Re: [PATCH v3 5/6] drm/gpusvm: keep a single DMA mapping inline for THP

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

On Sat, Sep 05, 2026 at 12:39:55AM +0800, Honglei Huang wrote:
> drm_gpusvm_get_pages() sizes the dma_addr array for one drm_pagemap_addr
> per page, but the mapping loop advances by page order, so a range backed
> by one huge page needs a single entry. For a 2 MiB THP that is an 8 KiB
> array holding 16 bytes of address.
> 
> Union that entry with the array pointer, discriminated by a new
> inline_dma_mapping flag. When drm_gpusvm_dma_map_pages() ends up with one
> entry it stores it inline and frees the array, after the last error
> unwind, which still walks the array form. An unchecked dma_addr read is
> now type confusion rather than a compile error, so reads go through the
> new drm_gpusvm_pages_first_dma() accessor, including the two
> xe_pt_stage_bind() paths.
> 
> Only get_pages() and the free path write the union, never the notifier,
> and both run under the driver lock that every address reader already
> holds. The unlocked short circuit in drm_gpusvm_pages_valid_unlocked()
> goes for the same reason: it cannot resolve the union, and every instance
> it rejects has to be reset before the allocation loop reuses it.
> 
> Suggested-by: Matthew Brost <matthew.brost@intel.com>
> Signed-off-by: Honglei Huang <honghuan@amd.com>

We have one build issue / CI failure on the Xe side, Sashiko also
flagged the issue (replied there) but other than that LGTM.

Thanks for doing this.

Matt

> ---
>  drivers/gpu/drm/drm_gpusvm.c | 48 ++++++++++++++++++++++++++++------
>  drivers/gpu/drm/xe/xe_pt.c   |  7 ++---
>  include/drm/drm_gpusvm.h     | 50 +++++++++++++++++++++++++++++++++---
>  3 files changed, 91 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gpusvm.c b/drivers/gpu/drm/drm_gpusvm.c
> index 7efc35507f1..2c7c4c89dc4 100644
> --- a/drivers/gpu/drm/drm_gpusvm.c
> +++ b/drivers/gpu/drm/drm_gpusvm.c
> @@ -1241,6 +1241,8 @@ static void __drm_gpusvm_unmap_pages(struct drm_gpusvm *gpusvm,
>  		struct drm_gpusvm_pages_flags flags = {
>  			.__flags = svm_pages->flags.__flags,
>  		};
> +		const struct drm_pagemap_addr *addrs =
> +			drm_gpusvm_pages_first_dma(svm_pages);
>  		bool use_iova = dma_use_iova(&svm_pages->state);
>  
>  		/*
> @@ -1253,12 +1255,12 @@ static void __drm_gpusvm_unmap_pages(struct drm_gpusvm *gpusvm,
>  			if (svm_pages->state_offset)
>  				dma_iova_unlink(dev, &svm_pages->state, 0,
>  						svm_pages->state_offset,
> -						svm_pages->dma_addr[0].dir, 0);
> +						addrs[0].dir, 0);
>  			dma_iova_free(dev, &svm_pages->state);
>  		}
>  
>  		for (i = 0, j = 0; i < npages; j++) {
> -			struct drm_pagemap_addr *addr = &svm_pages->dma_addr[j];
> +			const struct drm_pagemap_addr *addr = &addrs[j];
>  
>  			if (addr->proto == DRM_INTERCONNECT_SYSTEM) {
>  				/*
> @@ -1299,6 +1301,18 @@ static void __drm_gpusvm_free_pages(struct drm_gpusvm *gpusvm,
>  {
>  	lockdep_assert_held(&gpusvm->notifier_lock);
>  
> +	if (svm_pages->flags.inline_dma_mapping) {
> +		struct drm_gpusvm_pages_flags flags = {
> +			.__flags = svm_pages->flags.__flags,
> +		};
> +
> +		svm_pages->inline_addr = (struct drm_pagemap_addr){};
> +		flags.inline_dma_mapping = false;
> +		/* WRITE_ONCE pairs with READ_ONCE for opportunistic checks */
> +		WRITE_ONCE(svm_pages->flags.__flags, flags.__flags);
> +		return;
> +	}
> +
>  	if (svm_pages->dma_addr) {
>  		kvfree(svm_pages->dma_addr);
>  		svm_pages->dma_addr = NULL;
> @@ -1463,11 +1477,6 @@ static bool drm_gpusvm_pages_valid_unlocked(struct drm_gpusvm *gpusvm,
>  	bool pages_valid = true;
>  	unsigned int p;
>  
> -	for (p = 0; p < num_pages; ++p) {
> -		if (!svm_pages[p].dma_addr)
> -			return false;
> -	}
> -
>  	drm_gpusvm_notifier_lock(gpusvm);
>  	for (p = 0; p < num_pages; ++p) {
>  		if (drm_gpusvm_pages_valid(gpusvm, &svm_pages[p]))
> @@ -1480,6 +1489,21 @@ static bool drm_gpusvm_pages_valid_unlocked(struct drm_gpusvm *gpusvm,
>  	return pages_valid;
>  }
>  
> +/**
> + * drm_gpusvm_pages_inlinable() - Whether the dma address can be inlined
> + * @nentries: Number of entries the mapping loop produced
> + *
> + * A THP maps as one huge page, so the whole range needs a single device
> + * address: the dma_addr array can be freed and the address kept inline,
> + * which is where the memory saving comes from.
> + *
> + * Return: True if the mapping fits in a single drm_pagemap_addr.
> + */
> +static bool drm_gpusvm_pages_inlinable(unsigned long nentries)
> +{
> +	return nentries == 1;
> +}
> +
>  /**
>   * drm_gpusvm_dma_map_pages() - DMA map one drm_gpusvm_pages instance
>   * @gpusvm: Pointer to the GPU SVM structure
> @@ -1632,6 +1656,14 @@ static int drm_gpusvm_dma_map_pages(struct drm_gpusvm *gpusvm,
>  	if (pagemap)
>  		flags.has_devmem_pages = true;
>  
> +	if (drm_gpusvm_pages_inlinable(j)) {
> +		struct drm_pagemap_addr addr = svm_pages->dma_addr[0];
> +
> +		kvfree(svm_pages->dma_addr);
> +		svm_pages->inline_addr = addr;
> +		flags.inline_dma_mapping = true;
> +	}
> +
>  	/* WRITE_ONCE pairs with READ_ONCE for opportunistic checks */
>  	WRITE_ONCE(svm_pages->flags.__flags, flags.__flags);
>  
> @@ -1740,7 +1772,7 @@ int drm_gpusvm_get_pages(struct drm_gpusvm *gpusvm,
>  
>  	if (map_dma) {
>  		for (p = 0; p < num_pages; ++p) {
> -			if (svm_pages[p].dma_addr)
> +			if (drm_gpusvm_pages_first_dma(&svm_pages[p]))
>  				continue;
>  			svm_pages[p].dma_addr =
>  				kvzalloc_objs(*svm_pages[p].dma_addr, npages);
> diff --git a/drivers/gpu/drm/xe/xe_pt.c b/drivers/gpu/drm/xe/xe_pt.c
> index 5d990c1c374..96ea9735f34 100644
> --- a/drivers/gpu/drm/xe/xe_pt.c
> +++ b/drivers/gpu/drm/xe/xe_pt.c
> @@ -831,7 +831,7 @@ xe_pt_stage_bind(struct xe_tile *tile, struct xe_vma *vma,
>  			return -EAGAIN;
>  		}
>  		if (xe_svm_range_has_dma_mapping(range)) {
> -			xe_res_first_dma(range->pages.dma_addr, 0,
> +			xe_res_first_dma(drm_gpusvm_pages_first_dma(&range->pages), 0,
>  					 xe_svm_range_size(range),
>  					 &curs);
>  			xe_svm_range_debug(range, "BIND PREPARE - MIXED");
> @@ -866,8 +866,9 @@ xe_pt_stage_bind(struct xe_tile *tile, struct xe_vma *vma,
>  
>  	if (!xe_vma_is_null(vma) && !range && !is_purged) {
>  		if (xe_vma_is_userptr(vma))
> -			xe_res_first_dma(to_userptr_vma(vma)->userptr.pages.dma_addr, 0,
> -					 xe_vma_size(vma), &curs);
> +			xe_res_first_dma(drm_gpusvm_pages_first_dma
> +					 (&to_userptr_vma(vma)->userptr.pages),
> +					 0, xe_vma_size(vma), &curs);
>  		else if (xe_bo_is_vram(bo) || xe_bo_is_stolen(bo))
>  			xe_res_first(bo->ttm.resource, xe_vma_bo_offset(vma),
>  				     xe_vma_size(vma), &curs);
> diff --git a/include/drm/drm_gpusvm.h b/include/drm/drm_gpusvm.h
> index ec7b81957b1..aaad5c9b510 100644
> --- a/include/drm/drm_gpusvm.h
> +++ b/include/drm/drm_gpusvm.h
> @@ -10,6 +10,7 @@
>  #include <linux/kref.h>
>  #include <linux/interval_tree.h>
>  #include <linux/mmu_notifier.h>
> +#include <drm/drm_pagemap.h>
>  
>  struct dev_pagemap_ops;
>  struct drm_device;
> @@ -18,7 +19,6 @@ struct drm_gpusvm_notifier;
>  struct drm_gpusvm_ops;
>  struct drm_gpusvm_range;
>  struct drm_pagemap;
> -struct drm_pagemap_addr;
>  
>  /**
>   * struct drm_gpusvm_ops - Operations structure for GPU SVM
> @@ -112,6 +112,7 @@ struct drm_gpusvm_notifier {
>   * @unmapped: Flag indicating if the pages has been unmapped
>   * @has_devmem_pages: Flag indicating if the pages has devmem pages
>   * @has_dma_mapping: Flag indicating if the pages has a DMA mapping
> + * @inline_dma_mapping: Flag indicating if the pages have an inline DMA mapping
>   * @__flags: Flags for pages in u16 form (used for READ_ONCE)
>   */
>  struct drm_gpusvm_pages_flags {
> @@ -121,6 +122,7 @@ struct drm_gpusvm_pages_flags {
>  			u16 unmapped : 1;
>  			u16 has_devmem_pages : 1;
>  			u16 has_dma_mapping : 1;
> +			u16 inline_dma_mapping : 1;
>  		};
>  		u16 __flags;
>  	};
> @@ -130,17 +132,27 @@ struct drm_gpusvm_pages_flags {
>   * struct drm_gpusvm_pages - Structure representing a GPU SVM mapped pages
>   *
>   * @drm: The DRM device that owns the dma mappings
> - * @dma_addr: Device address array
> + * @dma_addr: Device address array, valid while @flags.inline_dma_mapping is
> + *            not set
> + * @inline_addr: Device address inline address, valid while
> + *               @flags.inline_dma_mapping is set
>   * @dpagemap: The struct drm_pagemap of the device pages we're dma-mapping.
>   *            Note this is assuming only one drm_pagemap per range is allowed.
>   * @state: DMA IOVA state for mapping.
>   * @state_offset: DMA IOVA offset for mapping.
>   * @notifier_seq: Notifier sequence number of the range's pages
>   * @flags: Flags for the range; see &struct drm_gpusvm_pages_flags
> + *
> + * @dma_addr and @inline_addr share storage, discriminated by
> + * @flags.inline_dma_mapping. Driver should use drm_gpusvm_pages_first_dma()
> + * to access the correct DMA address.
>   */
>  struct drm_gpusvm_pages {
>  	struct drm_device *drm;
> -	struct drm_pagemap_addr *dma_addr;
> +	union {
> +		struct drm_pagemap_addr *dma_addr;
> +		struct drm_pagemap_addr inline_addr;
> +	};
>  	struct drm_pagemap *dpagemap;
>  	struct dma_iova_state state;
>  	unsigned long state_offset;
> @@ -365,6 +377,38 @@ static inline void drm_gpusvm_init_pages(struct drm_gpusvm_pages *svm_pages,
>  	svm_pages->notifier_seq = LONG_MAX;
>  }
>  
> +/**
> + * drm_gpusvm_pages_first_dma() - Resolve the device address array
> + * @svm_pages: Pointer to the drm_gpusvm_pages.
> + *
> + * drm_gpusvm_pages use unions to optimize the storage of DMA addresses,
> + * this function abstracts the access to the first device address. The driver
> + * should use this helper instead of reading dma_addr directly to prevent
> + * array out of bounds access.
> + *
> + * Only get_pages() and the free path switch between the two union members.
> + * Both hold the notifier lock for read, so taking that lock does not stop
> + * them; callers need the driver lock that does, which every reader of the
> + * addresses holds anyway. The notifier never touches the union, so the
> + * pointer returned here stays good and can then be used under the notifier
> + * lock.
> + *
> + * Return: Pointer to the first device address, NULL if none is populated.
> + */
> +static inline const struct drm_pagemap_addr *
> +drm_gpusvm_pages_first_dma(const struct drm_gpusvm_pages *svm_pages)
> +{
> +	struct drm_gpusvm_pages_flags flags = {
> +		/* READ_ONCE pairs with the WRITE_ONCE of the flag writers */
> +		.__flags = READ_ONCE(svm_pages->flags.__flags),
> +	};
> +
> +	if (flags.inline_dma_mapping)
> +		return &svm_pages->inline_addr;
> +
> +	return READ_ONCE(svm_pages->dma_addr);
> +}
> +
>  /**
>   * 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.
> -- 
> 2.34.1
>