Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230728120054.12306-1-fmdefrancesco@gmail.com>
In-Reply-To: <20230728120054.12306-1-fmdefrancesco@gmail.com>
From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Mon, 7 Aug 2023 11:40:30 +0200
Message-ID: <CACRpkdYcAWDZV0GkdP6gEJv971wjYQ-8WweHkreTJ-w7sfKKQg@mail.gmail.com>
Subject: Re: [PATCH] Documentation/page_tables: Add info about MMU/TLB and
 Page Faults
To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Mike Rapoport <rppt@kernel.org>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

Hi Fabio,

I'm back from vacation! Overall this documentation looks good and
in line with the reset in this section.

On Fri, Jul 28, 2023 at 2:01=E2=80=AFPM Fabio M. De Francesco
<fmdefrancesco@gmail.com> wrote:

> +One cause of page faults is due to bugs (or maliciously crafted addresse=
s) and
> +happens when a process tries to access a range of memory that it doesn't=
 have
> +permission to. This could be because the memory is reserved for the kern=
el or
> +for another process, or because the process is trying to write to a read=
-only
> +section of memory. When this happens, the kernel sends a Segmentation Fa=
ult
> +(SIGSEGV) signal to the process, which usually causes the process to ter=
minate.

This "segmentation fault" (SIGSEGV reads "signal segmentation violation)
is actually a bit hard to understand for people not familiar
with the 1970ies hardware. The Wikipedia tries to explain it but gets a bit
long and confusing.
https://en.wikipedia.org/wiki/Segmentation_fault

The computers where the first Unix was developed (PDP machines) simply
named its MMU the "memory segmentation unit" so "segmentation fault"
is just a 1970ies way of saying "MMU access violation", which stuck inside
Unix and thus inside Linux. Here is the explanation:
https://wfjm.github.io/blogs/w11/2022-08-18-on-segments-and-pages.html

The binary loader would generously use the plentyful virtual memory
"segments" to split each executable into three segments when loading the bi=
nary,
still reflected in ELF binaries to this day:
https://en.wikipedia.org/wiki/Code_segment
https://en.wikipedia.org/wiki/Data_segment
https://en.wikipedia.org/wiki/.bss

Then the page table got special permissions set to each segment for read/wr=
ite
etc. Other programs and the kernel memory are also in inaccessible segments=
,
so accessing any of the own segments in the wrong way, or another programs
segment, or an unmapped segment (virtual memory) would all result in the
SIGSEGV opaque message "segmentation fault"

I don't know how to reflect this in a good way in the documentation
though, maybe
copy/paste/edit some of my text or I can try to write something as an addit=
ional
patch if you prefer.

Yours,
Linus Walleij