Re: [PATCH] drm/xe/dma-buf: Validate BOs on export to charge the exporter

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

Hi, Matt

Thanks for reviewing!

On Wed, 2026-11-11 at 12:18 +0000, Matthew Auld wrote:
> On 11/11/2026 10:09, Thomas Hellström wrote:
> > xe_gem_prime_export() calls ttm_bo_setup_export() to make sure the
> > exporter is charged for the memory backing an exported BO. However,
> > that only populates a TTM_PL_SYSTEM tt. A BO that has been evicted,
> > or
> > that has deferred backing, will instead get its memory allocated
> > when
> > the importer maps it. For p2p imports this happens through
> > xe_bo_validate() in xe_dma_buf_map(), so the importer, not the
> > exporter, is charged for the allocation.
> > 
> > Call xe_bo_validate() at export time instead, so that the BO is
> > moved
> > into its preferred placement and the exporter is charged.
> 
> Is it maybe also worth mentioning somewhere the unreclaimable pages 
> angle? I could be mistaken here, but I think that was maybe also the 
> issue I saw in a workload. For example:
> 
> 1. Create say 100G VRAM buffer. Use DEFERRED. This leaves lingering
> tt 
> structure.
> 
> 2. Validate it on native side. This is now resident in VRAM. But
> still 
> has lingering tt.
> 
> 3. Export it. This now populates 100G worth of TT pages due to
> lingering 
> tt. But we now have a populated VRAM buffer which is also populated
> with 
> 100G TT pages. That 100G TT pages is not reclaimable and is hidden
> from 
> shrinker since this still just looks like normal VRAM buffer.

Since we have another patch to fix that, which I hope will go in before
this one, I think mentioning it here will be unnecessary.

> 
> > 
> > Do the purgeable checks and the validation in the same
> > xe_validation_guard() transaction, to avoid locking the BO twice.
> > 
> > The commit message and code was drafted using LLM.
> > 
> > Link:
> > https://patchwork.freedesktop.org/patch/747480/?series=172524&rev=1
> > Fixes: 91494dee1091 ("xe: populate buffers before exporting them.")
> > Cc: Matthew Brost <matthew.brost@intel.com>
> > Cc: Matthew Auld <matthew.auld@intel.com>
> > Cc: Dave Airlie <airlied@redhat.com>
> > Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> > Cc: intel-xe@lists.freedesktop.org
> > Cc: <stable@vger.kernel.org> # v6.18+
> > Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> 
> Reviewed-by: Matthew Auld <matthew.auld@intel.com>

Thanks!

Thomas

> 
> > ---
> >   drivers/gpu/drm/xe/xe_dma_buf.c | 76 +++++++++++++++++-----------
> > -----
> >   1 file changed, 39 insertions(+), 37 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/xe/xe_dma_buf.c
> > b/drivers/gpu/drm/xe/xe_dma_buf.c
> > index 5bf13dc4b61d..90c9b90de552 100644
> > --- a/drivers/gpu/drm/xe/xe_dma_buf.c
> > +++ b/drivers/gpu/drm/xe/xe_dma_buf.c
> > @@ -263,60 +263,62 @@ struct drm_gem_object
> > *xe_dma_buf_to_gem(struct drm_device *dev,
> >   struct dma_buf *xe_gem_prime_export(struct drm_gem_object *obj,
> > int flags)
> >   {
> >   	struct xe_bo *bo = gem_to_xe_bo(obj);
> > +	struct xe_validation_ctx ctx;
> > +	struct drm_exec exec;
> >   	struct dma_buf *buf;
> > -	struct ttm_operation_ctx ctx = {
> > -		.interruptible = true,
> > -		.no_wait_gpu = true,
> > -		/* We opt to avoid OOM on system pages allocations
> > */
> > -		.gfp_retry_mayfail = true,
> > -		.allow_res_evict = false,
> > -	};
> > -	int ret;
> > +	int ret = 0;
> >   
> >   	if (bo->vm)
> >   		return ERR_PTR(-EPERM);
> >   
> > -	/*
> > -	 * Reject exporting purgeable BOs. DONTNEED BOs can be
> > purged
> > -	 * at any time, making the exported dma-buf unusable.
> > Purged BOs
> > -	 * have no backing store and are permanently invalid.
> > -	 */
> > -	ret = xe_bo_lock(bo, true);
> > -	if (ret)
> > -		return ERR_PTR(ret);
> > +	xe_validation_guard(&ctx, &xe_bo_device(bo)->val, &exec,
> > +			    (struct xe_val_flags) {.interruptible
> > = true}, ret) {
> > +		ret = drm_exec_lock_obj(&exec, &bo->ttm.base);
> > +		drm_exec_retry_on_contention(&exec);
> > +		if (ret)
> > +			break;
> >   
> > -	if (xe_bo_madv_is_dontneed(bo)) {
> > -		ret = -EBUSY;
> > -		goto out_unlock;
> > -	}
> > +		/*
> > +		 * Reject exporting purgeable BOs. DONTNEED BOs
> > can be purged
> > +		 * at any time, making the exported dma-buf
> > unusable. Purged BOs
> > +		 * have no backing store and are permanently
> > invalid.
> > +		 */
> > +		if (xe_bo_madv_is_dontneed(bo)) {
> > +			ret = -EBUSY;
> > +			break;
> > +		}
> >   
> > -	if (xe_bo_is_purged(bo)) {
> > -		ret = -EINVAL;
> > -		goto out_unlock;
> > -	}
> > +		if (xe_bo_is_purged(bo)) {
> > +			ret = -EINVAL;
> > +			break;
> > +		}
> >   
> > -	xe_bo_willneed_get_locked(bo);
> > -	xe_bo_unlock(bo);
> > +		/*
> > +		 * Make sure the BO is backed in its preferred
> > placement so that
> > +		 * the exporter, rather than the importer, is
> > charged for the
> > +		 * memory allocation.
> > +		 */
> > +		ret = xe_bo_validate(bo, NULL, false, &exec);
> > +		drm_exec_retry_on_contention(&exec);
> > +		xe_validation_retry_on_oom(&ctx, &ret);
> > +		if (ret)
> > +			break;
> >   
> > -	ret = ttm_bo_setup_export(&bo->ttm, &ctx);
> > +		xe_bo_willneed_get_locked(bo);
> > +	}
> >   	if (ret)
> > -		goto out_put;
> > +		return ERR_PTR(ret);
> >   
> >   	buf = drm_gem_prime_export(obj, flags);
> >   	if (IS_ERR(buf)) {
> > -		ret = PTR_ERR(buf);
> > -		goto out_put;
> > +		xe_bo_lock(bo, false);
> > +		xe_bo_willneed_put_locked(bo);
> > +		xe_bo_unlock(bo);
> > +		return buf;
> >   	}
> >   
> >   	buf->ops = &xe_dmabuf_ops;
> >   	return buf;
> > -
> > -out_put:
> > -	xe_bo_lock(bo, false);
> > -	xe_bo_willneed_put_locked(bo);
> > -out_unlock:
> > -	xe_bo_unlock(bo);
> > -	return ERR_PTR(ret);
> >   }
> >   
> >   static struct drm_gem_object *