[Intel-gfx] [CI] drm/i915/gem: Avoid iterating an empty list

[Chris Wilson]

Our __sgt_iter assumes that the scattergather list has at least one
element. But during construction we may fail in allocating the first
page, and so mark the first element as the terminator. This is
unexpected!

[22555.524752] RIP: 0010:shmem_get_pages+0x506/0x710 [i915]
[22555.524759] Code: 49 8b 2c 24 31 c0 66 89 44 24 40 48 85 ed 0f 84 62 01 00 00 4c 8b 75 00 8b 5d 08 44 8b 7d 0c 48 8b 0d 7e 34 07 e2 49 83 e6 fc <49> 8b 16 41 01 df 48 89 cf 48 89 d0 48 c1 e8 2d 48 85 c9 0f 84 c8
[22555.524765] RSP: 0018:ffffc9000053f9d0 EFLAGS: 00010246
[22555.524770] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff8881ffffa000
[22555.524774] RDX: fffffffffffffff4 RSI: ffffffffffffffff RDI: ffffffff821efe00
[22555.524778] RBP: ffff8881b099ab00 R08: 0000000000000000 R09: 00000000fffffff4
[22555.524782] R10: 0000000000000002 R11: 00000000ffec0a02 R12: ffff8881cd3c8d60
[22555.524786] R13: 00000000fffffff4 R14: 0000000000000000 R15: 0000000000000000
[22555.524790] FS:  00007f4fbeb9b9c0(0000) GS:ffff8881f8580000(0000) knlGS:0000000000000000
[22555.524795] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[22555.524799] CR2: 0000000000000000 CR3: 00000001ec7f0004 CR4: 00000000001606e0
[22555.524803] Call Trace:
[22555.524919]  __i915_gem_object_get_pages+0x4f/0x60 [i915]

Fixes: 85d1225ec066 ("drm/i915: Introduce & use new lightweight SGL iterators")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: <stable at vger.kernel.org> # v4.8+
Reviewed-by: Matthew Auld <matthew.auld at intel.com>
Reviewed-by: Maciej Patelczyk <maciej.patelczyk at intel.com>
---
 drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 15 +++++++++------
 drivers/gpu/drm/i915/i915_scatterlist.h   |  2 ++
 2 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
index 5d5d7eef3f43..7aff3514d97a 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
@@ -39,7 +39,6 @@ static int shmem_get_pages(struct drm_i915_gem_object *obj)
 	unsigned long last_pfn = 0;	/* suppress gcc warning */
 	unsigned int max_segment = i915_sg_segment_size();
 	unsigned int sg_page_sizes;
-	struct pagevec pvec;
 	gfp_t noreclaim;
 	int ret;
 
@@ -192,13 +191,17 @@ static int shmem_get_pages(struct drm_i915_gem_object *obj)
 	sg_mark_end(sg);
 err_pages:
 	mapping_clear_unevictable(mapping);
-	pagevec_init(&pvec);
-	for_each_sgt_page(page, sgt_iter, st) {
-		if (!pagevec_add(&pvec, page))
+	if (sg != st->sgl) {
+		struct pagevec pvec;
+
+		pagevec_init(&pvec);
+		for_each_sgt_page(page, sgt_iter, st) {
+			if (!pagevec_add(&pvec, page))
+				check_release_pagevec(&pvec);
+		}
+		if (pagevec_count(&pvec))
 			check_release_pagevec(&pvec);
 	}
-	if (pagevec_count(&pvec))
-		check_release_pagevec(&pvec);
 	sg_free_table(st);
 	kfree(st);
 
diff --git a/drivers/gpu/drm/i915/i915_scatterlist.h b/drivers/gpu/drm/i915/i915_scatterlist.h
index b7b59328cb76..5041c268f450 100644
--- a/drivers/gpu/drm/i915/i915_scatterlist.h
+++ b/drivers/gpu/drm/i915/i915_scatterlist.h
@@ -27,6 +27,8 @@ static __always_inline struct sgt_iter {
 } __sgt_iter(struct scatterlist *sgl, bool dma) {
 	struct sgt_iter s = { .sgp = sgl };
 
+	GEM_BUG_ON(sg_is_chain(s.sgp) || sg_is_last(s.sgp));
+
 	if (s.sgp) {
 		s.max = s.curr = s.sgp->offset;
 		s.max += s.sgp->length;
-- 
2.20.1