Re: [PATCH 0/10] fuse: An attempt to implement a write-back cache policy

[Nikolaus Rath <Nikolaus-BTH8mxji4b0@public.gmane.org>]

On 07/04/2012 10:33 AM, Pavel Emelyanov wrote:
>>>>> A good solution of this is switching the FUSE page cache into a write-back policy.
>>>>> >>>> With this file data are pushed to the userspace with big chunks (depending on the
>>>>> >>>> dirty memory limits, but this is much more than 128k) which lets the FUSE daemons
>>>>> >>>> handle the size updates in a more efficient manner.
>>>>> >>>>
>>>>> >>>> The writeback feature is per-connection and is explicitly configurable at the
>>>>> >>>> init stage (is it worth making it CAP_SOMETHING protected?)
>>>> >>>
>>>> >>> From your description it sounds as if the only effect of write-back is
>>>> >>> to increase the chunk size. Why the need to require special
>>>> >>> privileges for this?
>>> >>
>>> >> Provided I understand the code correctly: if FUSE daemon turns writeback on and sets
>>> >> per-bdi dirty limit too high it can cause a deadlock on the box. Thus then daemon
>>> >> should be trusted by the kernel, i.e. -- privileged.
>> > 
>> > Wouldn't it be more reasonable to enforce that the bdi dirty limit is
>> > not set too high then?
>
> Hardly, since if you have several mounts with small bdi limit each the sum of them
> can be still high.

Yeah, so isn't it possible to put a limit on the sum of them?


I think it would be much better if this could be made safe to use
instead of requiring special privileges. If that isn't possible,
wouldn't it be better to enable/disable the feature in /etc/fuse.conf,
similar to the user_allow_* options?

Having to explain how to assign capabilities in the documentation of a
fuse filesystem sounds quite dreadful to me...


Best,

   -Nikolaus


PS: My naive attempt to apply your patches to my Debian 3.2 kernel
failed. I'm compiling 3.5rc5 at the moment - will see if I can get it to
boot afterwards...


-- 
 »Time flies like an arrow, fruit flies like a Banana.«

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