linux-kernel - Re: [RFC][PATCH 2/5] add SWAP_HAS_CACHE flag to swap

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Message-Id: <20090528094157.5c39ac57.nishimura@mxp.nes.nec.co.jp>
Date:	Thu, 28 May 2009 09:41:57 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"hugh.dickins@...cali.co.uk" <hugh.dickins@...cali.co.uk>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [RFC][PATCH 2/5] add SWAP_HAS_CACHE flag to swap_map

> @@ -1969,17 +2017,33 @@ int swap_duplicate(swp_entry_t entry)
>  	offset = swp_offset(entry);
>  
>  	spin_lock(&swap_lock);
> -	if (offset < p->max && p->swap_map[offset]) {
> -		if (p->swap_map[offset] < SWAP_MAP_MAX - 1) {
> -			p->swap_map[offset]++;
> +
> +	if (unlikely(offset >= p->max))
> +		goto unlock_out;
> +
> +	count = swap_count(p->swap_map[offset]);
> +	has_cache = swap_has_cache(p->swap_map[offset]);
> +	if (cache) {
> +		/* set SWAP_HAS_CACHE if there is no cache and entry is used */
> +		if (!has_cache && count) {
Should we check !has_cache here ?

Concurrent read_swap_cache_async() might have set SWAP_HAS_CACHE, but not have added
a page to swap cache yet when find_get_page() was called.
add_to_swap_cache() would handle the race of concurrent read_swap_cache_async(),
but considering more, swapcache_free() at the end of the loop might dangerous in this case...
So I think it should be like:

	read_swap_cache_async()
		:
		valid = swapcache_prepare(entry);
		if (!valid)
			break;
		if (valid == -EAGAIN);
			continue;

to let the context that succeeded in swapcache_prepare() do add_to_swap_cache().


Thanks,
Daisuke Nishimura.

> +			p->swap_map[offset] = make_swap_count(count, 1);
> +			result = 1;
> +		}
> +	} else if (count || has_cache) {
> +		if (count < SWAP_MAP_MAX - 1) {
> +			p->swap_map[offset] = make_swap_count(count + 1,
> +							      has_cache);
>  			result = 1;
> -		} else if (p->swap_map[offset] <= SWAP_MAP_MAX) {
> +		} else if (count <= SWAP_MAP_MAX) {
>  			if (swap_overflow++ < 5)
> -				printk(KERN_WARNING "swap_dup: swap entry overflow\n");
> -			p->swap_map[offset] = SWAP_MAP_MAX;
> +				printk(KERN_WARNING
> +				       "swap_dup: swap entry overflow\n");
> +			p->swap_map[offset] = make_swap_count(SWAP_MAP_MAX,
> +							      has_cache);
>  			result = 1;
>  		}
>  	}
> +unlock_out:
>  	spin_unlock(&swap_lock);
>  out:
>  	return result;
--
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