ENOSPC returned by handle_mm_fault()

[Al Viro <viro@ZenIV.linux.org.uk>]

	When alloc_huge_page() runs afoul of quota, it returns ERR_PTR(-ENOSPC).
Callers do not expect that - hugetlb_cow() returns ENOSPC if it gets that
and so does hugetlb_no_page().  Eventually the thing propagates back to
hugetlb_fault() and is returned by it.

	Callers of hugetlb_fault() clearly expect a bitmap of VM_... and
not something from errno.h: one place is 
                        ret = hugetlb_fault(mm, vma, vaddr,
                                (flags & FOLL_WRITE) ? FAULT_FLAG_WRITE : 0);
                        spin_lock(&mm->page_table_lock);
                        if (!(ret & VM_FAULT_ERROR))
                                continue;
and another is handle_mm_fault(), which ends up returning ENOSPC and *its*
callers are definitely not ready to deal with that.

ENOSPC is 28, i.e. VM_FAULT_MAJOR | VM_FAULT_WRITE | VM_FAULT_HWPOISON;
it's also theoretically possible to get ENOMEM if region_chg() ends up
hitting
                nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
                if (!nrg)
                        return -ENOMEM;
region_chg() <- vma_needs_reservation() <- alloc_huge_page() and from that
point as with ENOSPC.  ENOMEM is 12, i.e. VM_FAULT_MAJOR | VM_FAULT_WRITE...

Am I right assuming that we want VM_FAULT_OOM in both cases?

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