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

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

Hi, Himal

On Fri, 2025-12-12 at 16:51 +0530, Ghimiray, Himal Prasad wrote:
> 
> 
> 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.

For the deferred series implementing the migrate_system_only migration
policy the usage looks like 

	if (migration_status == DRM_GPUSVM_SCAN_EQUAL ||
	    (!details->migrate_same_owner &&
	     (migration_status == DRM_GPUSVM_SCAN_OTHER ||
	      migration_status == DRM_GPUSVM_SCAN_MIXED_DEVICE))) {
		drm_info(dpagemap->drm, "Already migrated!\n");
		return 0;
	}

So not all are needed in the near future. I could look at simplifying a
bit, though, so that we actually only expose what we use currently.

/Thomas


> 
> > +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
>