Re: PROBLEM: kernel BUG at drivers/gpu/drm/drm_fops.c:146!

[Thomas Hellström]

Andrew Morton wrote:
> On Thu, 29 Jan 2009 19:50:17 -0800 Jesse Barnes <jbarnes@xxxxxxxxxxxxxxxx> wrote:
>
>   
> > On Thursday, January 29, 2009 5:43 pm Dave Airlie wrote:
> >     
> > > On Fri, Jan 30, 2009 at 11:20 AM, Andrew Morton
> > >       
> > > > hm, I'm a bit surprised to see the drm code using `struct
> > > > address_space' and read_mapping_page() and unmap_mapping_range() and
> > > > such.  I thought those only worked with regular files and pagecache :)
> > > >
> > > > Is it possible to briefly explain what's going on there?
> > > >
> > > > What instance of address_space_operations does ->dev_mapping actually
> > > > point at?
> > > >         
> > > Okay a bit tired and headache coming on but I'll try, maybe jbarnes
> > > can help out,
> > >
> > > We need to provide mappings to userspace that are backed by memory
> > > that can move around behind the mappings.
> > >
> > > So userspace wants a mapping for a GEM object via the AGP/GTT aperture
> > > instead of directly to the backing pages.
> > > Now as the GEM object is backed by shmem we can't use the shmem file
> > > descriptor we have to tie the mapping to without
> > > hacking up the shmem mmap functionality which seemed like a bad plan.
> > >
> > > So GEM uses the device inode to setup the mappings on. We just use a
> > > simple linear allocator to split up the device inodes address space
> > > and assign chunks to handles for different objects. The userspace app
> > > then uses the handle via mmap to get access to the VMAs. Now when GEM
> > > wants to move that object out of the GTT or to another area of the GTT
> > > we need some way to invalidate it, so we use unmap_mapping_range
> > > which destroys all the mappings for the object in all the VMA for all
> > > the processes mapping it currently
> > >
> > > GEM's read_mapping_page is distinct from this and is to do with the
> > > shmem interfaceing.
> > >
> > > Not sure if this explains it or just make it worse.
> > >       
> > Sounds right to me.  The offsets are just handles, not real file objects or 
> > backing store addresses.  We use them to take advantage of all the inode 
> > address mapping helpers, since they track stuff for us.
> >
> > That said, unmap_mapping_range may not be the best way to do this; basically 
> > we need a way to invalidate a given processes' mapping of a GTT range (which 
> > in turn is backed by real RAM).  If there's some other way we should be doing 
> > this I'm all ears.
> >     
>
> Well, we'd need to call in the big guns on this one - I've already
> stirred Hugh ;)
>
> unmap_mapping_range() is basically a truncate thing - it shoots down
> all mappings of a range of a *file*.  Across all processes in the
> machine which map that file.
>
> If that isn't what you want to do (and it sounds that way) then you'd
> want to use something which is mm_struct (or vma) centric, rather than
> file-centric.  zap_page_range(), methinks.
>
>   
I guess I was the one starting to use this function, so some explanation:

When the drm device is used to provide address space for buffers, 
user-space actually see it as a file with a distinct offset where 
buffers are laid out in a linear fashion, To access a certain buffer you 
need to lseek() to the correct offset and then read() write() or, the 
more common use, mmap / munmap.

When looking through its implementation, unmap_mapping_range() seemed to 
do exactly the thing I wanted, namely to kill all user-space mappings of 
all vmas of all processes mapping a part of the device address space. 
And it saves us from storing a list of all vmas mapping the device 
within the drm device.

What makes usage of unmap_mapping_range() on a device node with a well 
defined offset-to-data mapping different from using it on a file?

/Thomas
> ------------------------------------------------------------------------------
> This SF.net email is sponsored by:
> SourcForge Community
> SourceForge wants to tell your story.
> http://p.sf.net/sfu/sf-spreadtheword
> --
> _______________________________________________
> Dri-devel mailing list
> Dri-devel@xxxxxxxxxxxxxxxxxxxxx
> https://lists.sourceforge.net/lists/listinfo/dri-devel
>   



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/