Date: Tue, 2 Jun 2015 14:06:20 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Dan Streetman <ddstreet@ieee.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        David Vrabel <david.vrabel@citrix.com>, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCHv2] frontswap: allow multiple backends
Message-Id: <20150602140620.08465687d7c69f851cd2a10f@linux-foundation.org>
In-Reply-To: <1433168544-26301-1-git-send-email-ddstreet@ieee.org>
References: <20150529150705.5fd6b7c1545ef5829f7ace93@linux-foundation.org>
	<1433168544-26301-1-git-send-email-ddstreet@ieee.org>
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On Mon,  1 Jun 2015 10:22:24 -0400 Dan Streetman <ddstreet@ieee.org> wrote:

> Change frontswap single pointer to a singly linked list of frontswap
> implementations.  Update Xen tmem implementation as register no longer
> returns anything.
> 
> Frontswap only keeps track of a single implementation; any implementation
> that registers second (or later) will replace the previously registered
> implementation, and gets a pointer to the previous implementation that
> the new implementation is expected to pass all frontswap functions to
> if it can't handle the function itself.  However that method doesn't
> really make much sense, as passing that work on to every implementation
> adds unnecessary work to implementations; instead, frontswap should
> simply keep a list of all registered implementations and try each
> implementation for any function.  Most importantly, neither of the
> two currently existing frontswap implementations in the kernel actually
> do anything with any previous frontswap implementation that they
> replace when registering.
> 
> This allows frontswap to successfully manage multiple implementations
> by keeping a list of them all.
> 
> ...
>
> -struct frontswap_ops *frontswap_register_ops(struct frontswap_ops *ops)
> +void frontswap_register_ops(struct frontswap_ops *ops)
>  {
> -	struct frontswap_ops *old = frontswap_ops;
> -	int i;
> -
> -	for (i = 0; i < MAX_SWAPFILES; i++) {
> -		if (test_and_clear_bit(i, need_init)) {
> -			struct swap_info_struct *sis = swap_info[i];
> -			/* __frontswap_init _should_ have set it! */
> -			if (!sis->frontswap_map)
> -				return ERR_PTR(-EINVAL);
> -			ops->init(i);
> -		}
> +	DECLARE_BITMAP(a, MAX_SWAPFILES);
> +	DECLARE_BITMAP(b, MAX_SWAPFILES);
> +	struct swap_info_struct *si;
> +	unsigned int i;
> +
> +	spin_lock(&swap_lock);
> +	plist_for_each_entry(si, &swap_active_head, list) {
> +		if (!WARN_ON(!si->frontswap_map))
> +			set_bit(si->type, a);

umm, DECLARE_BITMAP() doesn't initialise the storage.  Either this
patch wasn't tested very well or you should buy me a lottery ticket!

>  	}
> -	/*
> -	 * We MUST have frontswap_ops set _after_ the frontswap_init's
> -	 * have been called. Otherwise __frontswap_store might fail. Hence
> -	 * the barrier to make sure compiler does not re-order us.
> +	spin_unlock(&swap_lock);
> +
> +	/* the new ops needs to know the currently active swap devices */
> +	for_each_set_bit(i, a, MAX_SWAPFILES)
> +		ops->init(i);
> +
> +	/* setting frontswap_ops must happen after the ops->init() calls
> +	 * above; cmpxchg implies smp_mb() which will ensure the init is
> +	 * complete at this point
> +	 */

Like this, please:

	/*
	 * Setting ...

and sentences start with capital letters ;)


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