Handle trashed queue pointers in vm_page_acquire_unlocked().

[markj]

Summary

vm_page_acquire_unlocked() relies heavily on type-stability of vm_page
structures, and assumes that the listq linkage pointers always point to
a vm_page or are NULL. QUEUE_MACRO_DEBUG_TRASH breaks that assumption.

Reported by: pho

[kib]

Isn't it too fragile still ? What if prev->q reused for something yet again ?

[markj]

In D24472#538433, @kib wrote:

Isn't it too fragile still ? What if prev->q reused for something yet again ?

That is handled by the subsequent checks, if I understood your question correctly.

[kib]

In D24472#538438, @markj wrote:

In D24472#538433, @kib wrote:

Isn't it too fragile still ? What if prev->q reused for something yet again ?

That is handled by the subsequent checks, if I understood your question correctly.

I mean that there were attempts to reuse m->q for pages not in the object queue, and there probably would be more. I believe it is somewhat mitigated by the requirement that prev is either busy or wired, this is why I agreed with the proposed patch.