[PATCH v2 5/6] proc: use down_read_killable mmap_sem for /proc/pid/map_files

[Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>]

Do not stuck forever if something wrong.
Killable lock allows to cleanup stuck tasks and simplifies investigation.

It seems ->d_revalidate() could return any error (except ECHILD) to
abort validation and pass error as result of lookup sequence.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Reviewed-by: Roman Gushchin <guro@xxxxxx>
Reviewed-by: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
Reviewed-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
---
 fs/proc/base.c |   27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 9c8ca6cd3ce4..515ab29c2adf 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1962,9 +1962,12 @@ static int map_files_d_revalidate(struct dentry *dentry, unsigned int flags)
 		goto out;
 
 	if (!dname_to_vma_addr(dentry, &vm_start, &vm_end)) {
-		down_read(&mm->mmap_sem);
-		exact_vma_exists = !!find_exact_vma(mm, vm_start, vm_end);
-		up_read(&mm->mmap_sem);
+		status = down_read_killable(&mm->mmap_sem);
+		if (!status) {
+			exact_vma_exists = !!find_exact_vma(mm, vm_start,
+							    vm_end);
+			up_read(&mm->mmap_sem);
+		}
 	}
 
 	mmput(mm);
@@ -2010,8 +2013,11 @@ static int map_files_get_link(struct dentry *dentry, struct path *path)
 	if (rc)
 		goto out_mmput;
 
+	rc = down_read_killable(&mm->mmap_sem);
+	if (rc)
+		goto out_mmput;
+
 	rc = -ENOENT;
-	down_read(&mm->mmap_sem);
 	vma = find_exact_vma(mm, vm_start, vm_end);
 	if (vma && vma->vm_file) {
 		*path = vma->vm_file->f_path;
@@ -2107,7 +2113,10 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
 	if (!mm)
 		goto out_put_task;
 
-	down_read(&mm->mmap_sem);
+	result = ERR_PTR(-EINTR);
+	if (down_read_killable(&mm->mmap_sem))
+		goto out_put_mm;
+
 	vma = find_exact_vma(mm, vm_start, vm_end);
 	if (!vma)
 		goto out_no_vma;
@@ -2118,6 +2127,7 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
 
 out_no_vma:
 	up_read(&mm->mmap_sem);
+out_put_mm:
 	mmput(mm);
 out_put_task:
 	put_task_struct(task);
@@ -2160,7 +2170,12 @@ proc_map_files_readdir(struct file *file, struct dir_context *ctx)
 	mm = get_task_mm(task);
 	if (!mm)
 		goto out_put_task;
-	down_read(&mm->mmap_sem);
+
+	ret = down_read_killable(&mm->mmap_sem);
+	if (ret) {
+		mmput(mm);
+		goto out_put_task;
+	}
 
 	nr_files = 0;