vm pqbatch: move unmanaged page assert under pagequeue lock

This KASSERT is overzealous because of the following race condition:
 1) A managed page which is currently in PQ_LAUNDRY is freed.
    vm_page_free_prep calls vm_page_dequeue_deferred()

    The page state is:
       PQ_LAUNDRY, PGA_DEQUEUE|PGA_ENQUEUED

 2) The laundry worker comes around and pick up the page and calls
    vm_pageout_defer(m, PQ_LAUNDRY, true) to check if page is still in the
    queue.  We do a vm_page_astate_load and get
       PQ_LAUNDRY, PGA_DEQUEUE|PGA_ENQUEUED
    as per above.

 3) The laundry worker is pre-empted and another thread allocates our page
    from the free pool.  For example vm_page_alloc_domain_after calls
    vm_page_dequeue() and sets VPO_UNMANAGED because we are allocating for
    an OBJT_UNMANAGED object.

    The page state is:
       PQ_NONE, 0 - VPO_UNMANAGED

 4) The laundry worker resumes, and processes vm_pageout_defer based on the
    stale astate which leads to a call to vm_page_pqbatch_submit, which will
    trip on the KASSERT.

Submitted by:	mlaier
Reviewed by:	markj, rlibby
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D28563

0 files changed, 0 insertions, 0 deletions