[Intel-gfx] [PATCH] Always mark GEM objects as dirty when written by the CPU

[Dave Gordon]

On 04/12/15 09:57, Daniel Vetter wrote:
> On Tue, Dec 01, 2015 at 01:21:07PM +0000, Dave Gordon wrote:
>> On 01/12/15 13:04, Chris Wilson wrote:
>>> On Tue, Dec 01, 2015 at 12:42:02PM +0000, Dave Gordon wrote:
>>>> In various places, one or more pages of a GEM object are mapped into CPU
>>>> address space and updated. In each such case, the object should be
>>>> marked dirty, to ensure that the modifications are not discarded if the
>>>> object is evicted under memory pressure.
>>>>
>>>> This is similar to commit
>>>> 	commit 51bc140431e233284660b1d22c47dec9ecdb521e
>>>> 	Author: Chris Wilson <chris at chris-wilson.co.uk>
>>>> 	Date:   Mon Aug 31 15:10:39 2015 +0100
>>>> 	drm/i915: Always mark the object as dirty when used by the GPU
>>>>
>>>> in which Chris ensured that updates by the GPU were not lost due to
>>>> eviction, but this patch applies instead to the multiple places where
>>>> object content is updated by the host CPU.
>>>
>>> Apart from that commit was to mask userspace bugs, here we are under
>>> control of when the pages are marked and have chosen a different
>>> per-page interface for CPU writes as opposed to per-object.
>>> -Chris
>>
>> The pattern
>> 	get_pages();
>> 	kmap(get_page())
>> 	write
>> 	kunmap()
>> occurs often enough that it might be worth providing a common function to do
>> that and mark only the specific page dirty (other cases touch the whole
>> object, so for those we can just set the obj->dirty flag and let put_pages()
>> take care of propagating that to all the individual pages).
>>
>> But can we be sure that all the functions touched by this patch will operate
>> only on regular (default) GEM objects (i.e. not phys, stolen, etc) 'cos some
>> of those don't support per-page tracking. What about objects with no backing
>> store -- can/should we mark those as dirty (which would prevent eviction)?
>
> I thought our special objects do clear obj->dirty on put_pages? Can you
> please elaborate on your concern?
>
> While we discuss all this: A patch at the end to document dirty (maybe
> even as a first stab at kerneldoc for i915_drm_gem_buffer_object) would be
> awesome.
> -Daniel

In general, obj->dirty means that (some or) all the pages of the object 
(may) have been modified since last time the object was read from 
backing store, and that the modified data should be written back rather 
than discarded.

Code that works only on default (gtt) GEM objects may be able to 
optimise writebacks by marking individual pages dirty, rather than the 
object as a whole. But not every GEM object has backing store, and even 
among those that do, some do not support per-page dirty tracking.

These are the GEM objects we may want to consider:

1. Default (gtt) object
    * Discontiguous, lives in page cache while pinned during use
    * Backed by shmfs (swap)
    * put_pages() transfers dirty status from object to each page
      before release
    * shmfs ensures that dirty unpinned pages are written out
      before deallocation
    * Could optimise by marking individual pages at point of use,
      rather than marking whole object and then pushing to all pages
      during put_pages()

2. Phys GEM object
    * Lives in physically-contiguous system memory, pinned during use
    * Backed by shmfs
    * if obj->dirty, put_pages() *copies* all pages back to shmfs via
      page cache RMW
    * No per-page tracking, cannot optimise

3. Stolen GEM object
    * Lives in (physically-contiguous) stolen memory, always pinned
    * No backing store!
    * obj->dirty is irrelevant (ignored)
    * put_pages() only called at end-of-life
    * No per-page tracking (not meaningful anyway)

4. Userptr GEM object
    * Discontiguous, lives in page cache while pinned during use
    * Backed by user process memory (which may then map to some
      arbitrary file mapping?)
    * put_pages() transfers dirty status from object to each page
      before release
    * dirty pages are still resident in user space, can be swapped
      out when not pinned
    * Could optimise by marking individual pages at point of use,
      rather than marking whole object and then pushing to all pages
      during put_pages()

Are there any more?

Given this diversity, it may be worth adding a dirty_page() vfunc, so 
that for those situations where a single page is dirtied AND the object 
type supports per-page tracking, we can take advantage of this to reduce 
copying. For objects that don't support per-page tracking, the 
implementation would just set obj->dirty.

For example:
     void (*dirty_page)(obj, pageno);
possibly with the additional semantic that pageno == -1 means 'dirty the 
whole object'.

A convenient further facility would be:
     struct page *i915_gem_object_get_dirty_page(obj, pageno);
which is just like i915_gem_object_get_page() but with the additional 
effect of marking the returned page dirty (by calling the above vfunc).
[Aside: can we call set_page_dirty() on a non-shmfs-backed page?].

This means that in all the places where I added 'obj->dirty = 1' after a 
kunmap() call, we would instead just change the earlier get_page() to 
get_dirty_page() instead, which provides better layering.

Together these changes mean that obj->dirty would then be a purely 
private member for use by implementations of get_pages/put_pages().

Opinions?

.Dave.