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Subject: Re: [PATCH v5 1/5] mm/sparse: abstract sparse buffer allocations
To:     Oscar Salvador <osalvador@techadventures.net>
Cc:     steven.sistare@oracle.com, daniel.m.jordan@oracle.com,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        kirill.shutemov@linux.intel.com, mhocko@suse.com,
        linux-mm@kvack.org, dan.j.williams@intel.com, jack@suse.cz,
        jglisse@redhat.com, jrdr.linux@gmail.com, bhe@redhat.com,
        gregkh@linuxfoundation.org, vbabka@suse.cz,
        richard.weiyang@gmail.com, dave.hansen@intel.com,
        rientjes@google.com, mingo@kernel.org, abdhalee@linux.vnet.ibm.com,
        mpe@ellerman.id.au
References: <20180712203730.8703-1-pasha.tatashin@oracle.com>
 <20180712203730.8703-2-pasha.tatashin@oracle.com>
 <20180713131749.GA16765@techadventures.net>
From:   Pavel Tatashin <pasha.tatashin@oracle.com>
Message-ID: <23f6e4e5-6e32-faf6-433d-67e50d2895a2@oracle.com>
Date:   Fri, 13 Jul 2018 09:24:44 -0400
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On 07/13/2018 09:17 AM, Oscar Salvador wrote:
> On Thu, Jul 12, 2018 at 04:37:26PM -0400, Pavel Tatashin wrote:
>> +static void *sparsemap_buf __meminitdata;
>> +static void *sparsemap_buf_end __meminitdata;
>> +
>> +void __init sparse_buffer_init(unsigned long size, int nid)
>> +{
>> +	BUG_ON(sparsemap_buf);
> 
> Why do we need a BUG_ON() here?
> Looking at the code I cannot really see how we can end up with sparsemap_buf being NULL.
> Is it just for over-protection?

This checks that we do not accidentally leak memory by calling sparse_buffer_init() consequently without sparse_buffer_fini() in-between.

> 
>> +	sparsemap_buf =
>> +		memblock_virt_alloc_try_nid_raw(size, PAGE_SIZE,
>> +						__pa(MAX_DMA_ADDRESS),
>> +						BOOTMEM_ALLOC_ACCESSIBLE, nid);
> 
> In your previous version, you didn't pass a required alignment when setting up sparsemap_buf.
> size is already PMD_SIZE aligned, do we need to align it also to PAGE_SIZE?
> 

I decided to add PAGE_SIZE alignment, because the implicit memblock alignment is SMP_CACHE_BYTES which is smaller than page size. While, in practice we will most likely get a page size aligned allocation, it is still possible that some ranges in memblock are not page size aligned if that the way they were passed from BIOS.

Thank you,
Pavel