Re: [patch] mm: pageable memory allocator (for DRM-GEM?)

[Jerome Glisse <glisse@freedesktop.org>]

On Tue, 23 Sep 2008 12:21:26 +0200
Thomas Hellstrom <thomas@tungstengraphics.com> wrote:

> Nick Piggin wrote:
> > Hi,
> >
> > So I promised I would look at this again, because I (and others) have some
> > issues with exporting shmem_file_setup for DRM-GEM to go off and do things
> > with.
> >
> > The rationale for using shmem seems to be that pageable "objects" are needed,
> > and they can't be created by userspace because that would be ugly for some
> > reason, and/or they are required before userland is running.
> >
> > I particularly don't like the idea of exposing these vfs objects to random
> > drivers because they're likely to get things wrong or become out of synch
> > or unreviewed if things change. I suggested a simple pageable object allocator
> > that could live in mm and hide the exact details of how shmem / pagecache
> > works. So I've coded that up quickly.
> >
> > Upon actually looking at how "GEM" makes use of its shmem_file_setup filp, I
> > see something strange... it seems that userspace actually gets some kind of
> > descriptor, a descriptor to an object backed by this shmem file (let's call it
> > a "file descriptor"). Anyway, it turns out that userspace sometimes needs to
> > pread, pwrite, and mmap these objects, but unfortunately it has no direct way
> > to do that, due to not having open(2)ed the files directly. So what GEM does
> > is to add some ioctls which take the "file descriptor" things, and derives
> > the shmem file from them, and then calls into the vfs to perform the operation.
> >
> > If my cursory reading is correct, then my allocator won't work so well as a
> > drop in replacement because one isn't allowed to know about the filp behind
> > the pageable object. It would also indicate some serious crack smoking by
> > anyone who thinks open(2), pread(2), mmap(2), etc is ugly in comparison...
> >
> > So please, nobody who worked on that code is allowed to use ugly as an
> > argument. Technical arguments are fine, so let's try to cover them.
> >
> >   
> Nick,
>  From my point of view, this is exactly what's needed, although there 
> might be some different opinions among the
> DRM developers. A question:
> 
> Sometimes it's desirable to indicate that a page / object is "cleaned", 
> which would mean data has moved and is backed by device memory. In that 
> case one could either free the object or indicate to it that it can 
> release it's pages. Is freeing / recreating such an object an expensive 
> operation? Would it, in that case, be possible to add an object / page 
> "cleaned" function?
> 
> /Thomas

Also what about a uncached page allocator ? As some drivers might need
them, there is no number but i think their was some concern that changing
PAT too often might be costly and that we would better have a poll of
such pages.

Cheers,
Jerome Glisse <glisse@freedesktop.org>

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