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

[Christoph Hellwig <hch@xxxxxx>]

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:

http://git.infradead.org/users/hch/misc.git/commitdiff/048522dfa26b6667adfb0371ff530dc263abe829

it needs extra prep patches from the series:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/alloc_vm_area

>>      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
allocation?