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sleepable scan, iteration over the shadow chain looking for a page could find an OBJ_DEAD object. Such state of the mapping is only transient, the dead object will be terminated and removed from the chain shortly. We must not return KERN_PROTECTION_FAILURE unless the object type is changed to OBJT_DEAD in the chain, indicating that paging on this address is really impossible. Returning KERN_PROTECTION_FAILURE prematurely causes spurious SIGSEGV delivered to processes, or kernel accesses to UVA spuriously failing with EFAULT. If the object with OBJ_DEAD flag is found, only return KERN_PROTECTION_FAILURE when object type is already OBJT_DEAD. Otherwise, sleep a tick and retry the fault handling. Ideally, we would wait until the OBJ_DEAD flag is resolved, e.g. by waiting until the paging on this object is finished. But to do so, we need to reference the dead object, while vm_object_collapse() insists on owning the final reference on the collapsed object. This could be fixed by e.g. changing the assert to shared reference release between vm_fault() and vm_object_collapse(), but it seems to be too much complications for rare boundary condition. PR: 204426 Tested by: pho Reviewed by: alc Sponsored by: The FreeBSD Foundation X-Differential revision: https://reviews.freebsd.org/D6085 MFC after: 2 weeks Approved by: re (gjb)
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