linux-kernel - Re: [PATCH RFC] drivers/char/mem: byte generating devices and poisoned mappings

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Message-ID: <CALYGNiONACddsH_6uaG5QyaWRaZ_5ZH0atZJyZVD=DowiKLJcw@mail.gmail.com>
Date:	Tue, 1 Apr 2014 21:27:47 +0400
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>,
	Yury Gribov <y.gribov@...sung.com>,
	Alexandr Andreev <aandreev@...allels.com>,
	Vassili Karpov <av1474@...tv.ru>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH RFC] drivers/char/mem: byte generating devices and
 poisoned mappings

On Tue, Apr 1, 2014 at 8:59 PM, One Thousand Gnomes
<gnomes@...rguk.ukuu.org.uk> wrote:
> On Tue, 01 Apr 2014 01:16:07 +0400
> Konstantin Khlebnikov <koct9i@...il.com> wrote:
>
>> This patch adds 256 virtual character devices: /dev/byte0, ..., /dev/byte255.
>> Each works like /dev/zero but generates memory filled with particular byte.
>
> More kernel code for an ultra-obscure corner case that can be done in
> user space
>
> I don't see the point

True. That was a long-planned joke.
But at the final moment I've found practical usage for it and overall
design became not such funny.

Currently I'm thinking about single-device model proposed by Kirill.

Let's call it /dev/poison. Application can open it, write a poison (up
to a page size) and after that this instance will generate pages
filled with this pattern. I don't see how this can be done in
userspace without major memory/cpu overhead caused by initial memset.

Default poison might be for example 0xff, so it still will be useful for 'dd'.
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