linux-kernel - Re: [PATCH for 5.8] mm: do_swap_page fix up the error code instantiation

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Message-ID: <20200617090238.GL9499@dhcp22.suse.cz>
Date:   Wed, 17 Jun 2020 11:02:38 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>
Cc:     Alex Shi <alex.shi@...ux.alibaba.com>,
        Joonsoo Kim <js1304@...il.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Roman Gushchin <guro@...com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH for 5.8] mm: do_swap_page fix up the error code
 instantiation

Damn, I forgot to commit my last change (s@err@ret@). Sorry about the
noise.

>From 50297dd026ebf71fe901e1945a9ce1e8d8aa083b Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.com>
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@...e.com>
---
 mm/memory.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index dc7f3543b1fd..1c632faa2611 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) {
+					ret = VM_FAULT_OOM;
 					goto out_page;
+				}
 
 				lru_cache_add(page);
 				swap_readpage(page, true);
-- 
2.26.2

-- 
Michal Hocko
SUSE Labs