K 10
svn:author
V 5
markj
K 8
svn:date
V 27
2019-12-28T19:03:46.341318Z
K 7
svn:log
V 1124
Generalize lazy dequeue logic for wired pages.

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

END