[PATCH v2] fs/proc: add VmTaskSize field to /proc/$$/status

[Joel Savitz <jsavitz@redhat.com>]

In the mainline kernel, there is no quick mechanism to get the virtual
memory size of the current process from userspace.

Despite the current state of affairs, this information is available to the
user through several means, one being a linear search of the entire address
space. This is an inefficient use of cpu cycles.

A component of the libhugetlb kernel test does exactly this, and as
systems' address spaces increase beyond 32-bits, this method becomes
exceedingly tedious.

For example, on a ppc64le system with a 47-bit address space, the linear
search causes the test to hang for some unknown amount of time. I
couldn't give you an exact number because I just ran it for about 10-20
minutes and went to go do something else, probably to get coffee or
something, and when I came back, I just killed the test and patched it
to use this new mechanism. I re-ran my new version of the test using a
kernel with this patch, and of course it passed through the previously
bottlenecking codepath nearly instantaneously.

This patched enabled me to upgrade an O(n) codepath to O(1) in an
architecture-independent manner.

Signed-off-by: Joel Savitz <jsavitz@redhat.com>
---
 Documentation/filesystems/proc.txt | 2 ++
 fs/proc/task_mmu.c                 | 5 ++++-
 2 files changed, 6 insertions(+), 1 deletion(-)

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                  size of entire virtual address space of a process
  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..0ddd51479f90 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -74,7 +74,10 @@ 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_puts(m, " kB\n");
+	SEQ_PUT_DEC(" kB\nVmSwap:\t", swap);
+	seq_put_decimal_ull_width(m,
+		    " kB\nVmTaskSize:\t", TASK_SIZE >> 10, 8);
+	seq_puts(m, " kB\n");
 	hugetlb_report_usage(m, mm);
 }
 #undef SEQ_PUT_DEC
-- 
2.18.1