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vm pqbatch: move unmanaged page assert under pagequeue lock

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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
This commit is contained in:
Max Laier 2021-02-24 15:56:16 -08:00 committed by Ryan Libby
parent 9a227a2fd6
commit 14b5a3c7d5
1 changed files with 2 additions and 5 deletions
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7
sys/vm/vm_page.c
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@ -3545,9 +3545,8 @@ vm_pqbatch_process_page(struct vm_pagequeue *pq, vm_page_t m, uint8_t queue)
counter_u64_add(queue_nops, 1);
break;
}
KASSERT(old.queue != PQ_NONE ||
(old.flags & PGA_QUEUE_STATE_MASK) == 0,
("%s: page %p has unexpected queue state", __func__, m));
KASSERT((m->oflags & VPO_UNMANAGED) == 0,
("%s: page %p is unmanaged", __func__, m));
new = old;
if ((old.flags & PGA_DEQUEUE) != 0) {
@ -3594,8 +3593,6 @@ vm_page_pqbatch_submit(vm_page_t m, uint8_t queue)
struct vm_pagequeue *pq;
int domain;
KASSERT((m->oflags & VPO_UNMANAGED) == 0,
("page %p is unmanaged", m));
KASSERT(queue < PQ_COUNT, ("invalid queue %d", queue));
domain = vm_page_domain(m);