Re: [Intel-gfx] [PATCH] drm/i915: Selectively enable self-reclaim

[Linus Torvalds]

On Wed, Jun 30, 2010 at 12:05 AM, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
>
> Reviewing the patch again, we no longer set the default gfpmask on the
> inode to contain NORETRY and instead add the NORETRY at the one spot in
> the code where we are trying to do a large allocation and our shrinker
> would be prevented from running (due to contention on struct_mutex).
>
> I do not know how this causes memory corruption across hibernate and would
> appreciate any insights.

Hmm. More likely is the __GFP_MOVABLE flag, I think.

That commit changes the page cache allocation to use

+                                          mapping_gfp_mask (mapping) |
+                                          __GFP_COLD |
+                                          gfpmask);

if I read it right. And the default mapping_gfp_mask() is
GFP_HIGHUSER_MOVABLE, so I think you get all of
(__GFP_WAIT | __GFP_IO | __GFP_FS | __GFP_HARDWALL | __GFP_HIGHMEM)
set by default.

The old code didn't just play games with ~__GFP_NORETRY and change
that at runtime (which was buggy - no locking, no protection, no
nothing), it also initialized the gfp mask. And that code also got
removed:

-       /* Basically we want to disable the OOM killer and handle ENOMEM
-        * ourselves by sacrificing pages from cached buffers.
-        * XXX shmem_file_[gs]et_gfp_mask()
-        */
-       mapping_set_gfp_mask(obj->filp->f_path.dentry->d_inode->i_mapping,
-                            GFP_HIGHUSER |
-                            __GFP_COLD |
-                            __GFP_FS |
-                            __GFP_RECLAIMABLE |
-                            __GFP_NORETRY |
-                            __GFP_NOWARN |
-                            __GFP_NOMEMALLOC);

(and note how it doesn't have __GFP_MOVABLE set).

So I wonder if we shouldn't re-instate that mapping_set_gfp_mask() for
the _initial_ setting when the file descriptor is created. That part
wasn't the bug - the bug was the code that used to try to do that
whole per-allocation dance with the bits incorrectly (ie this part of
the change:

-               gfp = i915_gem_object_get_page_gfp_mask(obj);
-               i915_gem_object_set_page_gfp_mask(obj, gfp & ~__GFP_NORETRY);
-               ret = i915_gem_object_get_pages(obj);
-               i915_gem_object_set_page_gfp_mask (obj, gfp);

in that patch).

I could easily see that something would get very unhappy and corrupt
memory if the suspend-to-disk phase ends up compacting memory and
moving the pages around from under the i915 driver.

I dunno. But that seems more likely than the __GFP_NORETRY flag, which
should have no semantic meaning (except making it more likely for
allocations to fail, of course, but that's what the i915 code
_wanted_).

                      Linus
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