Update the vm_page.h block comment to reflect recent changes.
Explain the new locking rules for per-page queue state updates. Reviewed by: jeff, kib Sponsored by: Netflix, Intel Differential Revision: https://reviews.freebsd.org/D22884
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2020-12-20 02:59:44 +00:00
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@ -171,34 +171,32 @@
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* The page daemon must therefore handle the possibility of a concurrent
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* free of the page.
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*
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* The queue field is the index of the page queue containing the page,
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* or PQ_NONE if the page is not enqueued. The queue lock of a page is
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* the page queue lock corresponding to the page queue index, or the
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* page lock (P) for the page if it is not enqueued. To modify the
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* queue field, the queue lock for the old value of the field must be
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* held. There is one exception to this rule: the page daemon may
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* transition the queue field from PQ_INACTIVE to PQ_NONE immediately
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* prior to freeing a page during an inactive queue scan. At that
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* point the page has already been physically dequeued and no other
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* references to that vm_page structure exist.
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* The queue state of a page consists of the queue and act_count fields of
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* its atomically updated state, and the subset of atomic flags specified
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* by PGA_QUEUE_STATE_MASK. The queue field contains the page's page queue
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* index, or PQ_NONE if it does not belong to a page queue. To modify the
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* queue field, the page queue lock corresponding to the old value must be
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* held, unless that value is PQ_NONE, in which case the queue index must
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* be updated using an atomic RMW operation. There is one exception to
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* this rule: the page daemon may transition the queue field from
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* PQ_INACTIVE to PQ_NONE immediately prior to freeing the page during an
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* inactive queue scan. At that point the page is already dequeued and no
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* other references to that vm_page structure can exist. The PGA_ENQUEUED
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* flag, when set, indicates that the page structure is physically inserted
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* into the queue corresponding to the page's queue index, and may only be
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* set or cleared with the corresponding page queue lock held.
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*
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* To avoid contention on page queue locks, page queue operations
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* (enqueue, dequeue, requeue) are batched using per-CPU queues. A
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* deferred operation is requested by inserting an entry into a batch
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* queue; the entry is simply a pointer to the page, and the request
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* type is encoded in the page's aflags field using the values in
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* PGA_QUEUE_STATE_MASK. The type-stability of struct vm_pages is
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* crucial to this scheme since the processing of entries in a given
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* batch queue may be deferred indefinitely. In particular, a page may
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* be freed before its pending batch queue entries have been processed.
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* The page lock (P) must be held to schedule a batched queue
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* operation, and the page queue lock must be held in order to process
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* batch queue entries for the page queue. There is one exception to
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* this rule: the thread freeing a page may schedule a dequeue without
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* holding the page lock. In this scenario the only other thread which
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* may hold a reference to the page is the page daemon, which is
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* careful to avoid modifying the page's queue state once the dequeue
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* has been requested by setting PGA_DEQUEUE.
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* To avoid contention on page queue locks, page queue operations (enqueue,
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* dequeue, requeue) are batched using fixed-size per-CPU queues. A
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* deferred operation is requested by setting one of the flags in
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* PGA_QUEUE_OP_MASK and inserting an entry into a batch queue. When a
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* queue is full, an attempt to insert a new entry will lock the page
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* queues and trigger processing of the pending entries. The
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* type-stability of vm_page structures is crucial to this scheme since the
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* processing of entries in a given batch queue may be deferred
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* indefinitely. In particular, a page may be freed with pending batch
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* queue entries. The page queue operation flags must be set using atomic
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* RWM operations.
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*/
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#if PAGE_SIZE == 4096
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