b7f30bff2f
Some recent work aims to remove the use of the page lock for synchronizing updates to page queue state. This change adds a mechanism to preserve the existing behaviour of lazily dequeuing wired pages, which was previously synchronized using the page lock. Handle this by setting PGA_DEQUEUE when a managed page's wire count transitions from 0 to 1. When the page daemon encounters a page with a flag in PGA_QUEUE_OP_MASK set, it creates a batch queue entry for that page, but in so doing it does not modify the page itself and thus racing with a concurrent free of the page is harmless. The flag is advisory; the page daemon still checks for wirings after acquiring the object and page xbusy locks. vm_page_unwire_managed() now clears PGA_DEQUEUE on a 1->0 transition. It must do this before dropping the reference to avoid a use-after-free but also handles races with concurrent wirings to ensure that PGA_DEQUEUE is not left unset on a wired page. Reviewed by: jeff Tested by: pho Sponsored by: Netflix, Intel Differential Revision: https://reviews.freebsd.org/D22882
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