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From: Cedric Blancher <cedric.blancher@gmail.com>
Date: Sun, 25 Sep 2016 19:59:22 +0200
Message-ID: <CALXu0UfuwGM+H0YnfSNW6O=hgcUrmws+ihHLVB=OJWOp8YCwgw@mail.gmail.com>
Subject: Re: [PATCH 2/2] radix-tree: Fix optimisation problem
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <mawilcox@linuxonhyperv.com>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Andrew Morton <akpm@linux-foundation.org>, Konstantin Khlebnikov <koct9i@gmail.com>, Ross Zwisler <ross.zwisler@linux.intel.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Linux MM <linux-mm@kvack.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Matthew Wilcox <mawilcox@microsoft.com>

On 25 September 2016 at 02:18, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Sat, Sep 24, 2016 at 4:35 PM, Cedric Blancher
> <cedric.blancher@gmail.com> wrote:
>>>
>>>         void *entry = parent->slots[offset];
>>>         int siboff = entry - parent->slots;
>>
>> If entry is a pointer to void, how can you do pointer arithmetic with it?
>
> It's actually void **.
>
> (That said, gcc has an extension that considers "void *" to be a byte
> pointer, so you can actually do arithmetic on them, and it acts like
> "char *")
>
>> Also, if you use pointer distances, the use of int is not valid, it
>> should then be ptrdiff_t siboff.
>
> The use of "int" is perfectly valid, since it's limited by
> RADIX_TREE_MAP_SIZE, so it's going to be a small integer.

A specific data type would be wise (aka radtr_mapsz_t) to prevent a
disaster as SystemV had early during development. It took AT&T TWO
fucking months to figure out that their avl tree implementation had a
small type problem with int vs long.
Since I'd expect no one cares I'm going to print this email so I can
send the scan as PDF each time you hit that problem in the future with
"told you so"


Ced
-- 
Cedric Blancher <cedric.blancher@gmail.com>
[https://plus.google.com/u/0/+CedricBlancher/]
Institute Pasteur

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