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

[Eric B Munson <emunson@mgebm.net>]

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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.



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