linux-kernel - Re: [PATCH 2/2] radix-tree: Fix optimisation problem

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Message-ID: <CA+55aFyRG=us-EKnomo=QPE0GR1Qdxyw1Ozmuzw0EJcSr7U3hQ@mail.gmail.com>
Date:   Sat, 24 Sep 2016 17:18:28 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Cedric Blancher <cedric.blancher@...il.com>
Cc:     Matthew Wilcox <mawilcox@...uxonhyperv.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Matthew Wilcox <mawilcox@...rosoft.com>
Subject: Re: [PATCH 2/2] radix-tree: Fix optimisation problem

On Sat, Sep 24, 2016 at 4:35 PM, Cedric Blancher
<cedric.blancher@...il.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.

             Linus