[PATCH v2 6/6] mm: use down_read_killable for locking mmap_sem in access_remote_vm

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

This function is used by ptrace and proc files like /proc/pid/cmdline and
/proc/pid/environ.

Access_remote_vm never returns error codes, all errors are ignored and
only size of successfully read data is returned. So, if current task was
killed we'll simply return 0 (bytes read).

Mmap_sem could be locked for a long time or forever if something wrong.
Killable lock allows to cleanup stuck tasks and simplifies investigation.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
---
 mm/memory.c |    4 +++-
 mm/nommu.c  |    3 ++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index ddf20bd0c317..9a4401d21e94 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4349,7 +4349,9 @@ int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
 	void *old_buf = buf;
 	int write = gup_flags & FOLL_WRITE;
 
-	down_read(&mm->mmap_sem);
+	if (down_read_killable(&mm->mmap_sem))
+		return 0;
+
 	/* ignore errors, just check how much was successfully transferred */
 	while (len) {
 		int bytes, ret, offset;
diff --git a/mm/nommu.c b/mm/nommu.c
index d8c02fbe03b5..b2823519f8cd 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1792,7 +1792,8 @@ int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
 	struct vm_area_struct *vma;
 	int write = gup_flags & FOLL_WRITE;
 
-	down_read(&mm->mmap_sem);
+	if (down_read_killable(&mm->mmap_sem))
+		return 0;
 
 	/* the access must start within one of the target process's mappings */
 	vma = find_vma(mm, addr);