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.