Re: [PATCH 1/2] drm/xe/userptr: fix EFAULT handling

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Thu, 2025-02-20 at 15:45 -0800, Matthew Brost wrote:
> From: 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 might explain 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. And from the logs it does
> seem
> like we hit this special EFAULT case before the explosions.
> 
> v2 (MattB):
>  - Move earlier
> 
> 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 d664f2e418b2..ea2e287e6526 100644
> --- a/drivers/gpu/drm/xe/xe_vm.c
> +++ b/drivers/gpu/drm/xe/xe_vm.c
> @@ -681,6 +681,18 @@ 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);