[Devel] Re: [-mm PATCH 1/8] Memory controller resource counters (v2)

[Pavel Emelianov]

Dave Hansen wrote:
> On Thu, 2007-07-05 at 22:20 -0700, Balbir Singh wrote:
>> +/*
>> + * the core object. the container that wishes to account for some
>> + * resource may include this counter into its structures and use
>> + * the helpers described beyond
>> + */
> 
> I'm going to nitpick a bit here.  Nothing major, I promise. ;)
> 
> Could we make these comments into nice sentences with capitalization?  I
> think it makes them easier to read in long comments.
> 
> How about something like this for the comment:
> 
> /*
>  * A container wishing to account for a resource should include this
>  * structure into one of its own.  It may use the helpers below.
>  */
> 
> The one above is worded a little bit strangely.
> 
>> +struct res_counter {
>> +	/*
>> +	 * the current resource consumption level
>> +	 */
>> +	unsigned long usage;
>> +	/*
>> +	 * the limit that usage cannot exceed
>> +	 */
>> +	unsigned long limit;
>> +	/*
>> +	 * the number of insuccessful attempts to consume the resource
>> +	 */
> 
> unsuccessful
> 
>> +	unsigned long failcnt;
>> +	/*
>> +	 * the lock to protect all of the above.
>> +	 * the routines below consider this to be IRQ-safe
>> +	 */
>> +	spinlock_t lock;
>> +};
> 
> Do we really need all of these comments?  Some of them are a wee bit
> self-explanatory.  I think we mostly know what a limit is. ;)

Since this is a new entities in the kernel and not many people
deal with the resource management, I think that nothing bad in
having them.

page->_count, signal_struct->shared_pending, mm_struct->mm_users and
others do not bother anyone with their comments either.

>> +/*
>> + * helpers to interact with userspace
>> + * res_counter_read/_write - put/get the specified fields from the
>> + * res_counter struct to/from the user
>> + *
>> + * @cnt:     the counter in question
>> + * @member:  the field to work with (see RES_xxx below)
>> + * @buf:     the buffer to opeate on,...
>> + * @nbytes:  its size...
>> + * @pos:     and the offset.
>> + */
>> +
>> +ssize_t res_counter_read(struct res_counter *cnt, int member,
>> +		const char __user *buf, size_t nbytes, loff_t *pos);
>> +ssize_t res_counter_write(struct res_counter *cnt, int member,
>> +		const char __user *buf, size_t nbytes, loff_t *pos);
>> +
>> +/*
>> + * the field descriptors. one for each member of res_counter
>> + */
>> +
>> +enum {
>> +	RES_USAGE,
>> +	RES_LIMIT,
>> +	RES_FAILCNT,
>> +};
>> +

[snip]

>> diff -puN /dev/null kernel/res_counter.c
>> --- /dev/null	2007-06-01 08:12:04.000000000 -0700
>> +++ linux-2.6.22-rc6-balbir/kernel/res_counter.c	2007-07-05 13:45:17.000000000 -0700
>> @@ -0,0 +1,121 @@
>> +/*
>> + * resource containers
>> + *
>> + * Copyright 2007 OpenVZ SWsoft Inc
>> + *
>> + * Author: Pavel Emelianov <xemul at openvz.org>
>> + *
>> + */
>> +
>> +#include <linux/types.h>
>> +#include <linux/parser.h>
>> +#include <linux/fs.h>
>> +#include <linux/res_counter.h>
>> +#include <linux/uaccess.h>
>> +
>> +void res_counter_init(struct res_counter *cnt)
>> +{
>> +	spin_lock_init(&cnt->lock);
>> +	cnt->limit = (unsigned long)LONG_MAX;
>> +}
>> +
>> +int res_counter_charge_locked(struct res_counter *cnt, unsigned long val)
>> +{
>> +	if (cnt->usage <= cnt->limit - val) {
>> +		cnt->usage += val;
>> +		return 0;
>> +	}
>> +
>> +	cnt->failcnt++;
>> +	return -ENOMEM;
>> +}
> 
> More nitpicking...
> 
> Can we leave the normal control flow in the lowest indentation level,
> and have only errors in the indented if(){} blocks?  Something like
> this:

As far as I know gcc usually makes the "true" branch to be 
in the straight code flow and in general case this does not 
trash the CPU pipeline.

>> +int res_counter_charge_locked(struct res_counter *cnt, unsigned long
> val)
>> +{
>> +	if (cnt->usage > cnt->limit - val) {
>> +		cnt->failcnt++;
>> +		return -ENOMEM;
>> +	}
>> +	cnt->usage += val;
>> +	return 0;
>> +}
> 
> Also, can you do my poor brain a favor an expand "cnt" to "counter"?
> You're not saving _that_ much typing ;)

Good catch. We use cnt for booth container and counter :)

>> +int res_counter_charge(struct res_counter *cnt, unsigned long val)
>> +{
>> +	int ret;
>> +	unsigned long flags;
>> +
>> +	spin_lock_irqsave(&cnt->lock, flags);
>> +	ret = res_counter_charge_locked(cnt, val);
>> +	spin_unlock_irqrestore(&cnt->lock, flags);
>> +	return ret;
>> +}
>> +
>> +void res_counter_uncharge_locked(struct res_counter *cnt, unsigned long val)
>> +{
>> +	if (unlikely(cnt->usage < val)) {
>> +		WARN_ON(1);
>> +		val = cnt->usage;
>> +	}
>> +
>> +	cnt->usage -= val;
>> +}
> 
> It actually looks like the WARN_ON() macros "return" values.  You should
> be able to:
> 
> 	if (WARN_ON(cnt->usage < val))
> 		val = count->usage;

Oh.. I do not trust these macros actually. One day some guy will
make CONFIG_OPTIMIZE_WARN_ON and will remove all these checks
out. Consider me a paranoiac.

>> +void res_counter_uncharge(struct res_counter *cnt, unsigned long val)
>> +{
>> +	unsigned long flags;
>> +
>> +	spin_lock_irqsave(&cnt->lock, flags);
>> +	res_counter_uncharge_locked(cnt, val);
>> +	spin_unlock_irqrestore(&cnt->lock, flags);
>> +}
>> +
>> +
>> +static inline unsigned long *res_counter_member(struct res_counter *cnt, int member)
>> +{
>> +	switch (member) {
>> +	case RES_USAGE:
>> +		return &cnt->usage;
>> +	case RES_LIMIT:
>> +		return &cnt->limit;
>> +	case RES_FAILCNT:
>> +		return &cnt->failcnt;
>> +	};
>> +
>> +	BUG();
>> +	return NULL;
>> +}
>>
>> +ssize_t res_counter_read(struct res_counter *cnt, int member,
>> +		const char __user *userbuf, size_t nbytes, loff_t *pos)
>> +{
>> +	unsigned long *val;
>> +	char buf[64], *s;
>> +
>> +	s = buf;
>> +	val = res_counter_member(cnt, member);
>> +	s += sprintf(s, "%lu\n", *val);
>> +	return simple_read_from_buffer((void __user *)userbuf, nbytes,
>> +			pos, buf, s - buf);
>> +}
> 
> Why do we need that cast?  

simple_read_from_buffer do not take const char * as the 1st arg

>> +ssize_t res_counter_write(struct res_counter *cnt, int member,
>> +		const char __user *userbuf, size_t nbytes, loff_t *pos)
>> +{
>> +	int ret;
>> +	char *buf, *end;
>> +	unsigned long tmp, *val;
>> +
>> +	buf = kmalloc(nbytes + 1, GFP_KERNEL);
> 
> Do we need some checking on nbytes?  Is it sanitized before it gets
> here?

I think we need some kind of simple_strtol_from_user() and
simple_strtol_to_user() instead. Since this code is the only user of
it I didn't make a separate patch for these yet.

>> +	ret = -ENOMEM;
>> +	if (buf == NULL)
>> +		goto out;
>> +
>> +	buf[nbytes] = 0;
> 
> Please use '\0'.  0 isn't a char. 
>  
>> +	ret = -EFAULT;
>> +	if (copy_from_user(buf, userbuf, nbytes))
>> +		goto out_free;
>> +
>> +	ret = -EINVAL;
>> +	tmp = simple_strtoul(buf, &end, 10);
>> +	if (*end != '\0')
>> +		goto out_free;
>> +
>> +	val = res_counter_member(cnt, member);
>> +	*val = tmp;
>> +	ret = nbytes;
>> +out_free:
>> +	kfree(buf);
>> +out:
>> +	return ret;
>> +}
>> _
>>
> -- Dave
> 
> -
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