Re: [PATCH v5 24/24] drm/xe/svm: Serialize migration to device if racing

[Matthew Brost <matthew.brost@intel.com>]

On Thu, Dec 18, 2025 at 05:21:01PM +0100, Thomas Hellström wrote:
> Introduce an rw-semaphore to serialize migration to device if
> it's likely that migration races with another device migration
> of the same CPU address space range.
> This is a temporary fix to attempt to mitigate a livelock that
> might happen if many devices try to migrate a range at the same
> time, and it affects only devices using the xe driver.
> A longer term fix is probably improvements in the core mm
> migration layer.

+1, yes we can likely do some core MM changes to make this work a bit
better.

Until then:
Reviewed-by: Matthew Brost <matthew.brost@intel.com>

> 
> Suggested-by: Matthew Brost <matthew.brost@intel.com>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
>  drivers/gpu/drm/xe/xe_svm.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_svm.c b/drivers/gpu/drm/xe/xe_svm.c
> index 7be4d129247d..dce7879b3133 100644
> --- a/drivers/gpu/drm/xe/xe_svm.c
> +++ b/drivers/gpu/drm/xe/xe_svm.c
> @@ -1616,10 +1616,12 @@ struct drm_pagemap *xe_vma_resolve_pagemap(struct xe_vma *vma, struct xe_tile *t
>  int xe_svm_alloc_vram(struct xe_svm_range *range, const struct drm_gpusvm_ctx *ctx,
>  		      struct drm_pagemap *dpagemap)
>  {
> +	static DECLARE_RWSEM(driver_migrate_lock);
>  	struct xe_vm *vm = range_to_vm(&range->base);
>  	enum drm_gpusvm_scan_result migration_state;
>  	struct xe_device *xe = vm->xe;
>  	int err, retries = 1;
> +	bool write_locked = false;
>  
>  	xe_assert(range_to_vm(&range->base)->xe, range->base.pages.flags.migrate_devmem);
>  	range_debug(range, "ALLOCATE VRAM");
> @@ -1638,16 +1640,32 @@ int xe_svm_alloc_vram(struct xe_svm_range *range, const struct drm_gpusvm_ctx *c
>  		drm_dbg(&xe->drm, "Request migration to device memory on \"%s\".\n",
>  			dpagemap->drm->unique);
>  
> +	err = down_read_interruptible(&driver_migrate_lock);
> +	if (err)
> +		return err;
>  	do {
>  		err = drm_pagemap_populate_mm(dpagemap, xe_svm_range_start(range),
>  					      xe_svm_range_end(range),
>  					      range->base.gpusvm->mm,
>  					      ctx->timeslice_ms);
>  
> -		if (err == -EBUSY && retries)
> -			drm_gpusvm_range_evict(range->base.gpusvm, &range->base);
> +		if (err == -EBUSY && retries) {
> +			if (!write_locked) {
> +				int lock_err;
>  
> +				up_read(&driver_migrate_lock);
> +				lock_err = down_write_killable(&driver_migrate_lock);
> +				if (lock_err)
> +					return lock_err;
> +				write_locked = true;
> +			}
> +			drm_gpusvm_range_evict(range->base.gpusvm, &range->base);
> +		}
>  	} while (err == -EBUSY && retries--);
> +	if (write_locked)
> +		up_write(&driver_migrate_lock);
> +	else
> +		up_read(&driver_migrate_lock);
>  
>  	return err;
>  }
> -- 
> 2.51.1
>