linux-kernel - Re: [PATCH] dma-debug: hash_bucket_find needs to allow for offsets within an entry

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Message-ID: <20110720103813.GL15108@amd.com>
Date:	Wed, 20 Jul 2011 12:38:13 +0200
From:	"Roedel, Joerg" <Joerg.Roedel@....com>
To:	Neil Horman <nhorman@...driver.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Divy LeRay <divy@...lsio.com>,
	Stanislaw Gruszka <sgruszka@...hat.com>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] dma-debug: hash_bucket_find needs to allow for offsets
 within an entry

Hi Neil,

thanks for your work on this :)

On Tue, Jul 19, 2011 at 01:41:18PM -0400, Neil Horman wrote:
>  lib/dma-debug.c |   17 +++++++++++++++--
>  1 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/dma-debug.c b/lib/dma-debug.c
> index db07bfd..333f0ee 100644
> --- a/lib/dma-debug.c
> +++ b/lib/dma-debug.c
> @@ -248,10 +248,23 @@ static struct dma_debug_entry *hash_bucket_find(struct hash_bucket *bucket,
>  {
>  	struct dma_debug_entry *entry, *ret = NULL;
>  	int matches = 0, match_lvl, last_lvl = 0;
> +	u64 estart, eend, rstart, rend;
> +
> +	rstart = ref->dev_addr;
> +	rend = rstart + ref->size;
>  
>  	list_for_each_entry(entry, &bucket->list, list) {
> -		if ((entry->dev_addr != ref->dev_addr) ||
> -		    (entry->dev != ref->dev))
> +		estart = entry->dev_addr;
> +		eend = estart + entry->size;
> +
> +		/*
> +		 * An entry matches if the address range specified by the ref
> +		 * dev_addr and size falls entirely within the range specified
> +		 * by the entries dev_addr and size and the devices match
> +		 */
> +		if (entry->dev != ref->dev)
> +			continue;
> +		if ((estart > rstart) || (eend < rend))
>  			continue;
>  
>  		/*

But it is not that easy. The hash_fn works on the dev_addr, this means
to search all entries which might match you need to scan multiple
hash-buckets potentially.
In fact, you need to scan

	hash_fn(estart) <= idx <= hash_fn(eend)

to be sure. The patch above also lifts up the check_unmap which is not
desired.

Thanks,

	Joerg

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