We added _NORECLAIM to request that kmem_alloc_contig_pages() not spend
time scanning physical memory for candidates to reclaim. In some
situations the scanning can induce large amounts of undesirable latency,
and it's less important that the request be satisfied than it is that we
not spend many milliseconds scanning.
The problem extends to vm_reserv_reclaim_contig(), which unlike
vm_reserv_reclaim() may have to scan the entire list of partially
populated reservations. Use VM_ALLOC_NORECLAIM to request that this
scan not be executed.[1]
As a side effect, this fixes a regression in 02fb0585e7b3 ("vm_page:
Drop handling of VM_ALLOC_NOOBJ in vm_page_alloc_contig_domain()")
where VM_ALLOC_CONTIG was not included in VPAC_FLAGS or VPANC_FLAGS even
though it is not masked by kmem_alloc_contig_pages().[2]
Reported by: gallatin [1], glebius [2]