[PATCH 3/5] shmem: shmem_writepage() split unlikely i915 THP

[Hugh Dickins <hughd@google.com>]

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;