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drm/xe/bo: Block CPU faults to purgeable buffer objects

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Block CPU page faults to buffer objects marked as purgeable (DONTNEED)
or already purged. Once a BO is marked DONTNEED, its contents can be
discarded by the kernel at any time, making access undefined behavior.
Return VM_FAULT_SIGBUS immediately to fail consistently instead of
allowing erratic behavior where access sometimes works (if not yet
purged) and sometimes fails (if purged).

For DONTNEED BOs:
- Block new CPU faults with SIGBUS to prevent undefined behavior.
- Existing CPU PTEs may still work until TLB flush, but new faults
  fail immediately.

For PURGED BOs:
- Backing store has been reclaimed, making CPU access invalid.
- Without this check, accessing existing mmap mappings would trigger
  xe_bo_fault_migrate() on freed backing store, causing kernel hangs
  or crashes.

The purgeable check is added to both CPU fault paths:
- Fastpath (xe_bo_cpu_fault_fastpath): Returns VM_FAULT_SIGBUS
  immediately under dma-resv lock, preventing attempts to
  migrate/validate DONTNEED/purged pages.
- Slowpath (xe_bo_cpu_fault): Returns -EFAULT under drm_exec lock,
  converted to VM_FAULT_SIGBUS.

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260326130843.3545241-5-arvind.yadav@intel.com
This commit is contained in:
Arvind Yadav 2026-03-26 18:38:30 +05:30 committed by Matthew Brost
parent ad9843aac9
commit 9a16fdf5dc
1 changed files with 19 additions and 0 deletions
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19
drivers/gpu/drm/xe/xe_bo.c
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@ -1982,6 +1982,16 @@ static vm_fault_t xe_bo_cpu_fault_fastpath(struct vm_fault *vmf, struct xe_devic
if (!dma_resv_trylock(tbo->base.resv))
goto out_validation;
/*
* Reject CPU faults to purgeable BOs. DONTNEED BOs can be purged
* at any time, and purged BOs have no backing store. Either case
* is undefined behavior for CPU access.
*/
if (xe_bo_madv_is_dontneed(bo) || xe_bo_is_purged(bo)) {
ret = VM_FAULT_SIGBUS;
goto out_unlock;
}
if (xe_ttm_bo_is_imported(tbo)) {
ret = VM_FAULT_SIGBUS;
drm_dbg(&xe->drm, "CPU trying to access an imported buffer object.\n");
@ -2072,6 +2082,15 @@ static vm_fault_t xe_bo_cpu_fault(struct vm_fault *vmf)
if (err)
break;
/*
* Reject CPU faults to purgeable BOs. DONTNEED BOs can be
* purged at any time, and purged BOs have no backing store.
*/
if (xe_bo_madv_is_dontneed(bo) || xe_bo_is_purged(bo)) {
err = -EFAULT;
break;
}
if (xe_ttm_bo_is_imported(tbo)) {
err = -EFAULT;
drm_dbg(&xe->drm, "CPU trying to access an imported buffer object.\n");