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

[Michal Hocko <mhocko@suse.cz>]

On Tue 04-01-11 05:21:46, CAI Qian wrote:
> 
> > > 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).
> That is strange - the test code merely did this,
> addr = mmap(ADDR, 2<<30, PROTECTION, FLAGS, fd, 0);

Didn't you want 1<<30 instead?

> 
> Do you know if overcommit was designed for 1GB pages? At least, read this
> from Documentation/kernel-parameters.txt,
> 
> hugepagesz=
>               ...
>              Note that 1GB pages can only be allocated at boot time
>              using hugepages= and not freed afterwards.
> 
> How does it allow to be overcommitted for only being able to allocate at
> boot time?

Sorry, I am not very much familiar with 1GB pages but the hugetlb code
is not page size specific AFAICS so if there are no other background
things than it should just work.

> 
> > > 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.
> I could double-check here, but it is not important if the fact is that
> overcommit is not supported for 1GB pages.

What is the complete test case?

> 
> > Btw. which kernel version are you using.
> mmotm 2010-12-02-16-34 version 2.6.37-rc4-mm1+. This problem is also present
> in 2.6.18.
> 
> Thanks.
> 
> CAI Qian
> 
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-- 
Michal Hocko
L3 team 
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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