Re: [PATCH -mm] procfs: add VmFlags field in smaps output v3

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 24 Oct 2012 16:27:30 +0400
Cyrill Gorcunov <gorcunov@openvz.org> wrote:

> During c/r sessions we've found that there is no way at the moment to
> fetch some VMA associated flags, such as mlock() and madvise().
> 
> This leads us to a problem -- we don't know if we should call for mlock()
> and/or madvise() after restore on the vma area we're bringing back to
> life.
> 
> This patch intorduces a new field into "smaps" output called VmFlags,
> where all set flags associated with the particular VMA is shown as two
> letter mnemonics.
> 
> [ Strictly speaking for c/r we only need mlock/madvise bits but it has been
>   said that providing just a few flags looks somehow inconsistent.  So all
>   flags are here now. ]
> 
> This feature is made available on CONFIG_CHECKPOINT_RESTORE=n kernels, as
> other applications may start to use these fields.
> 
> The data is encoded in a somewhat awkward two letters mnemonic form, to
> encourage userspace to be prepared for fields being added or removed in
> the future.
> 
> ...
>
> +	for_each_set_bit(i, &vma->vm_flags, BITS_PER_LONG) {

for_each_set_bit() seems to be rather sucky.  Going back to 

--- a/fs/proc/task_mmu.c~a-fix
+++ a/fs/proc/task_mmu.c
@@ -568,10 +568,11 @@ static void show_smap_vma_flags(struct s
 	size_t i;
 
 	seq_puts(m, "VmFlags: ");
-	for_each_set_bit(i, &vma->vm_flags, BITS_PER_LONG) {
-		seq_printf(m, "%c%c ",
-			   mnemonics[i][0],
-			   mnemonics[i][1]);
+	for (i = 0; i < BITS_PER_LONG; i++) {
+		if (vma->vm_flags & (1UL << i)) {
+			seq_printf(m, "%c%c ",
+				   mnemonics[i][0], mnemonics[i][1]);
+		}
 	}
 	seq_putc(m, '\n');
 }

saves 41 bytes.  That's rather a lot for such a small code sequence.