linux-kernel - Re: [PATCH] mm/page_alloc: Skip non present sections on zone initialization

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Date:   Fri, 10 Jan 2020 15:48:39 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...e.de>,
        "Jin, Zhi" <zhi.jin@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] mm/page_alloc: Skip non present sections on zone
 initialization

On 10.01.20 15:47, Kirill A. Shutemov wrote:
> On Fri, Jan 10, 2020 at 03:34:49PM +0100, David Hildenbrand wrote:
>> On 10.01.20 14:45, Kirill A. Shutemov wrote:
>>> On Fri, Jan 10, 2020 at 02:15:26PM +0100, David Hildenbrand wrote:
>>>> On 08.01.20 15:40, Michal Hocko wrote:
>>>>> On Mon 30-12-19 12:38:28, Kirill A. Shutemov wrote:
>>>>>> memmap_init_zone() can be called on the ranges with holes during the
>>>>>> boot. It will skip any non-valid PFNs one-by-one. It works fine as long
>>>>>> as holes are not too big.
>>>>>>
>>>>>> But huge holes in the memory map causes a problem. It takes over 20
>>>>>> seconds to walk 32TiB hole. x86-64 with 5-level paging allows for much
>>>>>> larger holes in the memory map which would practically hang the system.
>>>>>>
>>>>>> Deferred struct page init doesn't help here. It only works on the
>>>>>> present ranges.
>>>>>>
>>>>>> Skipping non-present sections would fix the issue.
>>>>>
>>>>> Makes sense to me.
>>>>>
>>>>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
>>>>>
>>>>> That pfn inc back and forth is quite ugly TBH but whatever.
>>>>
>>>> Indeed, can we please rewrite the loop to fix that?
>>>
>>> Any suggestions?
>>>
>>> I don't see an obvious way to not break readablity in another place.
>>>
>>
>> I'd probably do it like this (applied some other tweaks, untested)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index cb766aac6772..a96b1ad1d74b 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -5859,6 +5859,22 @@ overlap_memmap_init(unsigned long zone, unsigned long *pfn)
>>         return false;
>>  }
>>  
>> +static inline __meminit unsigned long next_present_pfn(unsigned long pfn)
>> +{
>> +#ifdef CONFIG_SPARSEMEM
> 
> I would rather keep it around function, but it's matter of taste.

Yes

> 
>> +       unsigned long section_nr = pfn_to_section_nr(pfn + 1);
>> +
>> +       /*
>> +        * Note: We don't check the subsection bitmap, so this can produce
>> +        * false positives when only subsections are present/valid. The
>> +        * caller should recheck if the returned pfn is valid.
>> +        */
>> +       if (!present_section_nr(section_nr))
>> +               return section_nr_to_pfn(next_present_section_nr(section_nr));
> 
> This won't compile. next_present_section_nr() is static to mm/sparse.c.

We should then move that to the header IMHO.

-- 
Thanks,

David / dhildenb