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In vm_object_backing_scan(), setting PG_REFERENCED on a page before

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sleeping on that page is nonsensical.  Doing so reduces the likelihood
that the page daemon will reclaim the page before the thread waiting in
vm_object_backing_scan() is reawakened.  However, it does not guarantee
that the page is not reclaimed, so vm_object_backing_scan() restarts
after reawakening.  More importantly, this muddles the meaning of
PG_REFERENCED.  There is no reason to believe that the caller of
vm_object_backing_scan() is going to use (i.e., access) the contents of
the page.  There is especially no reason to believe that an access is
more likely because vm_object_backing_scan() had to sleep on the page.

Discussed with:	kib
MFC after:	3 weeks
This commit is contained in:
alc 2010-04-17 18:35:07 +00:00
parent 8baf4dd12a
commit 7075c32b12
1 changed files with 0 additions and 3 deletions
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3
sys/vm/vm_object.c
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@ -1553,9 +1553,6 @@ vm_object_backing_scan(vm_object_t object, int op)
}
} else if (op & OBSC_COLLAPSE_WAIT) {
if ((p->oflags & VPO_BUSY) || p->busy) {
vm_page_lock_queues();
vm_page_flag_set(p, PG_REFERENCED);
vm_page_unlock_queues();
VM_OBJECT_UNLOCK(object);
p->oflags |= VPO_WANTED;
msleep(p, VM_OBJECT_MTX(backing_object),