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Subject[PATCH 3/5] shmem: shmem_writepage() split unlikely i915 THP
drivers/gpu/drm/i915/gem/i915_gem_shmem.c contains a shmem_writeback()
which calls shmem_writepage() from a shrinker: that usually works well
enough; but if /sys/kernel/mm/transparent_hugepage/shmem_enabled has
been set to "force" (documented as "Force the huge option on for all -
very useful for testing"), shmem_writepage() is surprised to be called
with a huge page, and crashes on the VM_BUG_ON_PAGE(PageCompound) (I
did not find out where the crash happens when CONFIG_DEBUG_VM is off).

LRU page reclaim always splits the shmem huge page first: I'd prefer not
to demand that of i915, so check and split compound in shmem_writepage().

Fixes: 2d6692e642e7 ("drm/i915: Start writeback from the shrinker")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org # v5.3+
---
I've marked this for stable just for the info, but the number of users
affected is very probably 1, so please feel free to delete that marking.

mm/shmem.c | 9 +++++++++
1 file changed, 9 insertions(+)

--- 5.9-rc2/mm/shmem.c 2020-08-16 17:32:50.693507198 -0700
+++ linux/mm/shmem.c 2020-08-28 17:35:08.326024349 -0700
@@ -1362,7 +1362,15 @@ static int shmem_writepage(struct page *
swp_entry_t swap;
pgoff_t index;

- VM_BUG_ON_PAGE(PageCompound(page), page);
+ /*
+ * If /sys/kernel/mm/transparent_hugepage/shmem_enabled is "force",
+ * then drivers/gpu/drm/i915/gem/i915_gem_shmem.c gets huge pages,
+ * and its shmem_writeback() needs them to be split when swapping.
+ */
+ if (PageTransCompound(page))
+ if (split_huge_page(page) < 0)
+ goto redirty;
+
BUG_ON(!PageLocked(page));
mapping = page->mapping;
index = page->index;
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