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Subject: Re: [RFC PATCH 12/15] mm: Make MAX_ORDER of buddy allocator
 configurable via Kconfig SET_MAX_ORDER.
To:     Zi Yan <ziy@nvidia.com>, David Hildenbrand <david@redhat.com>,
        linux-mm@kvack.org
Cc:     Matthew Wilcox <willy@infradead.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Michal Hocko <mhocko@kernel.org>,
        John Hubbard <jhubbard@nvidia.com>,
        linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>
References: <20210805190253.2795604-1-zi.yan@sent.com>
 <20210805190253.2795604-13-zi.yan@sent.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <1a5c17ac-7f63-6aae-b83b-4438bfa6fe34@suse.cz>
Date:   Fri, 6 Aug 2021 17:16:59 +0200
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On 8/5/21 9:02 PM, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
> +config SET_MAX_ORDER
> +	int "Set maximum order of buddy allocator"
> +    depends on SPARSEMEM_VMEMMAP && (ARCH_FORCE_MAX_ORDER = 0)
> +	range 11 255
> +	default "11"
> +	help
> +	  The kernel memory allocator divides physically contiguous memory
> +	  blocks into "zones", where each zone is a power of two number of
> +	  pages.  This option selects the largest power of two that the kernel
> +	  keeps in the memory allocator.  If you need to allocate very large
> +	  blocks of physically contiguous memory, then you may need to
> +	  increase this value.
> +
> +	  This config option is actually maximum order plus one. For example,
> +	  a value of 11 means that the largest free memory block is 2^10 pages.

It's enough that it's confusing for the devs, we could spare the users and add
+1 to the value they specify :)

>  config HAVE_MEMBLOCK_PHYS_MAP
>  	bool
>  
>