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

From: Christoph Hellwig
Date: Tue Sep 22 2020 - 12:33:52 EST

On Tue, Sep 22, 2020 at 05:13:45PM +0100, Tvrtko Ursulin wrote:
>> void *shmem_pin_map(struct file *file)
>> {
>> - const size_t n_pte = shmem_npte(file);
>> - pte_t *stack[32], **ptes, **mem;
> Chris can comment how much he'd miss the 32 page stack shortcut.

I'd like to see a profile that claim that kmalloc matters in a
path that does a vmap and reads pages through the page cache.
Especially when the kmalloc saves doing another page cache lookup
on the free side.

> Is there something in vmap() preventing us from freeing the pages array
> here? I can't spot anything that is holding on to the pointer. Or it was
> just a sketch before you realized we could walk the vm_area?
> Also, I may be totally misunderstanding something, but I think you need to
> assign area->pages manually so shmem_unpin_map can access it below.

We need area->pages to hold the pages for the free side. That being
said the patch I posted is broken because it never assigned to that.
As said it was a sketch. This is the patch I just rebooted into on
my Laptop:

it needs extra prep patches from the series:

>> mapping_clear_unevictable(file->f_mapping);
>> - __shmem_unpin_map(file, ptr, shmem_npte(file));
>> + for (i = 0; i < shmem_npages(file); i++)
>> + put_page(area->pages[i]);
>> + kvfree(area->pages);
>> + vunmap(ptr);
> Is the verdict from mm experts that we can't use vfree due __free_pages vs
> put_page differences?

Switched to vfree now.

> Could we get from ptes to pages, so that we don't have to keep the
> area->pages array allocated for the duration of the pin?

We could do vmalloc_to_page, but that is fairly expensive (not as bad
as reading from the page cache..). Are you really worried about the