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

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Date:	Wed, 20 Jul 2011 15:29:25 +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

On Wed, Jul 20, 2011 at 07:11:56AM -0400, Neil Horman wrote:
> On Wed, Jul 20, 2011 at 12:38:13PM +0200, Roedel, Joerg wrote:

> > 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)
> > 
> Ah, good point.  Its actually a bit more difficult that what you describe I
> think.  Given a ref->dev_addr, this check needs to find the entry in any bucket
> for a matching device that has an entry->dev_addr less than ref->dev_addr, where
> the former has a larger size than the latter.  And since we don't know the true
> size of the entry we are looking for, we could have crossed the HASH_FN_SHIFT
> many times over to get to the ref->dev addr that was passed in.  It almost
> sounds like the hash table for this needs to be accessible by device rather than
> by address.

You are right. We need to scan

	0 <= idx <= hash_fn(rstart)

Probably we can fix that with a better hash-function. Any ideas? Using
the device is not an option because then all entries would end up in
only a few buckets. This will impact scanning performance too much.

For now, the partial syncs seem to happen rarely enough so that we can
make it a slow-path. It is probably the best to do the exact scan first
and do the full scan only if exact-scan failed (until we come up with a
better solution).


> > to be sure. The patch above also lifts up the check_unmap which is not
> > desired.
> Hm, you're right, I suppose we need to add an exact map option to the passed in
> ref structure or some such, I'll rework this

Thanks,

	Joerg

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