From: Jason Gunthorpe <jgg@mellanox.com>
Date: Wed, 24 Jul 2019 08:52:58 +0200
Subject: mm/hmm: comment on VM_FAULT_RETRY semantics in handle_mm_fault
Patch-mainline: v5.4-rc1
Git-commit: e709accc7670a07b2e5186449f0640c2416662ec
References: jsc#SLE-15176, jsc#SLE-16387

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.de
Tested-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>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 mm/hmm.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -301,8 +301,10 @@ static int hmm_vma_do_fault(struct mm_wa
 	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;