Re: [RFC] /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_overcommit_hugepages

On Fri 31-12-10 06:08:21, CAI Qian wrote:

Hi,

Hi,

Problem: nr_overcommit_hugepages for 1gb hugepage went crazy.

Symptom:
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)

Hmm, you are trying to reserve/mmap a lot of memory (17179869182 1GB huge
pages).

# cat /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_overcommit_hugepages
18446744071589420672

As you can see from the above, it did not allow overcommit despite nr_overcommit_hugepages value.

You are trying to allocate much more than your overcommit allows.

Also, nr_overcommit_hugepages was overwritten with such a strange
value after overcommit failure. Should we just remove this file from
sysfs for simplicity?

This is strange. The value is set only in hugetlb_overcommit_handler
which is a sysctl handler.

Are you sure that you are not changing the value by the /sys interface
somewhere (there is no check for the value so you can set what-ever
value you like)? I fail to see any mmap code path which would change
this value.

Btw. which kernel version are you using.

Thanks.

CAI Qian

Regards
-- 
Michal Hocko
L3 team 
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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