Re: [PATCH v4 1/2] drm/ttm: Drop tt->restore after successful restore

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

On Tue, May 05, 2026 at 09:04:45AM +0200, Thomas Hellström wrote:
> On Mon, 2026-05-04 at 20:30 -0700, Matthew Brost wrote:
> > ttm_pool_restore_and_alloc() can successfully complete the restore
> > process via ttm_pool_restore_commit(), but tt->restore is not dropped
> > afterward. As a result, subsequent backup/restore flows observe what
> > appears to be a completed restore, while in reality shmem handles are
> > still installed in tt->pages, leading to the stack trace below.
> > 
> > Fix this by freeing and dropping tt->restore in
> > ttm_pool_restore_and_alloc() upon successful completion of the
> > restore.
> > 
> > 20545 [  309.784531] RIP:
> > 0010:sg_alloc_append_table_from_pages+0x38c/0x490
> > 20547 [  309.809570] RSP: 0018:ffffc9000623b838 EFLAGS: 00010206
> > 20548 [  309.814827] RAX: 0000000000001000 RBX: ffff88816e42a160 RCX:
> > 0000000000000000
> > 20549 [  309.821986] RDX: 0000000000002000 RSI: 0000000000000003 RDI:
> > 0000000000001000
> > 20550 [  309.829147] RBP: ffff88816e42a168 R08: 0000000000000002 R09:
> > 000000007ffff000
> > 20551 [  309.836310] R10: ffffc9000623b928 R11: 0000000000000000 R12:
> > 000000007ffff000
> > 20552 [  309.843471] R13: ffff88815ba5a100 R14: 0000000000000000 R15:
> > 0000000000000001
> > 20553 [  309.850634] FS:  00007f9ff305e700(0000)
> > GS:ffff888276c94000(0000) knlGS:0000000000000000
> > 20554 [  309.858749] CS:  0010 DS: 0000 ES: 0000 CR0:
> > 0000000080050033
> > 20555 [  309.864519] CR2: 00007f9fca701000 CR3: 00000001565e2005 CR4:
> > 0000000008f70ef0
> > 20556 [  309.871678] PKRU: 55555558
> > 20557 [  309.874403] Call Trace:
> > 20558 [  309.876866]  <TASK>
> > 20559 [  309.878988]  sg_alloc_table_from_pages_segment+0x60/0x100
> > 20560 [  309.884415]  ? ttm_resource_manager_usage+0x36/0x60 [ttm]
> > 20561 [  309.889845]  ? xe_tt_map_sg+0x7d/0xd0 [xe]
> > 20562 [  309.894045]  xe_tt_map_sg+0x7d/0xd0 [xe]
> > 20563 [  309.898037]  xe_bo_move+0x927/0xaa0 [xe]
> > 20564 [  309.902029]  ttm_bo_handle_move_mem+0xba/0x170 [ttm]
> > 20565 [  309.907022]  ttm_bo_validate+0xbe/0x190 [ttm]
> > 20566 [  309.911405]  xe_bo_validate+0x9a/0x120 [xe]
> > 20567 [  309.915663]  xe_gpuvm_validate+0xd9/0x140 [xe]
> > 20568 [  309.920206]  drm_gpuvm_validate+0x2f0/0x5b0 [drm_gpuvm]
> > 20569 [  309.925459]  ? drm_exec_lock_obj+0x63/0x210 [drm_exec]
> > 20570 [  309.930627]  xe_vm_validate_rebind+0x46/0xb0 [xe]
> > 20571 [  309.935428]  xe_exec_fn+0x20/0x40 [xe]
> > 20572 [  309.939249]  drm_gpuvm_exec_lock+0x78/0xc0 [drm_gpuvm]
> > 20573 [  309.944410]  xe_validation_exec_lock+0x5a/0xa0 [xe]
> > 20574 [  309.949385]  xe_exec_ioctl+0x806/0xc30 [xe]
> > 20575 [  309.953639]  ? ttwu_queue_wakelist+0xd9/0xf0
> > 20576 [  309.957935]  ? __pfx_xe_exec_fn+0x10/0x10 [xe]
> > 20577 [  309.962449]  ? __wake_up_common+0x73/0xa0
> > 20578 [  309.966482]  ? __pfx_xe_exec_ioctl+0x10/0x10 [xe]
> > 20579 [  309.971263]  drm_ioctl_kernel+0xa3/0x100
> > 20580 [  309.975209]  drm_ioctl+0x213/0x440
> > 20581 [  309.978637]  ? __pfx_xe_exec_ioctl+0x10/0x10 [xe]
> > 20582 [  309.983415]  xe_drm_ioctl+0x67/0xd0 [xe]
> > 20583 [  309.987408]  __x64_sys_ioctl+0x7f/0xd0
> > 
> > Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > Cc: Christian Koenig <christian.koenig@amd.com>
> > Cc: Huang Rui <ray.huang@amd.com>
> > Cc: Matthew Auld <matthew.auld@intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Maxime Ripard <mripard@kernel.org>
> > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > Cc: David Airlie <airlied@gmail.com>
> > Cc: Simona Vetter <simona@ffwll.ch>
> > Cc: dri-devel@lists.freedesktop.org
> > Cc: linux-kernel@vger.kernel.org
> > Cc: stable@vger.kernel.org
> > Fixes: b63d715b8090 ("drm/ttm/pool, drm/ttm/tt: Provide a helper to
> > shrink pages")
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > 
> > ---
> > 
> > v3:
> >  - Call ttm_pool_apply_caching after freeing local restore (sashiko)
> >  - Save alloc in snapshot on restore failure (sashiko)
> > v4:
> >  - Actual 'Save alloc in snapshot on restore failure (sashiko)'
> > ---
> >  drivers/gpu/drm/ttm/ttm_pool.c | 19 +++++++++++++++----
> >  1 file changed, 15 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/ttm/ttm_pool.c
> > b/drivers/gpu/drm/ttm/ttm_pool.c
> > index 278bbe7a11ad..c7aab60b7f01 100644
> > --- a/drivers/gpu/drm/ttm/ttm_pool.c
> > +++ b/drivers/gpu/drm/ttm/ttm_pool.c
> > @@ -902,6 +902,7 @@ int ttm_pool_restore_and_alloc(struct ttm_pool
> > *pool, struct ttm_tt *tt,
> >  {
> >  	struct ttm_pool_tt_restore *restore = tt->restore;
> >  	struct ttm_pool_alloc_state alloc;
> > +	int ret;
> >  
> >  	if (WARN_ON(!ttm_tt_is_backed_up(tt)))
> >  		return -EINVAL;
> > @@ -925,14 +926,24 @@ int ttm_pool_restore_and_alloc(struct ttm_pool
> > *pool, struct ttm_tt *tt,
> >  	} else {
> >  		alloc = restore->snapshot_alloc;
> >  		if (ttm_pool_restore_valid(restore)) {
> > -			int ret = ttm_pool_restore_commit(restore,
> > tt->backup,
> > -							  ctx,
> > &alloc);
> > +			ret = ttm_pool_restore_commit(restore, tt-
> > >backup,
> > +						      ctx, &alloc);
> >  
> > -			if (ret)
> > +			if (ret) {
> > +				restore->snapshot_alloc = alloc;
> >  				return ret;
> > +			}
> >  		}
> > -		if (!alloc.remaining_pages)
> > +		if (!alloc.remaining_pages) {
> > +			kfree(tt->restore);
> > +			tt->restore = NULL;
> > +
> > +			ret = ttm_pool_apply_caching(&alloc);
> 
> return ttm_pool_apply_caching(&alloc) ?
> 

Yes, will do.

> Otherwise LGTM.
> Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> 

Thanks.

Matt

> 
> > +			if (ret)
> > +				return ret;
> > +
> >  			return 0;
> > +		}
> >  	}
> >  
> >  	return __ttm_pool_alloc(pool, tt, ctx, &alloc, restore);