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

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Date:   Tue, 4 Feb 2020 14:38:51 +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 04.02.20 14:13, Segher Boessenkool wrote:
> On Tue, Feb 04, 2020 at 01:41:06PM +0100, David Hildenbrand wrote:
>> On 04.02.20 10:46, Oscar Salvador wrote:
>>> I have to confess that it took me while to wrap around my head
>>> with the new min() change, but looks ok:
>>
>> 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
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 ...

-- 
Thanks,

David / dhildenb