proc: revert /proc/<pid>/maps [stack:TID] annotation

Commit b76437579d13 ("procfs: mark thread stack correctly in
proc/<pid>/maps") added [stack:TID] annotation to /proc/<pid>/maps.

Finding the task of a stack VMA requires walking the entire thread list,
turning this into quadratic behavior: a thousand threads means a
thousand stacks, so the rendering of /proc/<pid>/maps needs to look at a
million combinations.

The cost is not in proportion to the usefulness as described in the
patch.

Drop the [stack:TID] annotation to make /proc/<pid>/maps (and
/proc/<pid>/numa_maps) usable again for higher thread counts.

The [stack] annotation inside /proc/<pid>/task/<tid>/maps is retained, as
identifying the stack VMA there is an O(1) operation.

Siddesh said:
 "The end users needed a way to identify thread stacks programmatically and
  there wasn't a way to do that.  I'm afraid I no longer remember (or have
  access to the resources that would aid my memory since I changed
  employers) the details of their requirement.  However, I did do this on my
  own time because I thought it was an interesting project for me and nobody
  really gave any feedback then as to its utility, so as far as I am
  concerned you could roll back the main thread maps information since the
  information is available in the thread-specific files"

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
Cc: Shaohua Li <shli@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 43 insertions, 72 deletions

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 4a0c31f904a6..fa95ab2d3674 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -259,23 +259,29 @@ static int do_maps_open(struct inode *inode, struct file *file,
                                 sizeof(struct proc_maps_private));
 }
 
-static pid_t pid_of_stack(struct proc_maps_private *priv,
-                                struct vm_area_struct *vma, bool is_pid)
+/*
+ * Indicate if the VMA is a stack for the given task; for
+ * /proc/PID/maps that is the stack of the main task.
+ */
+static int is_stack(struct proc_maps_private *priv,
+                    struct vm_area_struct *vma, int is_pid)
 {
-        struct inode *inode = priv->inode;
-        struct task_struct *task;
-        pid_t ret = 0;
+        int stack = 0;
+
+        if (is_pid) {
+                stack = vma->vm_start <= vma->vm_mm->start_stack &&
+                        vma->vm_end >= vma->vm_mm->start_stack;
+        } else {
+                struct inode *inode = priv->inode;
+                struct task_struct *task;
 
-        rcu_read_lock();
-        task = pid_task(proc_pid(inode), PIDTYPE_PID);
-        if (task) {
-                task = task_of_stack(task, vma, is_pid);
+                rcu_read_lock();
+                task = pid_task(proc_pid(inode), PIDTYPE_PID);
                 if (task)
-                        ret = task_pid_nr_ns(task, inode->i_sb->s_fs_info);
+                        stack = vma_is_stack_for_task(vma, task);
+                rcu_read_unlock();
         }
-        rcu_read_unlock();
-
-        return ret;
+        return stack;
 }
 
 static void
@@ -335,8 +341,6 @@ show_map_vma(struct seq_file *m, struct vm_area_struct *vma, int is_pid)
 
         name = arch_vma_name(vma);
         if (!name) {
-                pid_t tid;
-
                 if (!mm) {
                         name = "[vdso]";
                         goto done;
@@ -348,21 +352,8 @@ show_map_vma(struct seq_file *m, struct vm_area_struct *vma, int is_pid)
                         goto done;
                 }
 
-                tid = pid_of_stack(priv, vma, is_pid);
-                if (tid != 0) {
-                        /*
-                         * Thread stack in /proc/PID/task/TID/maps or
-                         * the main process stack.
-                         */
-                        if (!is_pid || (vma->vm_start <= mm->start_stack &&
-                            vma->vm_end >= mm->start_stack)) {
-                                name = "[stack]";
-                        } else {
-                                /* Thread stack in /proc/PID/maps */
-                                seq_pad(m, ' ');
-                                seq_printf(m, "[stack:%d]", tid);
-                        }
-                }
+                if (is_stack(priv, vma, is_pid))
+                        name = "[stack]";
         }
 
 done:
@@ -1618,19 +1609,8 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
                 seq_file_path(m, file, "\n\t= ");
         } else if (vma->vm_start <= mm->brk && vma->vm_end >= mm->start_brk) {
                 seq_puts(m, " heap");
-        } else {
-                pid_t tid = pid_of_stack(proc_priv, vma, is_pid);
-                if (tid != 0) {
-                        /*
-                         * Thread stack in /proc/PID/task/TID/maps or
-                         * the main process stack.
-                         */
-                        if (!is_pid || (vma->vm_start <= mm->start_stack &&
-                            vma->vm_end >= mm->start_stack))
-                                seq_puts(m, " stack");
-                        else
-                                seq_printf(m, " stack:%d", tid);
-                }
+        } else if (is_stack(proc_priv, vma, is_pid)) {
+                seq_puts(m, " stack");
         }
 
         if (is_vm_hugetlb_page(vma))
diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index e0d64c92e4f6..faacb0c0d857 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -123,23 +123,26 @@ unsigned long task_statm(struct mm_struct *mm,
         return size;
 }
 
-static pid_t pid_of_stack(struct proc_maps_private *priv,
-                                struct vm_area_struct *vma, bool is_pid)
+static int is_stack(struct proc_maps_private *priv,
+                    struct vm_area_struct *vma, int is_pid)
 {
-        struct inode *inode = priv->inode;
-        struct task_struct *task;
-        pid_t ret = 0;
-
-        rcu_read_lock();
-        task = pid_task(proc_pid(inode), PIDTYPE_PID);
-        if (task) {
-                task = task_of_stack(task, vma, is_pid);
+        struct mm_struct *mm = vma->vm_mm;
+        int stack = 0;
+
+        if (is_pid) {
+                stack = vma->vm_start <= mm->start_stack &&
+                        vma->vm_end >= mm->start_stack;
+        } else {
+                struct inode *inode = priv->inode;
+                struct task_struct *task;
+
+                rcu_read_lock();
+                task = pid_task(proc_pid(inode), PIDTYPE_PID);
                 if (task)
-                        ret = task_pid_nr_ns(task, inode->i_sb->s_fs_info);
+                        stack = vma_is_stack_for_task(vma, task);
+                rcu_read_unlock();
         }
-        rcu_read_unlock();
-
-        return ret;
+        return stack;
 }
 
 /*
@@ -181,21 +184,9 @@ static int nommu_vma_show(struct seq_file *m, struct vm_area_struct *vma,
         if (file) {
                 seq_pad(m, ' ');
                 seq_file_path(m, file, "");
-        } else if (mm) {
-                pid_t tid = pid_of_stack(priv, vma, is_pid);
-
-                if (tid != 0) {
-                        seq_pad(m, ' ');
-                        /*
-                         * Thread stack in /proc/PID/task/TID/maps or
-                         * the main process stack.
-                         */
-                        if (!is_pid || (vma->vm_start <= mm->start_stack &&
-                            vma->vm_end >= mm->start_stack))
-                                seq_printf(m, "[stack]");
-                        else
-                                seq_printf(m, "[stack:%d]", tid);
-                }
+        } else if (mm && is_stack(priv, vma, is_pid)) {
+                seq_pad(m, ' ');
+                seq_printf(m, "[stack]");
         }
 
         seq_putc(m, '\n');