Re: [PATCH 10/13] mm: Wire up MAP_SYNC

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Thu, Aug 17, 2017 at 06:08:12PM +0200, Jan Kara wrote:
> Pretty crude for now...
> 
> Signed-off-by: Jan Kara <jack@suse.cz>

One other thing that should probably be wired up before this is all said and
done is the VmFlag string in /proc/<pid>/smaps.  Right now when we set this
flag it ends up as ??:

7f44e6cbd000-7f44e6dbd000 rw-s 00000000 103:00 12              /root/dax/data
Size:               1024 kB
Rss:                   0 kB
Pss:                   0 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         0 kB
Referenced:            0 kB
Anonymous:             0 kB
LazyFree:              0 kB
AnonHugePages:         0 kB
ShmemPmdMapped:        0 kB
Shared_Hugetlb:        0 kB
Private_Hugetlb:       0 kB
Swap:                  0 kB
SwapPss:               0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Locked:                0 kB
VmFlags: rd wr sh mr mw me ms ?? mm hg 

The quick one-liner at the end of this patch changes that to:

7fe30e87f000-7fe30e97f000 rw-s 00000000 103:00 12               /root/dax/data
Size:               1024 kB
Rss:                   0 kB
Pss:                   0 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         0 kB
Referenced:            0 kB
Anonymous:             0 kB
LazyFree:              0 kB
AnonHugePages:         0 kB
ShmemPmdMapped:        0 kB
Shared_Hugetlb:        0 kB
Private_Hugetlb:       0 kB
Swap:                  0 kB
SwapPss:               0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Locked:                0 kB
VmFlags: rd wr sh mr mw me ms sf mm hg

I think that of software can rely on this flag for userspace flushing without
worrying about any new TOCTOU races because there isn't a way to unset the
VM_SYNC flag once it is set - it should be valid as long as the mmap() remains
open and the mmap'd address is valid.

--- 8< ---
 fs/proc/task_mmu.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index b836fd6..a2a82ed 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -650,6 +650,7 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
 		[ilog2(VM_ACCOUNT)]	= "ac",
 		[ilog2(VM_NORESERVE)]	= "nr",
 		[ilog2(VM_HUGETLB)]	= "ht",
+		[ilog2(VM_SYNC)]	= "sf",
 		[ilog2(VM_ARCH_1)]	= "ar",
 		[ilog2(VM_DONTDUMP)]	= "dd",
 #ifdef CONFIG_MEM_SOFT_DIRTY