Bug#761299: systemd: disrupts hugepages support

[Cyril Soldani <devmusings@legiasoft.com>]

On Fri, 12 Sep 2014 18:37:52 +0100
Ben Hutchings <ben@decadent.org.uk> wrote:
> > After investigations, it looks like systemd, when run as init,
> > mounts the hugepages in /dev/hugepages (IMHO, an unexpected place
> > for a mount point), before them being remounted on /mnt/huge_1GB as
> > per fstab. It looks like hugepages won't work when mounted twice.  
> [...]
> 
> Please explain 'won't work'.  I am able to create files on multiple
> hugetlbfs mounts.
> 
> I suspect that some other application may be automatically using
> hugepages in /dev/hugepages, whereas previously there was no default
> location available for it to use.

You are right.

I tried again and, indeed, our application is trying to use
/dev/hugepages instead of /mnt/huge_1GB, as it is doing
when /dev/hugepages is not present. Permissions on /dev/hugepages are
different and that causes hugepages mapping to fail.

So, there was no Linux bug here. However, it might still be nice to warn
users that have hugetlbfs entries in /etc/fstab on systemd-sysv install.

Thanks for helping clarifying that issue,
-- 
"Of all the enemies to public liberty, war is perhaps the most to be
 dreaded because it comprises and develops the germ of every other."
                                                         James Madison

Cyril Soldani <cyril.soldani@legiasoft.com>