[PATCH v3] fs/proc: add VmTaskSize field to /proc/$$/status

[Joel Savitz <jsavitz@redhat.com>]

There is currently no easy and architecture-independent way to find the
lowest unusable virtual address available to a process without
brute-force calculation. This patch allows a user to easily retrieve
this value via /proc/<pid>/status.

Using this patch, any program that previously needed to waste cpu cycles
recalculating a non-sensitive process-dependent value already known to
the kernel can now be optimized to use this mechanism.

Signed-off-by: Joel Savitz <jsavitz@redhat.com>
---
 Documentation/filesystems/proc.txt | 2 ++
 fs/proc/task_mmu.c                 | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 66cad5c86171..1c6a912e3975 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -187,6 +187,7 @@ read the file /proc/PID/status:
   VmLib:      1412 kB
   VmPTE:        20 kb
   VmSwap:        0 kB
+  VmTaskSize:	137438953468 kB
   HugetlbPages:          0 kB
   CoreDumping:    0
   THP_enabled:	  1
@@ -263,6 +264,7 @@ Table 1-2: Contents of the status files (as of 4.19)
  VmPTE                       size of page table entries
  VmSwap                      amount of swap used by anonymous private data
                              (shmem swap usage is not included)
+ VmTaskSize                  lowest unusable address in process virtual memory
  HugetlbPages                size of hugetlb memory portions
  CoreDumping                 process's memory is currently being dumped
                              (killing the process may lead to a corrupted core)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 95ca1fe7283c..0af7081f7b19 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -74,6 +74,8 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
 	seq_put_decimal_ull_width(m,
 		    " kB\nVmPTE:\t", mm_pgtables_bytes(mm) >> 10, 8);
 	SEQ_PUT_DEC(" kB\nVmSwap:\t", swap);
+	seq_put_decimal_ull_width(m,
+		    " kB\nVmTaskSize:\t", mm->task_size >> 10, 8);
 	seq_puts(m, " kB\n");
 	hugetlb_report_usage(m, mm);
 }
-- 
2.18.1