Re: [PATCH] drm/ttm: fix swapped-out resources never leaving their bulk_move range

[Thomas Hellström]

On Wed, 2026-09-09 at 23:50 +0300, Vadim Nikitushkin wrote:
> ttm_tt_swapout() returns the number of pages swapped out on success
> and
> a negative error code on failure; for a populated ttm it never
> returns
> zero. Commit b2ed01e7ad3d ("drm/ttm: Fix ttm_bo_swapout() infinite
> LRU
> walk on swapout failure") moved the bulk_move bookkeeping in
> ttm_bo_swapout_cb() under "if (!ret)", so the
> ttm_resource_del_bulk_move_unevictable() /
> ttm_resource_move_to_lru_tail()
> pair is now skipped on every successful swapout. The equivalent
> change
> for the shrinker in commit 1d59f36e95f7 ("drm/ttm: Fix
> ttm_bo_shrink()
> infinite LRU walk on backup failure") tests "lret > 0", which is what
> was intended here as well.
> 
> Before b2ed01e7ad3d the resource was taken off the bulk_move before
> the
> swapout; since then a swapped-out resource stays inside its BO's
> bulk_move range (and on the manager LRU) although it is unevictable.
> When it is later freed or the BO leaves the bulk_move
> (ttm_resource_free(), ttm_bo_set_bulk_move() via amdgpu_vm_bo_del()),
> ttm_resource_del_bulk_move() skips it because of its
> !ttm_resource_unevictable() guard, so a range endpoint in pos->first
> /
> pos->last is left pointing at freed memory. The next
> ttm_lru_bulk_move_tail() or ttm_resource_add_bulk_move() on that
> cursor
> is a use-after-free, seen as the resv WARN in
> ttm_lru_bulk_move_add(),
> "list_del corruption" in ttm_resource_move_to_lru_tail() or a NULL
> dereference in ttm_resource_manager_next() -- minutes to hours after
> a
> hibernation, or at process exit / reboot following one. Samuel
> Ainsworth's analysis of drm/amd issue 5387 (see Link) identified the
> dangling cursor; the missing removal at swapout time is the reason it
> dangles.
> 
> Testing the condition for success restores the removal. On an AMD
> Phoenix APU (ASUS UM3406GA, gfx1103) running suspend-then-hibernate
> on
> a 7.0.y stable kernel carrying the backport (Ubuntu 7.0.0-31) the bug
> crashed 5 of 18 hibernation cycles; a function profile of one
> hibernation showed 336 ttm_tt_swapout() calls and zero
> ttm_resource_del_bulk_move_unevictable() calls. With this change the
> removal happens for every swapped-out resource and 12 further cycles
> were clean.
> 
> Fixes: b2ed01e7ad3d ("drm/ttm: Fix ttm_bo_swapout() infinite LRU walk
> on swapout failure")
> Cc: stable@xxxxxxxxxxxxxxx # v7.1+
> Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/5387
> Link:
> https://lore.kernel.org/dri-devel/CAHYiNPa6aVacJoLOje-qZ1GyYx-9p0tN4NuP8D_eSL+UJeevXw@xxxxxxxxxxxxxx/
> Signed-off-by: Vadim Nikitushkin <bub4z0r@xxxxxxxxx>

Nice catch.

This also explains why https://patchwork.freedesktop.org/series/170311/
appeared to fix the issue. But that series actually kept the resource
on the bulk sublist until someone bumped the LRU or removed it.

Reviewed-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>

> ---
>  drivers/gpu/drm/ttm/ttm_bo.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c
> b/drivers/gpu/drm/ttm/ttm_bo.c
> index ef56c18..9b85b5f 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> @@ -1434,7 +1434,7 @@ ttm_bo_swapout_cb(struct ttm_lru_walk *walk,
> struct ttm_buffer_object *bo)
>  
>  	if (ttm_tt_is_populated(tt)) {
>  		ret = ttm_tt_swapout(bdev, tt, swapout_walk-
> >gfp_flags);
> -		if (!ret) {
> +		if (ret > 0) {
>  			spin_lock(&bdev->lru_lock);
>  			ttm_resource_del_bulk_move_unevictable(bo-
> >resource, bo);
>  			ttm_resource_move_to_lru_tail(bo->resource);