Re: [PATCH 1/2] rcfs core patch

[Herbert Poetzl]

On Sun, Mar 11, 2007 at 08:09:29PM +0300, Kirill Korotaev wrote:
> Herbert,
> 
> > sorry, I'm not in the lucky position that I get payed
> > for sending patches to LKML, so I have to think twice
> > before I invest time in coding up extra patches ...
> > 
> > i.e. you will have to live with my comments for now

> looks like you have no better argurments then that...

pardon?

if you want to make that personal, please do it
offline ... I'm sick of (lkml) folks wasting 
time for (political) hick hack instead of trying
to improve the kernel ...

>>> Looks like your main argument is non-intrusive...
>>> "working", "secure", "flexible" are not required to 
>>> people any more? :/

>> well, Linux-VServer is "working", "secure", "flexible"
>> _and_ non-intrusive ... it is quite natural that less
>> won't work for me ... and regarding patches, there
>> will be a 2.2 release soon, with all the patches ...

> ok. please check your dcache and slab accounting then
> (analyzed according to patch-2.6.20.1-vs2.3.0.11.diff):

development branch, good choice for new features
and code which is currently tested ...

> Both are full of races and problems. Some of them:
> 1. Slabs allocated from interrupt context are charged to 
>    current context.
>    So charged values contain arbitrary mess, since during
>    interrupts context can be arbitrary.

> 2. Due to (1) I guess you do not make any limiting of slabs.
>    So there are number of ways how to consume a lot of kernel
>    memory from inside container and
>    OOM killer will kill arbitrary tasks in case of 
>    memory-shortage after that.
>    Don't think it is secure... real DoS.

> 3. Dcache accounting simply doesn't work, since
>    charges/uncharges are done on current context (sic!!!),
>    which is arbitrary. i.e. lookup can be done in VE context,
>    while dcache shrink can be done from another context.
>    So the whole problem with dcache DoS is not solved at 
>    all, it is just hard to trigger.

> 4. Dcache accounting is racy, since your checks look like:
>    if (atomic_read(de->d_count))
>       charge();
>    which obviously races with other dput()'s/lookups.

> 5. Dcache accounting can be hit if someone does `find /`
>    inside container.
>    After that it is impossible to open something new,
>    since all the dentries for directories in dcache will 
>    have d_count > 0 (due it's children).
>    It is a BUG.

> 6. Counters can be non-zero on container stop due to all
>    of the above.

looks like for the the first time you are actually
looking at the code, or at least providing feedback
and/or suggestions for improvements (well, not many
of them, but hey, nobody is perfect :)

> There are more and more points which arise when such a 
> non-intrusive accounting is concerned. 

never claimed that Linux-VServer code is perfect,
(the Linux accounting isn't perfect either in many
ways) and Linux-VServer is constantly improving
(see my other email) ... but IIRC, we are _not_
discussing Linux-VServer code at all, we are talking
about a superior solution, which combines the best
of both worlds ...

> I'm really suprised, that you don't see them
> or try to behave as you don't see them :/

all I'm saying is that there is no point in achieving
perfect accounting and limits (and everything else)
when all you get is Xen performance and resource usage

> And, please, believe me, I would not suggest so much 
> complicated patches If everything was so easy and I 
> had no reasons simply to accept vserver code.

no, you are suggesting those patches, because that
is what your company came up with after being confronted
with the task (of creating OS-Level virtualization) and
the arising problems ... so it definitely _is_ a
solution to those problems, but not necessarily the
best and definitely not the only one :)

> > well, as you know, all current solutions use a syscall
> > interface to do most of the work, in the OpenVZ/Virtuozzo
> > case several, unassigned syscalls are used, while 
> > FreeVPS and Linux-VServer use a registered and versioned
> > (multiplexed) system call, which works quite fine for
> > all known purposes ...
> > 
> > I'm quite happy with the extensibility and flexibility
> > the versioned syscall interface has, the only thing I'd
> > change if I would redesign that interface is, that I
> > would add another pointer argument to eliminate 32/64bit
> > issues completely (i.e. use 4 args instead of the 3)

> Well, I would be happy with syscalls also.

> But my guess is that cpuset guys who already use fs 
> approach won't be happy :/

> Maybe we can use both?

I'm fine with either here, though my preference is
for syscalls (and we will probably keep the versioned
syscall commands for Linux-VServer anyway)

best,
Herbert

> Thanks,
> Kirill
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