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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 22/09/2020 07:22, 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.

+ Chris & Matt who were involved with this part of i915.

If I understood this sub-thread correctly, iterating and freeing the 
pages via the vmapped ptes, so no need for a
shmem_read_mapping_page_gfp loop in shmem_unpin_map looks plausible to me.

I did not get the reference to kernel/dma/remap.c though, and also not 
sure how to do the error unwind path in shmem_pin_map at which point the 
allocated vm area hasn't been fully populated yet. Hand-roll the loop 
walking vm area struct in there?

Regards,

Tvrtko