linux-kernel - Re: [PATCH] hugetlb: Add nohugepages parameter to prevent hugepages creation

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Message-ID: <92e8451a-3a64-cde9-a879-10c0c9e857aa@canonical.com>
Date:   Tue, 15 Oct 2019 01:50:07 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...onical.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        jay.vosburgh@...onical.com, kernel@...ccoli.net
Subject: Re: [PATCH] hugetlb: Add nohugepages parameter to prevent hugepages
 creation

On 14/10/2019 15:25, Mike Kravetz wrote:
>  [...]
> I don't know much about early_param(), so I will assume this works as you
> describe.  However, a quick grep shows hugepage options for ia64 also with
> early_param.
> 

Thanks a lot for the prompt and quite informative reply Mike.
I've checked this IA64 parameter after your mention, and it just sets
the hugepages size, I don't think it'll affect the purpose of this patch.


>> * The return when sysctl handler is prevented to progress due to
>> nohugepages is -EINVAL, but could be changed; I've just followed
>> present code there, but I'm OK changing that if we have suggestions.
> 
> It looks like you only have short circuited/prevented nr_hugepages via
> sysfs/sysctl.  Theoretically, one could set nr_overcommit_hugepages and
> still allocate hugetlb pages.  So, if you REALLY want to shut things down
> you need to stop this as well.
> 
> There is already a macro hugepages_supported() that can be set by arch
> specific code.  I wonder how difficult it would be to 'overwrite' the
> macro if nohugepages is specified.  Perhaps just a level of naming
> indirection.  This would use the existing code to prevent all hugetlb usage.
>

Outstanding! It's a much better idea to use hugepages_supported()
infrastructure, it prevents even the creation of hugepages-related sysfs
entries; I've worked a V2 with this modification, and it worked fine,
thanks for the suggestion.


> It seems like there may be some discussion about 'the right' way to
> do kdump.  I can't add to that discussion, but if such an option as
> nohugepages is needed, I can help.
> 

I think this parameter may be important/useful not only for kdump - it
is a legitimate way of disabling hugepages, something we don't currently
have on kernel. I'll submit a V2, for Ubuntu kdump specially this is
quite helpful!

Cheers,


Guilherme