linux-kernel - Re: [PATCH] mm/hugetlb: Fix incorrect proc nr

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Date:	Wed, 17 Feb 2016 11:59:35 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Vaishali Thakkar <vaishali.thakkar@...cle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	n-horiguchi@...jp.nec.com, mike.kravetz@...cle.com,
	hillf.zj@...baba-inc.com, kirill.shutemov@...ux.intel.com,
	dave.hansen@...ux.intel.com, paul.gortmaker@...driver.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: Fix incorrect proc nr_hugepages value

On Wed, Feb 17, 2016 at 01:13:26AM +0530, Vaishali Thakkar wrote:
> Currently incorrect default hugepage pool size is reported by proc
> nr_hugepages when number of pages for the default huge page size is
> specified twice.
> 
> When multiple huge page sizes are supported, /proc/sys/vm/nr_hugepages
> indicates the current number of pre-allocated huge pages of the default
> size. Basically /proc/sys/vm/nr_hugepages displays default_hstate->
> max_huge_pages and after boot time pre-allocation, max_huge_pages should
> equal the number of pre-allocated pages (nr_hugepages).
> 
> Test case:
> 
> Note that this is specific to x86 architecture.
> 
> Boot the kernel with command line option 'default_hugepagesz=1G
> hugepages=X hugepagesz=2M hugepages=Y hugepagesz=1G hugepages=Z'. After
> boot, 'cat /proc/sys/vm/nr_hugepages' and 'sysctl -a | grep hugepages'
> returns the value X.  However, dmesg output shows that Z huge pages were
> pre-allocated.
> 
> So, the root cause of the problem here is that the global variable
> default_hstate_max_huge_pages is set if a default huge page size is
> specified (directly or indirectly) on the command line. After the
> command line processing in hugetlb_init, if default_hstate_max_huge_pages
> is set, the value is assigned to default_hstae.max_huge_pages. However,
> default_hstate.max_huge_pages may have already been set based on the
> number of pre-allocated huge pages of default_hstate size.
> 
> The solution to this problem is if hstate->max_huge_pages is already set
> then it should not set as a result of global max_huge_pages value.
> Basically if the value of the variable hugepages is set multiple times
> on a command line for a specific supported hugepagesize then proc layer
> should consider the last specified value.
> 
> Signed-off-by: Vaishali Thakkar <vaishali.thakkar@...cle.com>
> ---
> The patch contains one line over 80 characters as I think limiting that
> line to 80 characters makes code look bit ugly. But if anyone is having
> issue with that then I am fine with limiting it to 80 chracters.

What about this?

	if (default_hstate_max_huge_pages && !default_hstate.max_huge_pages)
		default_hstate.max_huge_pages =	default_hstate_max_huge_pages;
> ---
>  mm/hugetlb.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 06ae13e..01f2b48 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2630,8 +2630,10 @@ static int __init hugetlb_init(void)
>  			hugetlb_add_hstate(HUGETLB_PAGE_ORDER);
>  	}
>  	default_hstate_idx = hstate_index(size_to_hstate(default_hstate_size));
> -	if (default_hstate_max_huge_pages)
> -		default_hstate.max_huge_pages = default_hstate_max_huge_pages;
> +	if (default_hstate_max_huge_pages) {
> +		if (!default_hstate.max_huge_pages)
> +			default_hstate.max_huge_pages = default_hstate_max_huge_pages;
> +	}
>  
>  	hugetlb_init_hstates();
>  	gather_bootmem_prealloc();
> -- 
> 2.1.4
> 
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-- 
 Kirill A. Shutemov