netdev - Re: [PATCH 12/30] mm: memory reserve management

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Message-ID: <18593.11340.609526.649904@notabene.brown>
Date:	Tue, 12 Aug 2008 16:23:08 +1000
From:	Neil Brown <neilb@...e.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	netdev@...r.kernel.org, trond.myklebust@....uio.no,
	Daniel Lezcano <dlezcano@...ibm.com>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH 12/30] mm: memory reserve management

On Thursday July 24, a.p.zijlstra@...llo.nl wrote:
> Generic reserve management code. 
> 
> It provides methods to reserve and charge. Upon this, generic alloc/free style
> reserve pools could be build, which could fully replace mempool_t
> functionality.

This looks quite different to last time I looked at the code (I
think).

You now have a more structured "kmalloc_reserve" interface which
returns a flag to say if the allocation was from an emergency pool.  I
think this will be a distinct improvement at the call sites, though I
haven't looked at them yet. :-)

> +
> +struct mem_reserve {
> +	struct mem_reserve *parent;
> +	struct list_head children;
> +	struct list_head siblings;
> +
> +	const char *name;
> +
> +	long pages;
> +	long limit;
> +	long usage;
> +	spinlock_t lock;	/* protects limit and usage */
                                            ^^^^^
> +
> +	wait_queue_head_t waitqueue;
> +};

....
> +static void __calc_reserve(struct mem_reserve *res, long pages, long limit)
> +{
> +	unsigned long flags;
> +
> +	for ( ; res; res = res->parent) {
> +		res->pages += pages;
> +
> +		if (limit) {
> +			spin_lock_irqsave(&res->lock, flags);
> +			res->limit += limit;
> +			spin_unlock_irqrestore(&res->lock, flags);
> +		}
> +	}
> +}

I cannot figure out why the spinlock is being used to protect updates
to 'limit'.
As far as I can see, mem_reserve_mutex already protects all those
updates.
Certainly we need the spinlock for usage, but why for limit??

> +
> +void *___kmalloc_reserve(size_t size, gfp_t flags, int node, void *ip,
> +			 struct mem_reserve *res, int *emerg)
> +{
....
> +	if (emerg)
> +		*emerg |= 1;

Why not just

	if (emerg)
		*emerg = 1.

I can't we where '*emerg' can have any value but 0 or 1, so the '|' is
pointless ???

Thanks,
NeilBrown
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