From: Ralph Campbell <rcampbell@nvidia.com>
Date: Wed, 31 Jan 2018 16:20:30 -0800
Subject: mm/hmm: fix uninitialized use of 'entry' in hmm_vma_walk_pmd()
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Git-commit: 8d63e4cd62b2583c7efe64f2ede406b3f44983f6
Patch-mainline: v4.16-rc1
References: HMM fixes, fate#325923, bsc#1104965

The variable 'entry' is used before being initialized in
hmm_vma_walk_pmd().

No bad effect (beside performance hit) so !non_swap_entry(0) evaluate to
true which trigger a fault as if CPU was trying to access migrated
memory and migrate memory back from device memory to regular memory.

This function (hmm_vma_walk_pmd()) is called when a device driver tries
to populate its own page table.  For migrated memory it should not
happen as the device driver should already have populated its page table
correctly during the migration.

Only case I can think of is multi-GPU where a second GPU triggers
migration back to regular memory.  Again this would just result in a
performance hit, nothing bad would happen.

Link: http://lkml.kernel.org/r/20180122185759.26286-1-jglisse@redhat.com
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/hmm.c |    4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -418,7 +418,7 @@ again:
 		}
 
 		if (!pte_present(pte)) {
-			swp_entry_t entry;
+			swp_entry_t entry = pte_to_swp_entry(pte);
 
 			if (!non_swap_entry(entry)) {
 				if (hmm_vma_walk->fault)
@@ -426,8 +426,6 @@ again:
 				continue;
 			}
 
-			entry = pte_to_swp_entry(pte);
-
 			/*
 			 * This is a special swap entry, ignore migration, use
 			 * device and report anything else as error.