[PATCH for 5.8] mm: do_swap_page fix up the error code instantiation

[Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

I hope I haven't missed anything but the patch should be the following.

From acd488c22b4bb2ee42526be8ca67145d5127b014 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
Date: Wed, 17 Jun 2020 10:40:47 +0200
Subject: [PATCH] mm: do_swap_page fix up the error code

do_swap_page returns error codes from the VM_FAULT* space. try_charge
might return -ENOMEM, though, and then do_swap_page simply returns 0
which means a success.

We almost never return ENOMEM for GFP_KERNEL single page charge. Except
for async OOM handling (oom_disabled v1). So this needs translation to
VM_FAULT_OOM otherwise the the page fault path will not notify the
userspace and wait for an action.

Fixes: 4c6355b25e8b ("mm: memcontrol: charge swapin pages on instantiation")
Signed-off-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
---
 mm/memory.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index dc7f3543b1fd..d944b7946b27 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3140,8 +3140,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 				err = mem_cgroup_charge(page, vma->vm_mm,
 							GFP_KERNEL);
 				ClearPageSwapCache(page);
-				if (err)
+				if (err) {
+					err = VM_FAULT_OOM;
 					goto out_page;
+				}
 
 				lru_cache_add(page);
 				swap_readpage(page, true);
-- 
2.26.2

-- 
Michal Hocko
SUSE Labs