Hi Rusty,
I am pathologically late in catching up lkml, so if I missed some
context here, I apologize in advance. I have just started looking
at mm6 ipc code and I want to point out a few things.
On Mon, Oct 28, 2002 at 02:20:04AM +0100, Rusty Russell wrote:
> Yes, nonsensical. Firstly, it's in violation of the standard to fail
> IPC_RMID under random circumstances. Secondly, failing to clean up is
> an unhandlable error, since you're quite possible in the failure path
> of the code already. This is a well known issue.
I am not sure how Ming/Hugh's current IPC changes affect IPC_RMID.
It affects only when you are trying to add a new ipc. In fact,
since it is a *add* operation (grow_ary()), it seems ok to fail it if rcu_head
allocation fails. Feel free to correct me if I missed something here.
AFAICS, the rcu stuff doesn't affect any freeing other than the IPC
id array.
> Two oom kills. Three oom kills. Four oom kills. Where's the bound
> here?
>
> Our allocator behavior for GFP_KERNEL has changed several times. Are
> you sure that it won't *ever* fail under other circomstances?
>
> > Okay (I expect, didn't review it) for just the ids arrays, but too much
> > memory waste if we have to allocate for each msq, sema, shm: if there's
> > a better solution available. mempool looks better to us.
>
> It's a hacky, fragile and incorrect solution. It's completely
> tasteless.
Yes, the mempool code is broken, but only because rcu_backup_pool
is created three times, one by each IPC mechanism init :-)
> > Not yet. You seem to have had a bad experience with something like
> > this (the mempools or the RCU or the combination), and you're warning
> > us away without actually telling us what you found.
>
> I *wrote* the RCU interface (though the implementation in 2.5 isn't
> mine). I thought it was pretty clear how it was supposed to be used.
> Obviously, I was wrong.
Yes, we went through this a long time ago and the general model
is to embedd the rcu_head thereby allocating it at the time
of allocation of the RCU protected data. This increases the
probability of recovery from low-memory situation as compared
to having to allocte during freeing.
That said, it seems that Ming/Hugh's patch does allocate
the rcu_head at the time of *growing* the array. It is just
that they allocate it for the freeing array rather than the
allocated array. I don't see how this is semantically different
from clubbing the two allocations other than the fact that
smaller number of allocation calls would likely reduce the
likelyhood of allocation failures.
> Patch below is against Mingming's mm4 release. Compiles, untested.
> Rusty.
Yes, this is the typical RCU model, except that in this case (IPC),
I am not quite sure if it is in effect that different from what Ming/Hugh
have done.
Thanks
Dipankar
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