[PATCH] drm/amdkfd: Use last + 1 of vm range to check 2MB huge page alignment

[Gerhard Schwanzer]

SVM ranges use inclusive page indices: prange->last is the last page in
the range. The split-remap logic introduced by commit 448ee45353ef
("drm/amdkfd: Use huge page size to check split svm range alignment")
uses ALIGN_DOWN(prange->last, 512) to determine whether the original
range can contain a 2MB huge-page mapping.

That aligns the last page itself down. Thus a range ending one page
before the next 2MB boundary is classified as if the final 2MB block did
not exist. When such a range is split inside that final block, the
split head or tail can be left off the remap list even though it was
derived from an original range that may have PMD mappings.

Use prange->last + 1 as the exclusive upper bound when computing the
original range's last 2MB-aligned boundary. For the head-side remap
check, compare the inclusive head->last directly against that exclusive
aligned boundary; this catches a split head ending one page before the
boundary while preserving the existing alignment check.

Xiaogang Chen identified this cause and posted the candidate fix in the
regression thread. With the culprit change active and the local revert not
applied, the unchanged C/HSA reproducer completes 10/10 runs with this
change on an RX 7600 XT.

Fixes: 448ee45353ef ("drm/amdkfd: Use huge page size to check split svm range alignment")
Cc: stable@xxxxxxxxxxxxxxx
Closes: https://gitlab.freedesktop.org/drm/amd/-/work_items/4914
Link: https://lore.kernel.org/stable/IA1PR12MB85172F7FE9157C092EDA46A0E3112@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Suggested-by: Xiaogang Chen <xiaogang.chen@xxxxxxx>
Signed-off-by: Gerhard Schwanzer <geschw@xxxxx>
---
 drivers/gpu/drm/amd/amdkfd/kfd_svm.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_svm.c b/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
index 72cfb4a..30693ed 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
@@ -1144,7 +1144,7 @@ static int
 svm_range_split_tail(struct svm_range *prange, uint64_t new_last,
 		     struct list_head *insert_list, struct list_head *remap_list)
 {
-	unsigned long last_align_down = ALIGN_DOWN(prange->last, 512);
+	unsigned long last_align_down = ALIGN_DOWN(prange->last + 1, 512);
 	unsigned long start_align = ALIGN(prange->start, 512);
 	bool huge_page_mapping = last_align_down > start_align;
 	struct svm_range *tail = NULL;
@@ -1168,7 +1168,7 @@ static int
 svm_range_split_head(struct svm_range *prange, uint64_t new_start,
 		     struct list_head *insert_list, struct list_head *remap_list)
 {
-	unsigned long last_align_down = ALIGN_DOWN(prange->last, 512);
+	unsigned long last_align_down = ALIGN_DOWN(prange->last + 1, 512);
 	unsigned long start_align = ALIGN(prange->start, 512);
 	bool huge_page_mapping = last_align_down > start_align;
 	struct svm_range *head = NULL;
@@ -1181,8 +1181,8 @@ svm_range_split_head(struct svm_range *prange, uint64_t new_start,
 
 	list_add(&head->list, insert_list);
 
-	if (huge_page_mapping && head->last + 1 > start_align &&
-	    head->last + 1 < last_align_down && (!IS_ALIGNED(head->last, 512)))
+	if (huge_page_mapping && head->last > start_align &&
+	    head->last < last_align_down && (!IS_ALIGNED(head->last, 512)))
 		list_add(&head->update_list, remap_list);
 
 	return 0;

base-commit: 2c7d5b0a5ec0fc713a7f350806553643e87e6f43
-- 
2.54.0