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

[Dave Hansen]

On Mon, 2007-07-09 at 11:16 +0400, Pavel Emelianov wrote:
> 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.

They waste space.  It makes the code harder to read.

> >> +/*
> >> + * 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.

It's not a big deal either way, but that's a pretty weak reason for
doing it that way.  Can you actually demonstrate a performance
difference?  If not, we should defer to the most readable form.

> >> +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

True, but we can pass char* to a function taking void* without a problem
and without an explicit cast.

What's the actual problem?  The "const"?  We're effectively throwing
away the information here that res_counter_read() expects userbuf to be
constant.  If simple_read_from_buffer() ever decided to write to
userbuf, we'd be in trouble.  If simple_read_from_buffer() will never
write, then _it_ should have a const first argument.

Also, what if "userbuf" changes type?  We'll never see warnings, just
weird runtime bugs.

I just worry that these kinds of casts shut up warnings that _are_ valid
and might find real bugs.  

-- Dave

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