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drm/gem: replace misleading comment

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shmem supports page-relocations during swapin since quite some time. It
was implemented in:

    commit bde05d1ccd
    Author: Hugh Dickins <hughd@google.com>
    Date:   Tue May 29 15:06:38 2012 -0700

        shmem: replace page if mapping excludes its zone

The gem-comment about wrongly placed DMA32 pages is no longer valid.
Replace it with a proper comment but keep the BUG_ON() to verify correct
shmem behavior.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>
This commit is contained in:
David Herrmann 2014-05-25 14:34:08 +02:00 committed by Dave Airlie
parent 89086bca67
commit 2123000b01
1 changed files with 4 additions and 15 deletions
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19
drivers/gpu/drm/drm_gem.c
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@ -474,21 +474,10 @@ struct page **drm_gem_get_pages(struct drm_gem_object *obj, gfp_t gfpmask)
goto fail;
pages[i] = p;
/* There is a hypothetical issue w/ drivers that require
* buffer memory in the low 4GB.. if the pages are un-
* pinned, and swapped out, they can end up swapped back
* in above 4GB. If pages are already in memory, then
* shmem_read_mapping_page_gfp will ignore the gfpmask,
* even if the already in-memory page disobeys the mask.
*
* It is only a theoretical issue today, because none of
* the devices with this limitation can be populated with
* enough memory to trigger the issue. But this BUG_ON()
* is here as a reminder in case the problem with
* shmem_read_mapping_page_gfp() isn't solved by the time
* it does become a real issue.
*
* See this thread: http://lkml.org/lkml/2011/7/11/238
/* Make sure shmem keeps __GFP_DMA32 allocated pages in the
* correct region during swapin. Note that this requires
* __GFP_DMA32 to be set in mapping_gfp_mask(inode->i_mapping)
* so shmem can relocate pages during swapin if required.
*/
BUG_ON((gfpmask & __GFP_DMA32) &&
(page_to_pfn(p) >= 0x00100000UL));