drm/amdkfd: Use exclusive bounds for SVM split alignment checks

drm/amdkfd: Use exclusive bounds for SVM split alignment checks

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. Then use the actual split
boundary for the head and tail alignment checks: tail->start for a tail
split, and new_start for a head split. new_start is equivalent to
head->last + 1 and directly names the exclusive end of the split head.

Using head->last for the head-side check can both remap a head that ends
exactly one page before a 2MB boundary and miss a head whose split
boundary is one page after such a boundary. Philip Yang pointed out in
the review of the original change that this condition should use
head->last + 1 or new_start.

Xiaogang Chen identified the inclusive-last 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")
Closes: https://gitlab.freedesktop.org/drm/amd/-/work_items/4914
Link: https://lore.kernel.org/stable/IA1PR12MB85172F7FE9157C092EDA46A0E3112@IA1PR12MB8517.namprd12.prod.outlook.com/
Link: https://lore.kernel.org/all/32ce2b72-aa16-4202-9f99-92e3cd4408bc@amd.com/
Suggested-by: Xiaogang Chen <xiaogang.chen@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Gerhard Schwanzer <geschw@pm.me>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit a60ea15807126b148a328051636977a33ad0e9bb)
Cc: stable@vger.kernel.org