linux-kernel - Re: [PATCH v6 10/10] mm/memory_hotplug: Cleanup __remove

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Message-ID: <32e06c13-222c-4486-3914-df2807cf2a2b@redhat.com>
Date:   Wed, 5 Feb 2020 14:17:47 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Oscar Salvador <osalvador@...e.de>, linux-s390@...r.kernel.org,
        Michal Hocko <mhocko@...e.com>, linux-ia64@...r.kernel.org,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        linux-sh@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Wei Yang <richardw.yang@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Dan Williams <dan.j.williams@...el.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v6 10/10] mm/memory_hotplug: Cleanup __remove_pages()

On 05.02.20 13:51, Segher Boessenkool wrote:
> On Tue, Feb 04, 2020 at 02:38:51PM +0100, David Hildenbrand wrote:
>> On 04.02.20 14:13, Segher Boessenkool wrote:
>>> On Tue, Feb 04, 2020 at 01:41:06PM +0100, David Hildenbrand wrote:
>>>> It's a pattern commonly used in compilers and emulators to calculate the
>>>> number of bytes to the next block/alignment. (we're missing a macro
>>>> (like we have ALIGN_UP/IS_ALIGNED) for that - but it's hard to come up
>>>> with a good name (e.g., SIZE_TO_NEXT_ALIGN) .
> 
>>> You can just write the easy to understand
>>>
>>>   ...  ALIGN_UP(x) - x  ...
>>
>> you mean
>>
>> ALIGN_UP(x, PAGES_PER_SECTION) - x
>>
>> but ...
>>
>>> which is better *without* having a separate name.  Does that not
>>> generate good machine code for you?
>>
>> 1. There is no ALIGN_UP. "SECTION_ALIGN_UP(x) - x" would be possible
> 
> Erm, you started it ;-)

Yeah, I was thinking in the wrong code base :)

> 
>> 2. It would be wrong if x is already aligned.
>>
>> e.g., let's use 4096 for simplicity as we all know that value by heart
>> (for both x and the block size).
>>
>> a) -(4096 | -4096) -> 4096
>>
>> b) #define ALIGN_UP(x, a) ((x + a - 1) & -(a))
>>
>> ALIGN_UP(4096, 4096) - 4096 -> 0
>>
>> Not as easy as it seems ...
> 
> If you always want to return a number >= 1, it it simply
>   ALIGN_UP(x + 1) - x


I'm sorry to have to correct you again for some corner cases:

ALIGN_UP(1, 4096) - 4096 = 0

Again, not as easy as it seems ...

-- 
Thanks,

David / dhildenb