[PATCH v3 2/3] drm/xe/userptr: fix EFAULT handling

[Matthew Auld <matthew.auld@intel.com>]

Currently we treat EFAULT from hmm_range_fault() as a non-fatal error
when called from xe_vm_userptr_pin() with the idea that we want to avoid
killing the entire vm and chucking an error, under the assumption that
the user just did an unmap or something, and has no intention of
actually touching that memory from the GPU.  At this point we have
already zapped the PTEs so any access should generate a page fault, and
if the pin fails there also it will then become fatal.

However it looks like it's possible for the userptr vma to still be on
the rebind list in preempt_rebind_work_func(), if we had to retry the
pin again due to something happening in the caller before we did the
rebind step, but in the meantime needing to re-validate the userptr and
this time hitting the EFAULT.

This explains an internal user report of hitting:

[  191.738349] WARNING: CPU: 1 PID: 157 at drivers/gpu/drm/xe/xe_res_cursor.h:158 xe_pt_stage_bind.constprop.0+0x60a/0x6b0 [xe]
[  191.738551] Workqueue: xe-ordered-wq preempt_rebind_work_func [xe]
[  191.738616] RIP: 0010:xe_pt_stage_bind.constprop.0+0x60a/0x6b0 [xe]
[  191.738690] Call Trace:
[  191.738692]  <TASK>
[  191.738694]  ? show_regs+0x69/0x80
[  191.738698]  ? __warn+0x93/0x1a0
[  191.738703]  ? xe_pt_stage_bind.constprop.0+0x60a/0x6b0 [xe]
[  191.738759]  ? report_bug+0x18f/0x1a0
[  191.738764]  ? handle_bug+0x63/0xa0
[  191.738767]  ? exc_invalid_op+0x19/0x70
[  191.738770]  ? asm_exc_invalid_op+0x1b/0x20
[  191.738777]  ? xe_pt_stage_bind.constprop.0+0x60a/0x6b0 [xe]
[  191.738834]  ? ret_from_fork_asm+0x1a/0x30
[  191.738849]  bind_op_prepare+0x105/0x7b0 [xe]
[  191.738906]  ? dma_resv_reserve_fences+0x301/0x380
[  191.738912]  xe_pt_update_ops_prepare+0x28c/0x4b0 [xe]
[  191.738966]  ? kmemleak_alloc+0x4b/0x80
[  191.738973]  ops_execute+0x188/0x9d0 [xe]
[  191.739036]  xe_vm_rebind+0x4ce/0x5a0 [xe]
[  191.739098]  ? trace_hardirqs_on+0x4d/0x60
[  191.739112]  preempt_rebind_work_func+0x76f/0xd00 [xe]

Followed by NPD, when running some workload, since the sg was never
actually populated but the vma is still marked for rebind when it should
be skipped for this special EFAULT case. This is confirmed to fix the
user report.

v2 (MattB):
 - Move earlier.
v3 (MattB):
 - Update the commit message to make it clear that this indeed fixes the
   issue.

Fixes: 521db22a1d70 ("drm/xe: Invalidate userptr VMA on page pin fault")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: <stable@vger.kernel.org> # v6.10+
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
---
 drivers/gpu/drm/xe/xe_vm.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
index 3dbfb20a7c60..0118a1147668 100644
--- a/drivers/gpu/drm/xe/xe_vm.c
+++ b/drivers/gpu/drm/xe/xe_vm.c
@@ -682,6 +682,18 @@ int xe_vm_userptr_pin(struct xe_vm *vm)
 		err = xe_vma_userptr_pin_pages(uvma);
 		if (err == -EFAULT) {
 			list_del_init(&uvma->userptr.repin_link);
+			/*
+			 * We might have already done the pin once already, but
+			 * then had to retry before the re-bind happened, due
+			 * some other condition in the caller, but in the
+			 * meantime the userptr got dinged by the notifier such
+			 * that we need to revalidate here, but this time we hit
+			 * the EFAULT. In such a case make sure we remove
+			 * ourselves from the rebind list to avoid going down in
+			 * flames.
+			 */
+			if (!list_empty(&uvma->vma.combined_links.rebind))
+				list_del_init(&uvma->vma.combined_links.rebind);
 
 			/* Wait for pending binds */
 			xe_vm_lock(vm, false);
-- 
2.48.1