Re: [PATCH] drm/xe: Fix shrinker accounting double-subtraction on nested external pins

[Matthew Auld <matthew.auld@intel.com>]

On 14/09/2026 14:45, Thomas Hellström wrote:
> xe_bo_pin_external() and xe_bo_unpin_external() support nested pins,
> since the underlying ttm_bo_pin()/ttm_bo_unpin() maintain a pin_count
> refcount rather than a boolean. However, the shrinker accounting calls
> xe_ttm_tt_account_subtract() and xe_ttm_tt_account_add() are invoked
> unconditionally on every pin_external()/unpin_external() call, instead
> of only on the transition into and out of the pinned state.
> 
> An external BO (dma-buf) can be pinned more than once while already
> pinned, for example when it has multiple attachments/importers, each
> independently calling xe_bo_pin_external(). Each such nested pin
> subtracts the BO's pages from the shrinker's accounting again, even
> though the BO's pages were already removed from consideration by the
> first pin. Symmetrically, each nested unpin adds them back again
> before the BO is actually unpinned. This desynchronizes the
> shrinker's page and object counts from reality, and since they are
> declared as signed long but interpreted as unsigned long in
> xe_shrinker_count(), the underflow can turn into an enormous
> shrinkable/purgeable page count, causing the shrinker to be invoked
> excessively under memory pressure.
> 
> Guard the accounting calls with the same pin-count transition checks
> already used to guard the pinned_link list maintenance, so accounting
> is only updated on the outermost pin and the final unpin.
> 
> Fixes: 00c8efc3180f ("drm/xe: Add a shrinker for xe bos")
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Cc: intel-xe@lists.freedesktop.org
> Cc: <stable@vger.kernel.org> # v6.15+
> Assisted-by: LLM
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

Reviewed-by: Matthew Auld <matthew.auld@intel.com>