b75fed1661
Since r354156 we may call release_page() without the page's object lock held, specifically following the page copy during a CoW fault. release_page() must therefore unbusy the page only after scheduling the requeue, to avoid racing with a free of the page. Previously, the object lock prevented this race from occurring. Add some assertions that were helpful in tracking this down. Reported by: pho, syzkaller Tested by: pho Reviewed by: alc, jeff, kib Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D22234
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