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

Yadav, Arvind
2026-01-21T08:24:42Z

On 20-01-2026 22:45, Matthew Brost wrote:
> On Tue, Jan 20, 2026 at 08:58:05AM -0800, Matthew Brost wrote:
>> On Tue, Jan 20, 2026 at 11:38:49AM +0530, Arvind Yadav wrote:
>>> 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);
>>> +
>> xe_bo_assert_held(bo);

Noted,  I will add xe_bo_assert_held() at the top of the purge helper

>>
>>> + if (ttm_bo->ttm) {
>>> + struct ttm_placement place = {};
>>> + int ret = ttm_bo_validate(ttm_bo, &place, ctx);
>>> +
>>> + drm_WARN_ON(&xe->drm, ret);
>> I think since 'xe' in available here, you should use xe_assert in place
>> of drm_WARN_ON.
Agreed. Switched this to xe_assert(xe, !ret) since this is Xe-specific
and we already have xe.
>>
>>> + if (!ret) {
>>> + if (xe_bo_madv_is_dontneed(bo)) {
>>> + bo->madv_purgeable = XE_MADV_PURGEABLE_PURGED;
>> Helper to set madv_purgeable state /w lockdep assert?
>>
>> Also perhaps assert valid state transitions in the helper (e.g., you
>> cannot tranistion out of XE_MADV_PURGEABLE_PURGED.

Noted,  I will xe_bo_set_purgeable_state() which asserts the BO lock and
enforces "once PURGED always PURGED" (reject transitions out of 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);
>> I think xe_bo_trigger_rebind is allowed to fail if ctx->no_wait_gpu is
>> set. In both the faulting fast path and certain parts of the shrinker we
>> set this. So I think any error returned from xe_bo_trigger_rebind needs
>> to propagte up the call stack.

Agreed. I will changed xe_ttm_bo_purge() to return int and propagate
errors. xe_bo_move() now returns the error when purge/rebind fails,
rather than warning and continuing.

>>> + }
>>> + }
>>> + }
>>> +}
>>> +
>>> 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);
>> With above, we need to send errors from xe_ttm_bo_purge up the call
>> stack.

Noted.

>>
>>> + 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)
>> Shouldn't this check be a vfunc in madvise_funcs?
>>
>> Also I think you can hook into xe_madvise_details for the return value /
>> final copy to user.

Yes. I will move purgeable handling into the madvise_funcs[] table as a
proper madvise vfunc (madvise_purgeable). The retained return is now
tracked via xe_madvise_details and copied back in
xe_madvise_details_fini(), so we keep the "copy_to_user after dropping
locks" rule without a special-case early return.

>>> +{
>>> + 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;
>> Use above suggested helper to set this state?

Yes, converted to xe_bo_set_purgeable_state().

>>
>>> + 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);
>>> +
>> Again use the existing vfuncs here.

Noted,

>>
>>> + /* 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)))
>> I don't think remained needs to be a u64 - maybe a u16? Will comment on
>> uAPI too.
>>
> Ignore this, I forgot purge_state_val.retained is a userptr so u64 is
> correct. Let me follow on if we are allowed to change IOCTLs from IOW ->
> IOWR. I am really unclear on the rules that part of the uAPI.

sure,

>
>>> + drm_warn(&vm->xe->drm, "Failed to copy retained value to user\n");
>> See above, use xe_madvise_details_fini for the final copy to user.

Noted -  The retained pointer is stored in details during init, and the
final copy happens in xe_madvise_details_fini().

Thanks,
Arvind

>>
>> Matt
>>
>>> + }
>>> +
>>> + /* Final cleanup for early return */
>>> + xe_vm_put(vm);
>>> + return 0;
>>> + }
>>> }
>>>
>>> if (madvise_range.has_svm_userptr_vmas) {
>>> --
>>> 2.43.0
>>>