[PATCH v6 6/6] swiotlb: Reinstate page-alignment for mappings >= PAGE_SIZE

[Will Deacon]

For swiotlb allocations >= PAGE_SIZE, the slab search historically
adjusted the stride to avoid checking unaligned slots. This had the
side-effect of aligning large mapping requests to PAGE_SIZE, but that
was broken by 0eee5ae10256 ("swiotlb: fix slot alignment checks").

Since this alignment could be relied upon drivers, reinstate PAGE_SIZE
alignment for swiotlb mappings >= PAGE_SIZE.

Reported-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 kernel/dma/swiotlb.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index c381a7ed718f..c5851034523f 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -992,6 +992,17 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
 	BUG_ON(!nslots);
 	BUG_ON(area_index >= pool->nareas);
 
+	/*
+	 * Historically, swiotlb allocations >= PAGE_SIZE were guaranteed to be
+	 * page-aligned in the absence of any other alignment requirements.
+	 * 'alloc_align_mask' was later introduced to specify the alignment
+	 * explicitly, however this is passed as zero for streaming mappings
+	 * and so we preserve the old behaviour there in case any drivers are
+	 * relying on it.
+	 */
+	if (!alloc_align_mask && !iotlb_align_mask && alloc_size >= PAGE_SIZE)
+		alloc_align_mask = PAGE_SIZE - 1;
+
 	/*
 	 * Ensure that the allocation is at least slot-aligned and update
 	 * 'iotlb_align_mask' to ignore bits that will be preserved when
@@ -1006,13 +1017,6 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
 	 */
 	stride = get_max_slots(max(alloc_align_mask, iotlb_align_mask));
 
-	/*
-	 * For allocations of PAGE_SIZE or larger only look for page aligned
-	 * allocations.
-	 */
-	if (alloc_size >= PAGE_SIZE)
-		stride = umax(stride, PAGE_SHIFT - IO_TLB_SHIFT + 1);
-
 	spin_lock_irqsave(&area->lock, flags);
 	if (unlikely(nslots > pool->area_nslabs - area->used))
 		goto not_found;
-- 
2.44.0.278.ge034bb2e1d-goog