Re: [PATCH] Always mark GEM objects as dirty when written by the CPU

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On 07/12/15 08:29, Daniel Vetter wrote:
On Fri, Dec 04, 2015 at 05:28:29PM +0000, Dave Gordon wrote:
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@xxxxxxxxxxxxxxxxxx>
	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?

Hm, I thought we've been careful with checking that an object is somehow
backed by struct pages, and only use the page-wise access if that's the
case. But looking at the execbuf relocate code we've probably already
screwed this up, or at least will when we expose stolen to userspace.
Userptr should still work (since ultimately it's struct page backed), and
phys gem object doesn't matter (if you but relocs into your cursor on
gen2-4.0 you get all the pieces). I think step one would be more nasty
test coverage, at least for the execbuf path.

The other page-wise access path seem all internal, so I'm much less
worried about those.
-Daniel

So does this mean that i915_pages_create_for_stolen() isn't really doing what it says? After that function has been called, obj->pages is filled in - but is it then valid to call i915_gem_object_get_page() ? That returns a pointer to the (preexisting) entry in the system page tables for the specified page, but isn't the anomalous thing about stolen memory the fact that the kernel doesn't know about it and doesn't include it in its page tables at all?

For kmap purposes, we don't really need the 'struct page' as we could use kmap_atomic_pfn() instead. So maybe to make stolen objects work in general without everything having to know they're different, we would need to move the kmap operation into the vfunc as well? That would mean the vfunc would be something like obj->kmap_page(obj, pageno, dirty) returning the vaddr of the mapped page.

This looks like it will need a bit more study and design so perhaps we could just take the quick fix of marking whole objects dirty for now (which will at least give *correct* behaviour) and then work out how to avoid marking whole objects dirty where possible.

.Dave.
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