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

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Message-ID: <20200205125149.GS22482@gate.crashing.org>
Date:   Wed, 5 Feb 2020 06:51:49 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     David Hildenbrand <david@...hat.com>
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 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 ;-)

> 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
(and replace 1 by any other minimum size required).  This *also* is
easy to read, without having to have any details (and quirks :-/ )
of those utility functions memorised.


Segher