drm/gpusvm: keep a single DMA mapping inline for THP

drm_gpusvm_get_pages() sizes the dma_addr array for one drm_pagemap_addr
per page, but the mapping loop advances by page order, so a range backed
by one huge page needs a single entry. For a 2 MiB THP that is an 8 KiB
array holding 16 bytes of address.

Union that entry with the array pointer, discriminated by a new
inline_dma_mapping flag. When drm_gpusvm_dma_map_pages() ends up with one
entry it stores it inline and frees the array, after the last error
unwind, which still walks the array form. An unchecked dma_addr read is
now type confusion rather than a compile error, so reads go through the
new drm_gpusvm_pages_first_dma() accessor, including the two
xe_pt_stage_bind() paths.

Only get_pages() and the free path write the union, never the notifier,
and both run under the driver lock that every address reader already
holds. The unlocked short circuit in drm_gpusvm_pages_valid_unlocked()
goes for the same reason: it cannot resolve the union, and every instance
it rejects has to be reset before the allocation loop reuses it.

Suggested-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Honglei Huang <honghuan@amd.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260905133142.3628027-6-honghuan@amd.com

1 files changed, 47 insertions, 3 deletions

diff --git a/include/drm/drm_gpusvm.h b/include/drm/drm_gpusvm.h
index ec7b81957b1d..aaad5c9b510c 100644
--- a/include/drm/drm_gpusvm.h
+++ b/include/drm/drm_gpusvm.h
@@ -10,6 +10,7 @@
 #include <linux/kref.h>
 #include <linux/interval_tree.h>
 #include <linux/mmu_notifier.h>
+#include <drm/drm_pagemap.h>
 
 struct dev_pagemap_ops;
 struct drm_device;
@@ -18,7 +19,6 @@ struct drm_gpusvm_notifier;
 struct drm_gpusvm_ops;
 struct drm_gpusvm_range;
 struct drm_pagemap;
-struct drm_pagemap_addr;
 
 /**
  * struct drm_gpusvm_ops - Operations structure for GPU SVM
@@ -112,6 +112,7 @@ struct drm_gpusvm_notifier {
  * @unmapped: Flag indicating if the pages has been unmapped
  * @has_devmem_pages: Flag indicating if the pages has devmem pages
  * @has_dma_mapping: Flag indicating if the pages has a DMA mapping
+ * @inline_dma_mapping: Flag indicating if the pages have an inline DMA mapping
  * @__flags: Flags for pages in u16 form (used for READ_ONCE)
  */
 struct drm_gpusvm_pages_flags {
@@ -121,6 +122,7 @@ struct drm_gpusvm_pages_flags {
 			u16 unmapped : 1;
 			u16 has_devmem_pages : 1;
 			u16 has_dma_mapping : 1;
+			u16 inline_dma_mapping : 1;
 		};
 		u16 __flags;
 	};
@@ -130,17 +132,27 @@ struct drm_gpusvm_pages_flags {
  * struct drm_gpusvm_pages - Structure representing a GPU SVM mapped pages
  *
  * @drm: The DRM device that owns the dma mappings
- * @dma_addr: Device address array
+ * @dma_addr: Device address array, valid while @flags.inline_dma_mapping is
+ *            not set
+ * @inline_addr: Device address inline address, valid while
+ *               @flags.inline_dma_mapping is set
  * @dpagemap: The struct drm_pagemap of the device pages we're dma-mapping.
  *            Note this is assuming only one drm_pagemap per range is allowed.
  * @state: DMA IOVA state for mapping.
  * @state_offset: DMA IOVA offset for mapping.
  * @notifier_seq: Notifier sequence number of the range's pages
  * @flags: Flags for the range; see &struct drm_gpusvm_pages_flags
+ *
+ * @dma_addr and @inline_addr share storage, discriminated by
+ * @flags.inline_dma_mapping. Driver should use drm_gpusvm_pages_first_dma()
+ * to access the correct DMA address.
  */
 struct drm_gpusvm_pages {
 	struct drm_device *drm;
-	struct drm_pagemap_addr *dma_addr;
+	union {
+		struct drm_pagemap_addr *dma_addr;
+		struct drm_pagemap_addr inline_addr;
+	};
 	struct drm_pagemap *dpagemap;
 	struct dma_iova_state state;
 	unsigned long state_offset;
@@ -366,6 +378,38 @@ static inline void drm_gpusvm_init_pages(struct drm_gpusvm_pages *svm_pages,
 }
 
 /**
+ * drm_gpusvm_pages_first_dma() - Resolve the device address array
+ * @svm_pages: Pointer to the drm_gpusvm_pages.
+ *
+ * drm_gpusvm_pages use unions to optimize the storage of DMA addresses,
+ * this function abstracts the access to the first device address. The driver
+ * should use this helper instead of reading dma_addr directly to prevent
+ * array out of bounds access.
+ *
+ * Only get_pages() and the free path switch between the two union members.
+ * Both hold the notifier lock for read, so taking that lock does not stop
+ * them; callers need the driver lock that does, which every reader of the
+ * addresses holds anyway. The notifier never touches the union, so the
+ * pointer returned here stays good and can then be used under the notifier
+ * lock.
+ *
+ * Return: Pointer to the first device address, NULL if none is populated.
+ */
+static inline const struct drm_pagemap_addr *
+drm_gpusvm_pages_first_dma(const struct drm_gpusvm_pages *svm_pages)
+{
+	struct drm_gpusvm_pages_flags flags = {
+		/* READ_ONCE pairs with the WRITE_ONCE of the flag writers */
+		.__flags = READ_ONCE(svm_pages->flags.__flags),
+	};
+
+	if (flags.inline_dma_mapping)
+		return &svm_pages->inline_addr;
+
+	return READ_ONCE(svm_pages->dma_addr);
+}
+
+/**
  * enum drm_gpusvm_scan_result - Scan result from the drm_gpusvm_scan_mm() function.
  * @DRM_GPUSVM_SCAN_UNPOPULATED: At least one page was not present or inaccessible.
  * @DRM_GPUSVM_SCAN_EQUAL: All pages belong to the struct dev_pagemap indicated as