Re: [PATCH] hugetlb: remove overcommit sysfs for 1GB pages

[CAI Qian <caiqian@redhat.com>]

----- Original Message -----
> On Tue, 04 Jan 2011, CAI Qian wrote:
> 
> > 1GB pages cannot be over-commited, attempting to do so results in
> > corruption,
> > so remove those files for simplicity.
> >
> > Symptoms:
> > 1) setup 1gb hugepages.
> >
> > cat /proc/cmdline
> > ...default_hugepagesz=1g hugepagesz=1g hugepages=1...
> >
> > cat /proc/meminfo
> > ...
> > HugePages_Total: 1
> > HugePages_Free: 1
> > HugePages_Rsvd: 0
> > HugePages_Surp: 0
> > Hugepagesize: 1048576 kB
> > ...
> >
> > 2) set nr_overcommit_hugepages
> >
> > echo 1
> > >/sys/kernel/mm/hugepages/hugepages-1048576kB/nr_overcommit_hugepages
> > cat
> > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_overcommit_hugepages
> > 1
> >
> > 3) overcommit 2gb hugepages.
> >
> > mmap(NULL, 18446744071562067968, PROT_READ|PROT_WRITE, MAP_SHARED,
> > 3,
> > 	   0) = -1 ENOMEM (Cannot allocate memory)
> >
> > cat
> > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_overcommit_hugepages
> > 18446744071589420672
> >
> > Signed-off-by: CAI Qian <caiqian@redhat.com>
> 
> There are a couple of issues here: first, I think the overcommit value
> being overwritten
> is a bug and this needs to be addressed and fixed before we cover it
> by removing the sysfs
> file.
> 
> Second, will it be easier for userspace to work with some huge page
> sizes having the
> overcommit file and others not or making the kernel hand EINVAL back
> when nr_overcommit is
> is changed for an unsupported page size?
> 
> Finally, this is a problem for more than 1GB pages on x86_64. It is
> true for all pages >
> 1 << MAX_ORDER. Once the overcommit bug is fixed and the second issue
> is answered, the
> solution that is used (either EINVAL or no overcommit file) needs to
> happen for all cases
> where it applies, not just the 1GB case.
I have a new patch ready to return EINVAL for both sysfs/procfs, and will
reject changing of nr_hugepages. Do you know if nr_hugepages_mempolicy
is supposed to be able to change in this case? It is not possible currently.

# cat /proc/sys/vm/nr_hugepages_mempolicy
1
# echo 0 >/proc/sys/vm/nr_hugepages_mempolicy 
# cat /proc/sys/vm/nr_hugepages_mempolicy
1

Thanks.

CAI Qian

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