Re: [ckrm-tech] [RFC][PATCH][2/4] Add RSS accounting and control

[Balbir Singh <balbir@in.ibm.com>]

Andrew Morton wrote:
> On Mon, 19 Feb 2007 16:07:44 +0530 Balbir Singh <balbir@in.ibm.com> wrote:
>
> > > > +void memctlr_mm_free(struct mm_struct *mm)
> > > > +{
> > > > +	kfree(mm->counter);
> > > > +}
> > > > +
> > > > +static inline void memctlr_mm_assign_container_direct(struct mm_struct *mm,
> > > > +							struct container *cont)
> > > > +{
> > > > +	write_lock(&mm->container_lock);
> > > > +	mm->container = cont;
> > > > +	write_unlock(&mm->container_lock);
> > > > +}
> > > More weird locking here.
> > The container field of the mm_struct is protected by a read write spin lock.
>
> That doesn't mean anything to me.
>
> What would go wrong if the above locking was simply removed?  And how does
> the locking prevent that fault?

Some pages could charged to the wrong container. Apart from that I do not
see anything going bad (I'll double check that).

> > > > +void memctlr_mm_assign_container(struct mm_struct *mm, struct task_struct *p)
> > > > +{
> > > > +	struct container *cont = task_container(p, &memctlr_subsys);
> > > > +	struct memctlr *mem = memctlr_from_cont(cont);
> > > > +
> > > > +	BUG_ON(!mem);
> > > > +	write_lock(&mm->container_lock);
> > > > +	mm->container = cont;
> > > > +	write_unlock(&mm->container_lock);
> > > > +}
> > > And here.
> > Ditto.
>
> ditto ;)

:-)

> > > > +/*
> > > > + * Update the rss usage counters for the mm_struct and the container it belongs
> > > > + * to. We do not fail rss for pages shared during fork (see copy_one_pte()).
> > > > + */
> > > > +int memctlr_update_rss(struct mm_struct *mm, int count, bool check)
> > > > +{
> > > > +	int ret = 1;
> > > > +	struct container *cont;
> > > > +	long usage, limit;
> > > > +	struct memctlr *mem;
> > > > +
> > > > +	read_lock(&mm->container_lock);
> > > > +	cont = mm->container;
> > > > +	read_unlock(&mm->container_lock);
> > > > +
> > > > +	if (!cont)
> > > > +		goto done;
> > > And here.  I mean, if there was a reason for taking the lock around that
> > > read, then testing `cont' outside the lock just invalidated that reason.
> > We took a consistent snapshot of cont. It cannot change outside the lock,
> > we check the value outside. I am sure I missed something.
>
> If it cannot change outside the lock then we don't need to take the lock!

We took a snapshot that we thought was consistent. We check for the value
outside. I guess there is no harm, the worst thing that could happen
is wrong accounting during mm->container changes (when a task changes
container).

> > MEMCTLR_DONT_CHECK_LIMIT exists for the following reasons
> >
> > 1. Pages are shared during fork, fork() is not failed at that point
> >     since the pages are shared anyway, we allow the RSS limit to be
> >     exceeded.
> > 2. When ZERO_PAGE is added, we don't check for limits (zeromap_pte_range).
> > 3. On reducing RSS (passing -1 as the value)
>
> OK, that might make a nice comment somewhere (if it's not already there).

Yes, thanks for keeping us humble and honest, I'll add it.

--
	Warm Regards,
	Balbir Singh
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