[PATCH v2 4/4] drm/xe: Lower BO TTM priority to lowest when fully unmapped from VM

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

An UNMAP (or the unmap half of a REMAP) can leave a BO with no
remaining VMA mappings anywhere. Such a BO is unlikely to be touched
again soon, so rather than leaving it at whatever priority band it had
while still in active use, drop it to XE_BO_PRIORITY_LOW, the lowest
level, so it becomes one of the first candidates considered for
eviction/shrinking. This applies both to BOs private to a VM and to
extobjs shared across multiple VMs (bo->vm == NULL) -- an extobj with
no bindings left anywhere is just as good an eviction/shrink candidate
as an unmapped private BO. Imported bos are excluded, since their
vma_count is never tracked (xe_bo_vma_count_inc/dec_locked() are
no-ops for them), so it would never reflect anything meaningful.

This has to be done inside xe_vma_destroy() itself, right after
xe_bo_vma_count_dec_locked(), rather than by its callers after the
fact: xe_vma_destroy() may drop the last reference to the BO via
xe_bo_put() in xe_vma_destroy_late() (called either synchronously, or
asynchronously via a fence callback), so a caller that stashed a
pointer to the BO before calling xe_vma_destroy() and then dereferences
it afterwards risks a use-after-free. Doing the check and update
before that possible final put avoids this.

bo->purgeable.vma_count, which xe_bo_vma_count_dec_locked() just
updated, tells us whether the BO has become fully unmapped. A REMAP
that only shrinks a mapping still leaves vma_count > 0 via the
prev/next VMA(s) created earlier in the bind, so those are correctly
left untouched. This runs under the BO's dma-resv, already asserted
held via xe_bo_assert_held() just above.

Cc: Carlos Santa <carlos.santa@intel.com>
Cc: Ryan Neph <ryanneph@google.com>
Assisted-by: GitHub_Copilot:claude-sonnet-5
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
---
 drivers/gpu/drm/xe/xe_vm.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
index d42390e7ae2f..8f3827f957ea 100644
--- a/drivers/gpu/drm/xe/xe_vm.c
+++ b/drivers/gpu/drm/xe/xe_vm.c
@@ -1282,6 +1282,22 @@ static void xe_vma_destroy(struct xe_vma *vma, struct dma_fence *fence)
 		xe_bo_vma_count_dec_locked(bo);
 		if (vma->attr.purgeable_state == XE_MADV_PURGEABLE_WILLNEED)
 			xe_bo_willneed_put_locked(bo);
+
+		/*
+		 * @bo has just lost a VMA mapping. If it now has none left
+		 * anywhere, it is unlikely to be used again soon, so lower
+		 * its TTM priority to the lowest level, making it one of the
+		 * first buffers considered for eviction/shrinking. This
+		 * applies both to BOs private to @vm and to extobjs shared
+		 * across multiple VMs (bo->vm == NULL), but not to imported
+		 * bos, whose vma_count is never tracked. Must be done here,
+		 * rather than by the caller, as this may be the last
+		 * reference to @bo (see xe_vma_destroy_late()).
+		 */
+		if (!drm_gem_is_imported(&bo->ttm.base) &&
+		    !bo->purgeable.vma_count &&
+		    bo->ttm.priority != XE_BO_PRIORITY_LOW)
+			xe_bo_update_ttm_priority(bo, XE_BO_PRIORITY_LOW);
 	}
 
 	xe_vm_assert_held(vm);
-- 
2.34.1