mm/hmm: comment on VM_FAULT_RETRY semantics in handle_mm_fault

The magic dropping of mmap_sem when handle_mm_fault returns VM_FAULT_RETRY
is rather subtile.  Add a comment explaining it.

Link: https://lore.kernel.org/r/20190724065258.16603-8-hch@lst.deTested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
[hch: wrote a changelog]
Signed-off-by: Christoph Hellwig <hch@lst.de>

mm/hmm.c

@@ -301,8 +301,10 @@ static int hmm_vma_do_fault(struct mm_walk *walk, unsigned long addr,
 	flags |= hmm_vma_walk->block ? 0 : FAULT_FLAG_ALLOW_RETRY;
 	flags |= write_fault ? FAULT_FLAG_WRITE : 0;
 	ret = handle_mm_fault(vma, addr, flags);
-	if (ret & VM_FAULT_RETRY)
+	if (ret & VM_FAULT_RETRY) {
+		/* Note, handle_mm_fault did up_read(&mm->mmap_sem)) */
 		return -EAGAIN;
+	}
 	if (ret & VM_FAULT_ERROR) {
 		*pfn = range->values[HMM_PFN_ERROR];
 		return -EFAULT;