From: Michal Hocko <mhocko@xxxxxxxx>
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@xxxxxxxx>
---
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 50681f0286de..a8575b880baf 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -980,6 +980,7 @@ 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().
*
@@ -1247,6 +1248,10 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
}
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 48ba9729062e..1965e2681877 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);
}
--
2.25.1
