From 2d3a36a47964371101d9a71691c18d59ee611e87 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Wed, 3 Jun 2020 16:03:25 -0700
Subject: [PATCH] mm, mempolicy: fix up gup usage in lookup_node
ba841078cd05 ("mm/mempolicy: Allow lookup_node() to handle fatal signal")
has added a special casing for 0 return value because that was a possible
gup return value when interrupted by fatal signal. This has been fixed by
ae46d2aa6a7f ("mm/gup: Let __get_user_pages_locked() return -EINTR for
fatal signal") in the mean time so ba841078cd05 can be reverted.
This patch however doesn't go all the way to revert it because the check
for 0 is wrong and confusing here. Firstly it is inherently unsafe to
access the page when get_user_pages_locked returns 0 (aka no page
returned).
Fortunatelly this will not happen because get_user_pages_locked will not
return 0 when nr_pages > 0 unless FOLL_NOWAIT is specified which is not
the case here. Document this potential error code in gup code while we
are at it.
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>
Link: http://lkml.kernel.org/r/20200421071026.18394-1-mhocko@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
mm/gup.c | 5 +++++
mm/mempolicy.c | 5 +----
2 files changed, 6 insertions(+), 4 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index 61ee405ec55882..e19ff770eb4c87 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -989,6 +989,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
* -- If nr_pages is >0, but no pages were pinned, returns -errno.
* -- If nr_pages is >0, and some pages were pinned, returns the number of
* pages pinned. Again, this may be less than nr_pages.
+ * -- 0 return value is possible when the fault would need to be retried.
*
* The caller is responsible for releasing returned @pages, via put_page().
*
@@ -1265,6 +1266,10 @@ retry:
}
EXPORT_SYMBOL_GPL(fixup_user_fault);
+/*
+ * Please note that this function, unlike __get_user_pages will not
+ * return 0 for nr_pages > 0 without FOLL_NOWAIT
+ */
static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
struct mm_struct *mm,
unsigned long start,
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 48ba9729062ea6..1965e268187735 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -927,10 +927,7 @@ static int lookup_node(struct mm_struct *mm, unsigned long addr)
int locked = 1;
err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked);
- if (err == 0) {
- /* E.g. GUP interrupted by fatal signal */
- err = -EFAULT;
- } else if (err > 0) {
+ if (err > 0) {
err = page_to_nid(p);
put_page(p);
}
--
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