Re: [PATCH 3/6] drm/i915: use vmap in shmem_pin_map

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Tue, Sep 22, 2020 at 08:22:49AM +0200, Christoph Hellwig wrote:
> On Mon, Sep 21, 2020 at 08:11:57PM +0100, Matthew Wilcox wrote:
> > This is awkward.  I'd like it if we had a vfree() variant which called
> > put_page() instead of __free_pages().  I'd like it even more if we
> > used release_pages() instead of our own loop that called put_page().
> 
> Note that we don't need a new vfree variant, we can do this manually if
> we want, take a look at kernel/dma/remap.c.  But I thought this code
> intentionally doesn't want to do that to avoid locking in the memory
> for the pages array.  Maybe the i915 maintainers can clarify.

Actually, vfree() will work today; I cc'd you on a documentation update
to make it clear that this is permitted.

>From my current experience with the i915 shmem code, I think that the
i915 maintainers are experts at graphics, and are unfamiliar with the MM.
There are a number of places where they do things the hard way.