shmem info issue

Hugh Dickins hughd at
Fri Jul 12 18:58:38 EDT 2013

On Fri, 12 Jul 2013, naveen yadav wrote:
>  Hi All
> I am working on tmpfs. During code analysis I found shmem driver
> register itself as tmpfs .
> cat /proc/meminfo Shmem field read NR_SHMEM enum filed and shows used
> memory in tmpfs
> [root at localhost linux-3.8.2]# cat /proc/meminfo | grep -r Shmem
> Shmem:               704 kB
> [root at localhost linux-3.8.2]#
> include/linux/mmzone.h
> enum zone_stat_item {
> -----cut here-------
>  NR_SHMEM,               /* shmem pages (included tmpfs/GEM pages
>  -----cut here
> }
> I have the following query
> 1-> cat /proc/meminfo | grep -r Shmem show only the used tmpfs memory
> or it show used  tmpfs memory + mata data also (because I found Shmem
> is bit larger then tempfs used memory )

"Shmem" includes only data, not metadata, nor memory currently swapped
out.  But it includes tmpfs memory, SysV shared memory (from ipc/shm.c),
POSIX shared memory (under /dev/shm), and shared anonymous mappings
(from mmap of /dev/zero with MAP_SHARED: see call to shmem_zero_setup()
from drivers/char/mem.c): whatever allocates pages through mm/shmem.c.

> 2-> as per the developer comments NR_SHMEM included tmpfs and GEM
> pages whct is Gem pages

Ah yes, and the Graphics Execution Manager uses shmem for objects shared
with the GPU: see use of shmem_read_mapping_page*() in drivers/gpu/drm/.

> 3-> is my above code analysis is correct?

I think so.


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