse, as you say.
> 
> If we assume that applications are going to want to read
> /proc/self/mount* a log, we probably need to make it faster.
> I performed a simple experiment where I mounted 1000 tmpfs filesystems,
> copied /proc/self/mountinfo to /tmp/mountinfo, then
> ran 4 for loops in parallel catting one of these files to /dev/null 1000 times.
> On a single CPU VM:
>   For /tmp/mountinfo, each group of 1000 cats took about 3 seconds.
>   For /proc/self/mountinfo, each group of 1000 cats took about 14 seconds.
> On a 4 CPU VM
>   /tmp/mountinfo: 1.5secs
>   /proc/self/mountinfo: 3.5 secs
> 
> Using "perf record" it appears that most of the cost is repeated calls
> to prepend_path, with a small contribution from the fact that each read
> only returns 4K rather than the 128K that cat asks for.
> 
> If we could hang a cache off struct mnt_namespace and use it instead of
> iterating the mount table - using rcu and ns->event to ensure currency -
> we should be able to minimize the cost of this increased use of
> /proc/self/mount*.
> 
> I suspect that the best approach would be implement a cache at the
> seq_file level.
> 
> One possible problem might be if applications assume that a read will
> always return a whole number of lines (it currently does).  To be
> sure we remain safe, we would only be able to use the cache for
> a read() syscall which reads the whole file.
> How big do people see /proc/self/mount* getting?  What size reads
> does 'strace' show the various programs using to read it?

Buffer size almost always has a significant impact on IO so that's
likely a big factor but the other aspect of this is notification
of changes.

The risk is improving the IO efficiency might just allow a higher
rate of processing of change notifications and similar symptoms
to what we have now.

The suggestion is that a system that allows for incremental (diff
type) update notification is needed to allow mount table propagation
to scale well.

That implies some as yet undefined user <-> kernel communication
protocol.

Ian