Re: [RFC 1/2]swap: add a simple buddy allocator

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 22 Oct 2012 10:30:51 +0800
Shaohua Li <shli@kernel.org> wrote:

> I'm using a fast SSD to do swap. scan_swap_map() sometimes uses up to 20~30%
> CPU time (when cluster is hard to find), which becomes a bottleneck.
> scan_swap_map() scans a byte array to search a 256 page cluster, which is very
> slow.
> 
> Here I introduced a simple buddy allocator. Since we only care about 256 pages
> cluster, we can just use a counter to implement the buddy allocator. Every 256
> pages use one int to store the counter, so searching cluster is very efficient.
> With this, scap_swap_map() overhead disappears.
> 
> This might help low end SD card swap too. Because if the cluster is aligned, SD
> firmware can do flash erase more efficiently.
> 
> The downside is the cluster must be aligned to 256 pages, which will reduce the
> chance to find a cluster.
> 

hm.  How serious is this downside?

>
> ...
>
> @@ -2020,12 +2052,19 @@ SYSCALL_DEFINE2(swapon, const char __use
>  		goto bad_swap;
>  	}
>  
> +	swap_cluster_count = vzalloc(DIV_ROUND_UP(maxpages, SWAPFILE_CLUSTER) *
> +					sizeof(int));

[ Actually sizeof(unsigned int).  Or, probably safer, sizeof(*swap_cluster_count)]

How large is this allocation?  swap-size-in-bytes/256k, methinks.  So
64kbytes for a 16G swap partition?  That sounds acceptable.  Something
like lib/flex_array.c could be used here perhaps, although that would
involve memory allocations at awkward times.

> +	if (!swap_cluster_count) {
> +		error = -ENOMEM;
> +		goto bad_swap;
> +	}

I shall await Hugh review on this patchset ;)

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