Re: mlock() confusing 1 half of system RAM limit
From: Alex Richman
Date: Fri Jun 01 2018 - 08:26:15 EST
I am using a shm MAP_SHARED, along these lines:
> shm_fd = shm_open(handle, (O_RDWR | O_CREAT), (S_IRWXU | S_IRWXG |
S_IRWXO));
> ftruncate(shm_fd, channel->sled_size)
> channel->sled = mmap(NULL, channel->sled_size, (PROT_READ |
PROT_WRITE), (MAP_SHARED | MAP_NORESERVE), shm_fd, 0);
> mlock(channel->sled, channel->sled_size) /* Fails with ENOMEM. */
But shmmax is unlimited on my box:
# sysctl -a | grep shm
kernel.shm_next_id = -1
kernel.shm_rmid_forced = 0
kernel.shmall = 18446744073692774399
kernel.shmmax = 18446744073692774399
kernel.shmmni = 4096
Any ideas?
Many thanks,
- Alex.
On 01/06/18 12:44, Michal Hocko wrote:
On Fri 01-06-18 10:53:57, Alex Richman wrote:
Hi,
I'm seeing a 1/2 of system RAM limit when calling mlock(). mlock(2) says:
ENOMEM (Linux 2.4 and earlier) the calling process tried to lock more than
half of RAM.
Which implies to me that the 1 half of system RAM limit should be only Linux
2.4 and earlier, but I'm running linux 4.4.127 and still seeing the limit.
This is on a system with 66007168 KB total memory, locking 33794621440 bytes
works, locking 33795653632 bytes does not work.
I assume you are doing mlock on MAP_SHARED resp. on mmap of shmem
file. You are likely to hit the shmem limit. This is half of the
available memory by default.
--
Alex Richman
alex.r@xxxxxxxxxxxx
Engineering
GB Labs
2 Orpheus House,
Calleva park,
Reading
RG7 8TA
Tel:+44 (0)118 455 5000
www.gblabs.com
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