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

[Neil Horman <nhorman@tuxdriver.com>]

On Wed, Jul 20, 2011 at 04:59:01PM +0200, Roedel, Joerg wrote:
> On Wed, Jul 20, 2011 at 10:32:22AM -0400, Neil Horman wrote:
> > On Wed, Jul 20, 2011 at 03:29:25PM +0200, Roedel, Joerg wrote:
> > > 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.
> > > 
> > Unfortunately I don't have any ideas for a better hash function here, but I had
> > been thinking about fixing this in add_dma_entry.  We could detect there that a
> > debug entry to be added crossed one or more hash bucket boundaries, and, if it
> > did, split it along those boundaries into multiple entries, hashing each of them
> > in separately.  The check_unmap and check_sync routines would of course then
> > potentially have to do multiple lookups as well to ensure that they found all of
> > the correct entries to validate/remove.  It would work in all cases, but it
> > might be overkill.  What do you think?
> 
> Interesting. I discussed that with a colleague an hour ago and he came
> up with the same idea :-)
> I like it because we still need to scan only one hash-bucket, so this
> seems like the best solution.
> 
> > > 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).
> > > 
> > Agreed, if you don't like my above idea, I'll get to work on this this
> > afternoon.
> 
> I think this idea is a better solution.
> 
Copy that, i'll try implement it later today, thanks!
Neil

> Thanks,
> 
> 	Joerg
> 
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