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

From: Balbir Singh
Date: Mon Feb 19 2007 - 06:10:19 EST

Andrew Morton wrote:
On Mon, 19 Feb 2007 16:07:44 +0530 Balbir Singh <balbir@xxxxxxxxxx> 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 ;)


+ * 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

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
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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