Author: markj Date: Sun Feb 3 18:43:20 2019 New Revision: 343720 URL: https://svnweb.freebsd.org/changeset/base/343720
Log: Allow vm_page_free_prep() to dequeue pages without the page lock. This is a step towards being able to free pages without the page lock held. The approach is simply to add an implementation of vm_page_dequeue_deferred() which does not assert that the page lock is held. Formally, the page lock is required to set PGA_DEQUEUE, but in the case of vm_page_free_prep() we get the same mutual exclusion for free by virtue of the fact that no other references to the page may exist. No functional change intended. Reviewed by: kib (previous version) MFC after: 2 weeks Sponsored by: Netflix Differential Revision: https://reviews.freebsd.org/D19065 Modified: head/sys/vm/vm_page.c head/sys/vm/vm_page.h Modified: head/sys/vm/vm_page.c ============================================================================== --- head/sys/vm/vm_page.c Sun Feb 3 18:38:58 2019 (r343719) +++ head/sys/vm/vm_page.c Sun Feb 3 18:43:20 2019 (r343720) @@ -3175,7 +3175,11 @@ vm_pqbatch_submit_page(vm_page_t m, uint8_t queue) struct vm_pagequeue *pq; int domain; - vm_page_assert_locked(m); + KASSERT((m->oflags & VPO_UNMANAGED) == 0, + ("page %p is unmanaged", m)); + KASSERT(mtx_owned(vm_page_lockptr(m)) || + (m->object == NULL && (m->aflags & PGA_DEQUEUE) != 0), + ("missing synchronization for page %p", m)); KASSERT(queue < PQ_COUNT, ("invalid queue %d", queue)); domain = vm_phys_domain(m); @@ -3197,8 +3201,9 @@ vm_pqbatch_submit_page(vm_page_t m, uint8_t queue) /* * The page may have been logically dequeued before we acquired the - * page queue lock. In this case, the page lock prevents the page - * from being logically enqueued elsewhere. + * page queue lock. In this case, since we either hold the page lock + * or the page is being freed, a different thread cannot be concurrently + * enqueuing the page. */ if (__predict_true(m->queue == queue)) vm_pqbatch_process_page(pq, m); @@ -3290,6 +3295,30 @@ vm_page_dequeue_deferred(vm_page_t m) } /* + * A variant of vm_page_dequeue_deferred() that does not assert the page + * lock and is only to be called from vm_page_free_prep(). It is just an + * open-coded implementation of vm_page_dequeue_deferred(). Because the + * page is being freed, we can assume that nothing else is scheduling queue + * operations on this page, so we get for free the mutual exclusion that + * is otherwise provided by the page lock. + */ +static void +vm_page_dequeue_deferred_free(vm_page_t m) +{ + uint8_t queue; + + KASSERT(m->object == NULL, ("page %p has an object reference", m)); + + if ((m->aflags & PGA_DEQUEUE) != 0) + return; + atomic_thread_fence_acq(); + if ((queue = m->queue) == PQ_NONE) + return; + vm_page_aflag_set(m, PGA_DEQUEUE); + vm_pqbatch_submit_page(m, queue); +} + +/* * vm_page_dequeue: * * Remove the page from whichever page queue it's in, if any. @@ -3474,7 +3503,7 @@ vm_page_free_prep(vm_page_t m) * dequeue. */ if ((m->oflags & VPO_UNMANAGED) == 0) - vm_page_dequeue_deferred(m); + vm_page_dequeue_deferred_free(m); m->valid = 0; vm_page_undirty(m); Modified: head/sys/vm/vm_page.h ============================================================================== --- head/sys/vm/vm_page.h Sun Feb 3 18:38:58 2019 (r343719) +++ head/sys/vm/vm_page.h Sun Feb 3 18:43:20 2019 (r343720) @@ -351,8 +351,10 @@ extern struct mtx_padalign pa_lock[]; * queue, and cleared when the dequeue request is processed. A page may * have PGA_DEQUEUE set and PGA_ENQUEUED cleared, for instance if a dequeue * is requested after the page is scheduled to be enqueued but before it is - * actually inserted into the page queue. The page lock must be held to set - * this flag, and the queue lock for the page must be held to clear it. + * actually inserted into the page queue. For allocated pages, the page lock + * must be held to set this flag, but it may be set by vm_page_free_prep() + * without the page lock held. The page queue lock must be held to clear the + * PGA_DEQUEUE flag. * * PGA_REQUEUE is set when the page is scheduled to be enqueued or requeued * in its page queue. The page lock must be held to set this flag, and the _______________________________________________ svn-src-all@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
