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vm_page: Fix a logic bug in vm_page_unwire_managed()

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When releasing a page reference, we have logic for various cases, based
on the value of the counter.  But, the implementation fails to take into
account the possibility that the VPRC_BLOCKED flag is set, which is ORed
into the counter for short windows when removing mappings of a page.  If
the flag is set while the last reference is being released, we may fail
to add the page to a page queue when the last wiring reference is
released.

Fix the problem by performing comparisons with VPRC_BLOCKED masked off.
While here, add a related assertion.

Reviewed by:	dougm, kib
Tested by:	pho
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D46944
This commit is contained in:
Mark Johnston 2024-10-07 20:50:49 +00:00
parent d8b32da235
commit c59166e5b4
1 changed files with 8 additions and 3 deletions
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11
sys/vm/vm_page.c
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@ -4275,10 +4275,13 @@ vm_page_unwire_managed(vm_page_t m, uint8_t nqueue, bool noreuse)
*/
old = atomic_load_int(&m->ref_count);
do {
u_int count;
KASSERT(VPRC_WIRE_COUNT(old) > 0,
("vm_page_unwire: wire count underflow for page %p", m));
if (old > VPRC_OBJREF + 1) {
count = old & ~VPRC_BLOCKED;
if (count > VPRC_OBJREF + 1) {
/*
* The page has at least one other wiring reference. An
* earlier iteration of this loop may have called
@ -4287,7 +4290,7 @@ vm_page_unwire_managed(vm_page_t m, uint8_t nqueue, bool noreuse)
*/
if ((vm_page_astate_load(m).flags & PGA_DEQUEUE) == 0)
vm_page_aflag_set(m, PGA_DEQUEUE);
} else if (old == VPRC_OBJREF + 1) {
} else if (count == VPRC_OBJREF + 1) {
/*
* This is the last wiring. Clear PGA_DEQUEUE and
* update the page's queue state to reflect the
@ -4296,7 +4299,7 @@ vm_page_unwire_managed(vm_page_t m, uint8_t nqueue, bool noreuse)
* clear leftover queue state.
*/
vm_page_release_toq(m, nqueue, noreuse);
} else if (old == 1) {
} else if (count == 1) {
vm_page_aflag_clear(m, PGA_DEQUEUE);
}
} while (!atomic_fcmpset_rel_int(&m->ref_count, &old, old - 1));
@ -4572,6 +4575,8 @@ vm_page_try_blocked_op(vm_page_t m, void (*op)(vm_page_t))
do {
KASSERT(old != 0,
("vm_page_try_blocked_op: page %p has no references", m));
KASSERT((old & VPRC_BLOCKED) == 0,
("vm_page_try_blocked_op: page %p blocks wirings", m));
if (VPRC_WIRE_COUNT(old) != 0)
return (false);
} while (!atomic_fcmpset_int(&m->ref_count, &old, old | VPRC_BLOCKED));