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

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Message-ID: <20180601114425.GA15686@dhcp22.suse.cz>
Date:   Fri, 1 Jun 2018 13:44:25 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Alex Richman <alex.r@...abs.co.uk>
Cc:     linux-man@...r.kernel.org, linux-mm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: mlock() confusing 1 half of system RAM limit

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.
-- 
Michal Hocko
SUSE Labs