linux-kernel - Re: mlock() confusing 1 half of system RAM limit

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Date:   Fri, 1 Jun 2018 13:26:59 +0100
From:   Alex Richman <alex.r@...abs.co.uk>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-man@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: mlock() confusing 1 half of system RAM limit

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@...abs.co.uk
Engineering
GB Labs
2 Orpheus House,
Calleva park,
Reading
RG7 8TA
Tel:+44 (0)118 455 5000
www.gblabs.com



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