[PATCH v4 3/8] drm/xe/madvise: Implement purgeable buffer object support

Arvind Yadav
2026-01-20T06:09:22Z

This allows userspace applications to provide memory usage hints to
the kernel for better memory management under pressure:

Add the core implementation for purgeable buffer objects, enabling memory
reclamation of user-designated DONTNEED buffers during eviction.

This patch implements the purge operation and state machine transitions:

Purgeable States (from xe_madv_purgeable_state):
- WILLNEED (0): BO should be retained, actively used
- DONTNEED (1): BO eligible for purging, not currently needed
- PURGED (2): BO backing store reclaimed, permanently invalid

Design Rationale:
- Async TLB invalidation via trigger_rebind (no blocking xe_vm_invalidate_vma)
- i915 compatibility: retained field, "once purged always purged" semantics
- Shared BO protection prevents multi-process memory corruption
- Scratch PTE reuse avoids new infrastructure, safe for fault mode

v2:
- Use xe_bo_trigger_rebind() for async TLB invalidation (Thomas Hellström)
- Add NULL rebind with scratch PTEs for fault mode (Thomas Hellström)
- Implement i915-compatible retained field logic (Thomas Hellström)
- Skip BO validation for purged BOs in page fault handler (crash fix)
- Add scratch VM check in page fault path (non-scratch VMs fail fault)
- Force clear_pt for non-scratch VMs to avoid phys addr 0 mapping (review fix)
- Add !is_purged check to resource cursor setup to prevent stale access

v3:
- Rebase as xe_gt_pagefault.c is gone upstream and replaced
with xe_pagefault.c (Matthew Brost)
- Xe specific warn on (Matthew Brost)
- Call helpers for madv_purgeable access(Matthew Brost)
- Remove bo NULL check(Matthew Brost)
- Use xe_bo_assert_held instead of dma assert(Matthew Brost)
- Move the xe_bo_is_purged check under the dma-resv lock( by Matt)
- Drop is_purged from xe_pt_stage_bind_entry and just set is_null to true
for purged BO rename s/is_null/is_null_or_purged (by Matt)
- UAPI rule should not be changed.(Matthew Brost)
- Make 'retained' a userptr (Matthew Brost)

v4:
- @madv_purgeable atomic_t → u32 change across all relevant patches. (Matt)

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
---
drivers/gpu/drm/xe/xe_bo.c | 61 +++++++++++++++++----
drivers/gpu/drm/xe/xe_pagefault.c | 12 ++++
drivers/gpu/drm/xe/xe_pt.c | 38 +++++++++++--
drivers/gpu/drm/xe/xe_vm.c | 11 +++-
drivers/gpu/drm/xe/xe_vm_madvise.c | 88 ++++++++++++++++++++++++++++++
5 files changed, 191 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
index 408c74216fdf..d0a6d340b255 100644
--- a/drivers/gpu/drm/xe/xe_bo.c
+++ b/drivers/gpu/drm/xe/xe_bo.c
@@ -836,6 +836,43 @@ static int xe_bo_move_notify(struct xe_bo *bo,
return 0;
}

+/**
+ * xe_ttm_bo_purge() - Purge buffer object backing store
+ * @ttm_bo: The TTM buffer object to purge
+ * @ctx: TTM operation context
+ *
+ * This function purges the backing store of a BO marked as DONTNEED and
+ * triggers rebind to invalidate stale GPU mappings. For fault-mode VMs,
+ * this zaps the PTEs. The next GPU access will trigger a page fault and
+ * perform NULL rebind (scratch pages or clear PTEs based on VM config).
+ */
+static void xe_ttm_bo_purge(struct ttm_buffer_object *ttm_bo, struct ttm_operation_ctx *ctx)
+{
+ struct xe_device *xe = ttm_to_xe_device(ttm_bo->bdev);
+ struct xe_bo *bo = ttm_to_xe_bo(ttm_bo);
+
+ if (ttm_bo->ttm) {
+ struct ttm_placement place = {};
+ int ret = ttm_bo_validate(ttm_bo, &place, ctx);
+
+ drm_WARN_ON(&xe->drm, ret);
+ if (!ret) {
+ if (xe_bo_madv_is_dontneed(bo)) {
+ bo->madv_purgeable = XE_MADV_PURGEABLE_PURGED;
+
+ /*
+ * Trigger rebind to invalidate stale GPU mappings.
+ * - Non-fault mode: Marks VMAs for rebind
+ * - Fault mode: Zaps PTEs (sets to 0), next access triggers fault
+ * and NULL rebind with scratch/clear PTEs per VM config
+ */
+ ret = xe_bo_trigger_rebind(xe, bo, ctx);
+ XE_WARN_ON(ret);
+ }
+ }
+ }
+}
+
static int xe_bo_move(struct ttm_buffer_object *ttm_bo, bool evict,
struct ttm_operation_ctx *ctx,
struct ttm_resource *new_mem,
@@ -855,6 +892,15 @@ static int xe_bo_move(struct ttm_buffer_object *ttm_bo, bool evict,
ttm && ttm_tt_is_populated(ttm)) ? true : false;
int ret = 0;

+ /*
+ * Purge only non-shared BOs explicitly marked DONTNEED by userspace.
+ * The move_notify callback will handle invalidation asynchronously.
+ */
+ if (evict && xe_bo_madv_is_dontneed(bo)) {
+ xe_ttm_bo_purge(ttm_bo, ctx);
+ return 0;
+ }
+
/* Bo creation path, moving to system or TT. */
if ((!old_mem && ttm) && !handle_system_ccs) {
if (new_mem->mem_type == XE_PL_TT)
@@ -1604,18 +1650,6 @@ static void xe_ttm_bo_delete_mem_notify(struct ttm_buffer_object *ttm_bo)
}
}

-static void xe_ttm_bo_purge(struct ttm_buffer_object *ttm_bo, struct ttm_operation_ctx *ctx)
-{
- struct xe_device *xe = ttm_to_xe_device(ttm_bo->bdev);
-
- if (ttm_bo->ttm) {
- struct ttm_placement place = {};
- int ret = ttm_bo_validate(ttm_bo, &place, ctx);
-
- drm_WARN_ON(&xe->drm, ret);
- }
-}
-
static void xe_ttm_bo_swap_notify(struct ttm_buffer_object *ttm_bo)
{
struct ttm_operation_ctx ctx = {
@@ -2196,6 +2230,9 @@ struct xe_bo *xe_bo_init_locked(struct xe_device *xe, struct xe_bo *bo,
#endif
INIT_LIST_HEAD(&bo->vram_userfault_link);

+ /* Initialize purge advisory state */
+ bo->madv_purgeable = XE_MADV_PURGEABLE_WILLNEED;
+
drm_gem_private_object_init(&xe->drm, &bo->ttm.base, size);

if (resv) {
diff --git a/drivers/gpu/drm/xe/xe_pagefault.c b/drivers/gpu/drm/xe/xe_pagefault.c
index 6bee53d6ffc3..e3ace179e9cf 100644
--- a/drivers/gpu/drm/xe/xe_pagefault.c
+++ b/drivers/gpu/drm/xe/xe_pagefault.c
@@ -59,6 +59,18 @@ static int xe_pagefault_begin(struct drm_exec *exec, struct xe_vma *vma,
if (!bo)
return 0;

+ /*
+ * Check if BO is purged (under dma-resv lock).
+ * For purged BOs:
+ * - Scratch VMs: Skip validation, rebind will use scratch PTEs
+ * - Non-scratch VMs: FAIL the page fault (no scratch page available)
+ */
+ if (unlikely(xe_bo_is_purged(bo))) {
+ if (!xe_vm_has_scratch(vm))
+ return -EACCES;
+ return 0;
+ }
+
return need_vram_move ? xe_bo_migrate(bo, vram->placement, NULL, exec) :
xe_bo_validate(bo, vm, true, exec);
}
diff --git a/drivers/gpu/drm/xe/xe_pt.c b/drivers/gpu/drm/xe/xe_pt.c
index 6703a7049227..c8c66300e25b 100644
--- a/drivers/gpu/drm/xe/xe_pt.c
+++ b/drivers/gpu/drm/xe/xe_pt.c
@@ -533,20 +533,26 @@ xe_pt_stage_bind_entry(struct xe_ptw *parent, pgoff_t offset,
/* Is this a leaf entry ?*/
if (level == 0 || xe_pt_hugepte_possible(addr, next, level, xe_walk)) {
struct xe_res_cursor *curs = xe_walk->curs;
- bool is_null = xe_vma_is_null(xe_walk->vma);
- bool is_vram = is_null ? false : xe_res_is_vram(curs);
+ struct xe_bo *bo = xe_vma_bo(xe_walk->vma);
+ bool is_null_or_purged = xe_vma_is_null(xe_walk->vma) ||
+ (bo && xe_bo_is_purged(bo));
+ bool is_vram = is_null_or_purged ? false : xe_res_is_vram(curs);

XE_WARN_ON(xe_walk->va_curs_start != addr);

if (xe_walk->clear_pt) {
pte = 0;
} else {
- pte = vm->pt_ops->pte_encode_vma(is_null ? 0 :
+ /*
+ * For purged BOs, treat like null VMAs - pass address 0.
+ * The pte_encode_vma will set XE_PTE_NULL flag for scratch mapping.
+ */
+ pte = vm->pt_ops->pte_encode_vma(is_null_or_purged ? 0 :
xe_res_dma(curs) +
xe_walk->dma_offset,
xe_walk->vma,
pat_index, level);
- if (!is_null)
+ if (!is_null_or_purged)
pte |= is_vram ? xe_walk->default_vram_pte :
xe_walk->default_system_pte;

@@ -570,7 +576,7 @@ xe_pt_stage_bind_entry(struct xe_ptw *parent, pgoff_t offset,
if (unlikely(ret))
return ret;

- if (!is_null && !xe_walk->clear_pt)
+ if (!is_null_or_purged && !xe_walk->clear_pt)
xe_res_next(curs, next - addr);
xe_walk->va_curs_start = next;
xe_walk->vma->gpuva.flags |= (XE_VMA_PTE_4K << level);
@@ -723,6 +729,26 @@ xe_pt_stage_bind(struct xe_tile *tile, struct xe_vma *vma,
};
struct xe_pt *pt = vm->pt_root[tile->id];
int ret;
+ bool is_purged = false;
+
+ /*
+ * Check if BO is purged:
+ * - Scratch VMs: Use scratch PTEs (XE_PTE_NULL) for safe zero reads
+ * - Non-scratch VMs: Clear PTEs to zero (non-present) to avoid mapping to phys addr 0
+ *
+ * For non-scratch VMs, we force clear_pt=true so leaf PTEs become completely
+ * zero instead of creating a PRESENT mapping to physical address 0.
+ */
+ if (bo && xe_bo_is_purged(bo)) {
+ is_purged = true;
+
+ /*
+ * For non-scratch VMs, a NULL rebind should use zero PTEs
+ * (non-present), not a present PTE to phys 0.
+ */
+ if (!xe_vm_has_scratch(vm))
+ xe_walk.clear_pt = true;
+ }

if (range) {
/* Move this entire thing to xe_svm.c? */
@@ -762,7 +788,7 @@ xe_pt_stage_bind(struct xe_tile *tile, struct xe_vma *vma,
if (!range)
xe_bo_assert_held(bo);

- if (!xe_vma_is_null(vma) && !range) {
+ 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);
diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
index 694f592a0f01..c3a5fe76ff96 100644
--- a/drivers/gpu/drm/xe/xe_vm.c
+++ b/drivers/gpu/drm/xe/xe_vm.c
@@ -1359,6 +1359,9 @@ static u64 xelp_pte_encode_bo(struct xe_bo *bo, u64 bo_offset,
static u64 xelp_pte_encode_vma(u64 pte, struct xe_vma *vma,
u16 pat_index, u32 pt_level)
{
+ struct xe_bo *bo = xe_vma_bo(vma);
+ struct xe_vm *vm = xe_vma_vm(vma);
+
pte |= XE_PAGE_PRESENT;

if (likely(!xe_vma_read_only(vma)))
@@ -1367,7 +1370,13 @@ static u64 xelp_pte_encode_vma(u64 pte, struct xe_vma *vma,
pte |= pte_encode_pat_index(pat_index, pt_level);
pte |= pte_encode_ps(pt_level);

- if (unlikely(xe_vma_is_null(vma)))
+ /*
+ * NULL PTEs redirect to scratch page (return zeros on read).
+ * Set for: 1) explicit null VMAs, 2) purged BOs on scratch VMs.
+ * Never set NULL flag without scratch page - causes undefined behavior.
+ */
+ if (unlikely(xe_vma_is_null(vma) ||
+ (bo && xe_bo_is_purged(bo) && xe_vm_has_scratch(vm))))
pte |= XE_PTE_NULL;

return pte;
diff --git a/drivers/gpu/drm/xe/xe_vm_madvise.c b/drivers/gpu/drm/xe/xe_vm_madvise.c
index add9a6ca2390..dfeab9e24a09 100644
--- a/drivers/gpu/drm/xe/xe_vm_madvise.c
+++ b/drivers/gpu/drm/xe/xe_vm_madvise.c
@@ -179,6 +179,56 @@ static void madvise_pat_index(struct xe_device *xe, struct xe_vm *vm,
}
}

+/*
+ * Handle purgeable buffer object advice for DONTNEED/WILLNEED/PURGED.
+ * Returns true if any BO was purged, false otherwise.
+ * Caller must copy retained value to userspace after releasing locks.
+ */
+static bool xe_vm_madvise_purgeable_bo(struct xe_device *xe, struct xe_vm *vm,
+ struct xe_vma **vmas, int num_vmas,
+ struct drm_xe_madvise *op)
+{
+ bool has_purged_bo = false;
+ int i;
+
+ xe_assert(vm->xe, op->type == DRM_XE_VMA_ATTR_PURGEABLE_STATE);
+
+ for (i = 0; i < num_vmas; i++) {
+ struct xe_bo *bo = xe_vma_bo(vmas[i]);
+
+ if (!bo)
+ continue;
+
+ /* BO must be locked before modifying madv state */
+ xe_bo_assert_held(bo);
+
+ /*
+ * Once purged, always purged. Cannot transition back to WILLNEED.
+ * This matches i915 semantics where purged BOs are permanently invalid.
+ */
+ if (xe_bo_is_purged(bo)) {
+ has_purged_bo = true;
+ continue;
+ }
+
+ switch (op->purge_state_val.val) {
+ case DRM_XE_VMA_PURGEABLE_STATE_WILLNEED:
+ bo->madv_purgeable = XE_MADV_PURGEABLE_WILLNEED;
+ break;
+ case DRM_XE_VMA_PURGEABLE_STATE_DONTNEED:
+ bo->madv_purgeable = XE_MADV_PURGEABLE_DONTNEED;
+ break;
+ default:
+ drm_warn(&vm->xe->drm, "Invalid madvice value = %d\n",
+ op->purge_state_val.val);
+ return false;
+ }
+ }
+
+ /* Return whether any BO was purged; caller will copy to user after unlocking */
+ return has_purged_bo;
+}
+
typedef void (*madvise_func)(struct xe_device *xe, struct xe_vm *vm,
struct xe_vma **vmas, int num_vmas,
struct drm_xe_madvise *op,
@@ -306,6 +356,16 @@ static bool madvise_args_are_sane(struct xe_device *xe, const struct drm_xe_madv
return false;
break;
}
+ case DRM_XE_VMA_ATTR_PURGEABLE_STATE:
+ {
+ u32 val = args->purge_state_val.val;
+
+ if (XE_IOCTL_DBG(xe, !(val == DRM_XE_VMA_PURGEABLE_STATE_WILLNEED ||
+ val == DRM_XE_VMA_PURGEABLE_STATE_DONTNEED)))
+ return false;
+
+ break;
+ }
default:
if (XE_IOCTL_DBG(xe, 1))
return false;
@@ -465,6 +525,34 @@ int xe_vm_madvise_ioctl(struct drm_device *dev, void *data, struct drm_file *fil
goto err_fini;
}
}
+ if (args->type == DRM_XE_VMA_ATTR_PURGEABLE_STATE) {
+ bool has_purged_bo;
+
+ has_purged_bo = xe_vm_madvise_purgeable_bo(xe, vm, madvise_range.vmas,
+ madvise_range.num_vmas, args);
+
+ /* Release BO locks */
+ drm_exec_fini(&exec);
+ kfree(madvise_range.vmas);
+ up_write(&vm->lock);
+
+ /*
+ * Set retained flag to indicate if backing store still exists.
+ * Matches i915: retained = 1 if not purged, 0 if purged.
+ * Must copy_to_user AFTER releasing ALL locks to avoid circular dependency.
+ */
+ if (args->purge_state_val.retained) {
+ u32 retained = !has_purged_bo;
+
+ if (copy_to_user(u64_to_user_ptr(args->purge_state_val.retained),
+ &retained, sizeof(retained)))
+ drm_warn(&vm->xe->drm, "Failed to copy retained value to user\n");
+ }
+
+ /* Final cleanup for early return */
+ xe_vm_put(vm);
+ return 0;
+ }
}

if (madvise_range.has_svm_userptr_vmas) {
--
2.43.0