Re: [PATCH v4 19/22] drm/gpusvm: Introduce a function to scan the current migration state

["Ghimiray, Himal Prasad" <himal.prasad.ghimiray@intel.com>]

On 11-12-2025 22:29, Thomas Hellström wrote:
> With multi-device we are much more likely to have multiple
> drm-gpusvm ranges pointing to the same struct mm range.
> 
> To avoid calling into drm_pagemap_populate_mm(), which is always
> very costly, introduce a much less costly drm_gpusvm function,
> drm_gpusvm_scan_mm() to scan the current migration state.
> The device fault-handler and prefetcher can use this function to
> determine whether migration is really necessary.
> 
> There are a couple of performance improvements that can be done
> for this function if it turns out to be too costly. Those are
> documented in the code.
> 
> v3:
> - New patch.
> 
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
>   drivers/gpu/drm/drm_gpusvm.c | 121 +++++++++++++++++++++++++++++++++++
>   include/drm/drm_gpusvm.h     |  29 +++++++++
>   2 files changed, 150 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_gpusvm.c b/drivers/gpu/drm/drm_gpusvm.c
> index 4c7474a331bc..aa9a0b60e727 100644
> --- a/drivers/gpu/drm/drm_gpusvm.c
> +++ b/drivers/gpu/drm/drm_gpusvm.c
> @@ -743,6 +743,127 @@ static bool drm_gpusvm_check_pages(struct drm_gpusvm *gpusvm,
>   	return err ? false : true;
>   }
>   
> +/**
> + * drm_gpusvm_scan_mm() - Check the migration state of a drm_gpusvm_range
> + * @range: Pointer to the struct drm_gpusvm_range to check.
> + * @dev_private_owner: The struct dev_private_owner to use to determine
> + * compatible device-private pages.
> + * @pagemap: The struct dev_pagemap pointer to use for pagemap-specific
> + * checks.
> + *
> + * Scan the CPU address space corresponding to @range and return the
> + * current migration state. Note that the result may be invalid as
> + * soon as the function returns. It's an advisory check.
> + *
> + * TODO: Bail early and call hmm_range_fault() for subranges.
> + *
> + * Return: See &enum drm_gpusvm_scan_result.
> + */
> +enum drm_gpusvm_scan_result drm_gpusvm_scan_mm(struct drm_gpusvm_range *range,
> +					       void *dev_private_owner,
> +					       const struct dev_pagemap *pagemap)
> +{
> +	struct mmu_interval_notifier *notifier = &range->notifier->notifier;
> +	unsigned long start = drm_gpusvm_range_start(range);
> +	unsigned long end = drm_gpusvm_range_end(range);
> +	struct hmm_range hmm_range = {
> +		.default_flags = 0,
> +		.notifier = notifier,
> +		.start = start,
> +		.end = end,
> +		.dev_private_owner = dev_private_owner,
> +	};
> +	unsigned long timeout =
> +		jiffies + msecs_to_jiffies(HMM_RANGE_DEFAULT_TIMEOUT);
> +	enum drm_gpusvm_scan_result state = DRM_GPUSVM_SCAN_UNPOPULATED, new_state;
> +	unsigned long *pfns;
> +	unsigned long npages = npages_in_range(start, end);
> +	const struct dev_pagemap *other = NULL;
> +	int err, i;
> +
> +	pfns = kvmalloc_array(npages, sizeof(*pfns), GFP_KERNEL);
> +	if (!pfns)
> +		return DRM_GPUSVM_SCAN_UNPOPULATED;
> +
> +	hmm_range.hmm_pfns = pfns;
> +
> +retry:
> +	hmm_range.notifier_seq = mmu_interval_read_begin(notifier);
> +	mmap_read_lock(range->gpusvm->mm);
> +
> +	while (true) {
> +		err = hmm_range_fault(&hmm_range);
> +		if (err == -EBUSY) {
> +			if (time_after(jiffies, timeout))
> +				break;
> +
> +			hmm_range.notifier_seq =
> +				mmu_interval_read_begin(notifier);
> +			continue;
> +		}
> +		break;
> +	}
> +	mmap_read_unlock(range->gpusvm->mm);
> +	if (err)
> +		goto err_free;
> +
> +	drm_gpusvm_notifier_lock(range->gpusvm);
> +	if (mmu_interval_read_retry(notifier, hmm_range.notifier_seq)) {
> +		drm_gpusvm_notifier_unlock(range->gpusvm);
> +		goto retry;
> +	}
> +
> +	for (i = 0; i < npages;) {
> +		struct page *page;
> +		const struct dev_pagemap *cur = NULL;
> +
> +		if (!(pfns[i] & HMM_PFN_VALID)) {
> +			state = DRM_GPUSVM_SCAN_UNPOPULATED;
> +			goto err_free;
> +		}
> +
> +		page = hmm_pfn_to_page(pfns[i]);
> +		if (is_device_private_page(page) ||
> +		    is_device_coherent_page(page))
> +			cur = page_pgmap(page);
> +
> +		if (cur == pagemap) {
> +			new_state = DRM_GPUSVM_SCAN_EQUAL;
> +		} else if (cur && (cur == other || !other)) {
> +			new_state = DRM_GPUSVM_SCAN_OTHER;
> +			other = cur;
> +		} else if (cur) {
> +			new_state = DRM_GPUSVM_SCAN_MIXED_DEVICE;
> +		} else {
> +			new_state = DRM_GPUSVM_SCAN_SYSTEM;
> +		}
> +
> +		/*
> +		 * TODO: Could use an array for state
> +		 * transitions, and caller might want it
> +		 * to bail early for some results.
> +		 */
> +		if (state == DRM_GPUSVM_SCAN_UNPOPULATED) {
> +			state = new_state;
> +		} else if (state != new_state) {
> +			if (new_state == DRM_GPUSVM_SCAN_SYSTEM ||
> +			    state == DRM_GPUSVM_SCAN_SYSTEM)
> +				state = DRM_GPUSVM_SCAN_MIXED;
> +			else if (state != DRM_GPUSVM_SCAN_MIXED)
> +				state = DRM_GPUSVM_SCAN_MIXED_DEVICE;
> +		}
> +
> +		i += 1ul << drm_gpusvm_hmm_pfn_to_order(pfns[i], i, npages);
> +	}
> +
> +err_free:
> +	drm_gpusvm_notifier_unlock(range->gpusvm);
> +
> +	kvfree(pfns);
> +	return state;
> +}
> +EXPORT_SYMBOL(drm_gpusvm_scan_mm);
> +
>   /**
>    * drm_gpusvm_range_chunk_size() - Determine chunk size for GPU SVM range
>    * @gpusvm: Pointer to the GPU SVM structure
> diff --git a/include/drm/drm_gpusvm.h b/include/drm/drm_gpusvm.h
> index 632e100e6efb..2578ac92a8d4 100644
> --- a/include/drm/drm_gpusvm.h
> +++ b/include/drm/drm_gpusvm.h
> @@ -328,6 +328,35 @@ void drm_gpusvm_free_pages(struct drm_gpusvm *gpusvm,
>   			   struct drm_gpusvm_pages *svm_pages,
>   			   unsigned long npages);
>   
> +/**
> + * enum drm_gpusvm_scan_result - Scan result from the drm_gpusvm_scan_mm() function.
> + * @DRM_GPUSVM_SCAN_UNPOPULATED: At least one page was not present or inaccessible.
> + * @DRM_GPUSVM_SCAN_EQUAL: All pages belong to the struct dev_pagemap indicated as
> + * the @pagemap argument to the drm_gpusvm_scan_mm() function.
> + * @DRM_GPUSVM_SCAN_OTHER: All pages belong to exactly one dev_pagemap, which is
> + * *NOT* the @pagemap argument to the drm_gpusvm_scan_mm(). All pages belong to
> + * the same device private owner.
> + * @DRM_GPUSVM_SCAN_SYSTEM: All pages are present and system pages.
> + * @DRM_GPUSVM_SCAN_MIXED_DEVICE: All pages are device pages and belong to at least
> + * two different struct dev_pagemaps. All pages belong to the same device private
> + * owner.
> + * @DRM_GPUSVM_SCAN_MIXED: Pages are present and are a mix of system pages
> + * and device-private pages. All device-private pages belong to the same device
> + * private owner.
> + */
> +enum drm_gpusvm_scan_result {
> +	DRM_GPUSVM_SCAN_UNPOPULATED,
> +	DRM_GPUSVM_SCAN_EQUAL,
> +	DRM_GPUSVM_SCAN_OTHER,
> +	DRM_GPUSVM_SCAN_SYSTEM,
> +	DRM_GPUSVM_SCAN_MIXED_DEVICE,
> +	DRM_GPUSVM_SCAN_MIXED,
> +};
> +

Do we really need these enums, wont simply returning whether all pages 
in range has same pgmap be sufficient ? Return true or false and use to 
decide range needs migration or not.

If we are expecting some further uses cases for these enums, then this 
looks OK though.

> +enum drm_gpusvm_scan_result drm_gpusvm_scan_mm(struct drm_gpusvm_range *range,
> +					       void *dev_private_owner,
> +					       const struct dev_pagemap *pagemap);
> +
>   #ifdef CONFIG_LOCKDEP
>   /**
>    * drm_gpusvm_driver_set_lock() - Set the lock protecting accesses to GPU SVM