Change Details

These two fields encode the "queue state" of a page. The page lock is required to set PGA_{DEQUEUE,REQUEUE,REQUEUE_HEAD}, and the page queue lock corresponding to the "queue" field's value is required to clear them. The page queue lock is also required when modifying the "queue" field or setting and clearing PGA_ENQUEUED. My plan is to support freeing pages without the page lock. However, vm_page_free_prep() must set PGA_DEQUEUE in vm_page_dequeue_deferred_free(). Since the page is being freed, the scope of possible races is limited to the page daemon scanning the page. Introduce a new function, vm_page_aflag_queue_cmpset(), which atomically updates both fields containing queue state. It relies on "aflags" and "queue" being in the same self-aligned 32-bit word. This primitive is used by the page daemon in vm_page_swapqueue() and vm_page_dequeue_deferred(). It allows the page daemon to avoid modifying queue state after vm_page_dequeue_deferred_free() has already set PGA_DEQUEUE. Once the page daemon has acquire the object lock, it has a stable reference to the page and no longer needs to handle races.

These two fields encode the "queue state" of a page. The page lock is required to set `PGA_{DEQUEUE,REQUEUE,REQUEUE_HEAD}`, and the page queue lock corresponding to the "queue" field's value is required to clear them. The page queue lock is also required when modifying the "queue" field or setting and clearing `PGA_ENQUEUED`. My goal is to support freeing pages without the page lock. However, `vm_page_free_prep()` must set `PGA_DEQUEUE` in `vm_page_dequeue_deferred_free()`. Since the page is being freed, the scope of possible races is limited to the page daemon scanning the page. Introduce a new function, `vm_page_aflag_queue_cmpset()`, which atomically updates both fields containing queue state. It relies on "aflags" and "queue" being in the same self-aligned 32-bit word. This primitive is used by the page daemon in `vm_page_swapqueue()` and `vm_page_dequeue_deferred()`. It allows the page daemon to avoid modifying queue state after `vm_page_dequeue_deferred_free()` has already set `PGA_DEQUEUE`. Once the page daemon has acquire the object lock, it has a stable reference to the page and no longer needs to handle races, so the only modifications to the page queue scans occur in places where the object lock is not held. `vm_pageout_reinsert_inactive_page()` does not require modification since it takes the page queue lock and is thus serialized with the `vm_page_dequeue()` calls in the page allocator functions.

These two fields encode the "queue state" of a page. The page lock is required to set `PGA_{DEQUEUE,REQUEUE,REQUEUE_HEAD}`, and the page queue lock corresponding to the "queue" field's value is required to clear them. The page queue lock is also required when modifying the "queue" field or setting and clearing `PGA_ENQUEUED`. My planMy goal is to support freeing pages without the page lock. However, `vm_page_free_prep()` must set `PGA_DEQUEUE in` in `vm_page_dequeue_deferred_free()`. Since the page is being freed, the scope of possible races is limited to the page daemon scanning the page. Introduce a new function, `vm_page_aflag_queue_cmpset()`, which atomically updates both fields containing queue state. It relies on "aflags" and "queue" being in the same self-aligned 32-bit word. This primitive is used by the page daemon in `vm_page_swapqueue()` and `vm_page_dequeue_deferred()`. It allows the page daemon to avoid modifying queue state after `vm_page_dequeue_deferred_free()` has already set `PGA_DEQUEUE`. Once the page daemon has acquire the object lock, it has a stable reference to the page and no longer needs to handle races, so the only modifications to the page queue scans occur in places where the object lock is not held. `vm_pageout_reinsert_inactive_page()` does not require modification since it takes the page queue lock and is thus serialized with the `vm_page_dequeue()` calls in the page allocator functions.