linux-kernel - Re: Gem GTT mmaps..

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Message-ID: <498CBC3B.6010609@shipmail.org>
Date:	Fri, 06 Feb 2009 23:39:55 +0100
From:	Thomas Hellström <thomas@...pmail.org>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
CC:	Eric Anholt <eric@...olt.net>,
	DRI <dri-devel@...ts.sourceforge.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: Gem GTT mmaps..

Jesse Barnes wrote:
> On Friday, February 6, 2009 1:35 pm Thomas Hellström wrote:
>   
>> Jesse Barnes wrote:
>>     
>>> On Thursday, February 5, 2009 10:37 am Jesse Barnes wrote:
>>>       
>>>> So if we leave the lookup reference around from the GTT mapping ioctl,
>>>> that would take care of new mappings.  And if we added/removed
>>>> references at VM open/close time, we should be covered for fork.  But is
>>>> it ok to add a new unref in the finish ioctl for GTT mapped objects?  I
>>>> don't think so, because we don't know for sure if the caller was the one
>>>> that created the new fake offset (which would be one way of detecting
>>>> whether it was GTT mapped). Seems like we need a new unmap ioctl?  Or we
>>>> could put the mapping ref/unref in libdrm, where it would be tracked on
>>>> a per-process basis...
>>>>         
>>> Ah but maybe we should just tear down the fake offset at unmap time; then
>>> we'd be able to use it as an existence test for the mapping and get the
>>> refcounting right.  The last thing I thought of was whether we'd be ok in
>>> a map_gtt -> crash case.  I *think* the vm_close code will deal with
>>> that, if we do a deref there?
>>>       
>> Yes, an mmap() is always paired with a vm_close(), and the vm_close()
>> also happens in a crash situation.
>>     
>
> This one should cover the cases you found.
>   - ref at map time will keep the object around so fault shouldn't fail
>   - additional threads will take their refs in vm_open/close
>   - unmap will unref and remove mmap_offset allowing object to be freed
>
>   
Jesse,

Yes, it looks OK to me.
A short question, though, when is i915_gem_sw_finish_ioctl called? Is it 
possible that the client may still think its mmap offset is valid?

Thomas




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