Re: [RFC PATCH] drm/xe/bo: Honor madvise(2) advices

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

On 28/11/2025 10:46, Thomas Hellström wrote:
> The user can give advices as to how the CPU will access an
> address range. Use those advices to determine the number of
> bo pages to prefault on a page-fault.
> 
> Do this regardless of whether we can find a way to avoid the
> fairly slow vm_insert_pfn_prot() to populate buffer
> object maps.
> 
> Initially, fault up to 512 pages on sequential access and
> a single page on random access.
> 
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
>   drivers/gpu/drm/xe/xe_bo.c | 18 +++++++++++++++++-
>   1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> index 6fd6ce6c6586..07d0d954f826 100644
> --- a/drivers/gpu/drm/xe/xe_bo.c
> +++ b/drivers/gpu/drm/xe/xe_bo.c
> @@ -1821,15 +1821,31 @@ static int xe_bo_fault_migrate(struct xe_bo *bo, struct ttm_operation_ctx *ctx,
>   	return err;
>   }
>   
> +/*
> + * Number of prefaulted pages for the MADV_SEQUENTIAL and
> + * MADV_RANDOM madvise() advices.
> + */
> +#define XE_BO_VM_NUM_PREFAULT_SEQ  512
> +#define XE_BO_VM_NUM_PREFAULT_RAND 1
> +
>   /* Call into TTM to populate PTEs, and register bo for PTE removal on runtime suspend. */
>   static vm_fault_t __xe_bo_cpu_fault(struct vm_fault *vmf, struct xe_device *xe, struct xe_bo *bo)
>   {
> +	const struct vm_area_struct *vma = vmf->vma;
> +	pgoff_t num_prefault;
>   	vm_fault_t ret;
>   
>   	trace_xe_bo_cpu_fault(bo);
>   
> +	if (vma->vm_flags & VM_SEQ_READ)
> +		num_prefault = XE_BO_VM_NUM_PREFAULT_SEQ;
> +	else if (vma->vm_flags & VM_RAND_READ)
> +		num_prefault = XE_BO_VM_NUM_PREFAULT_RAND;
> +	else
> +		num_prefault = TTM_BO_VM_NUM_PREFAULT;

Ah, interesting. Do we know if any UMD is making use of these special 
flags today? Just wondering if this might be a visible change or not? 
Also would it make sense to document/advertise this somewhere for UMD 
folks, in case this has an immediate benefit for them?

I guess would be good to add an IGT which uses both flags, if we don't 
already?

Anyway, I think change makes sense,
Reviewed-by: Matthew Auld <matthew.auld@intel.com>

> +
>   	ret = ttm_bo_vm_fault_reserved(vmf, vmf->vma->vm_page_prot,
> -				       TTM_BO_VM_NUM_PREFAULT);
> +				       num_prefault);
>   	/*
>   	 * When TTM is actually called to insert PTEs, ensure no blocking conditions
>   	 * remain, in which case TTM may drop locks and return VM_FAULT_RETRY.