linux-kernel - Re: [PATCH] arm: remove [_text, _stext) from kernel code resource

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Date:   Wed, 8 Dec 2021 18:22:37 +0800
From:   Mark-PK Tsai <mark-pk.tsai@...iatek.com>
To:     <rppt@...nel.org>
CC:     <akpm@...ux-foundation.org>, <arnd@...db.de>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>, <linux@...linux.org.uk>,
        <linux@...ck-us.net>, <mark-pk.tsai@...iatek.com>,
        <matthias.bgg@...il.com>, <rmk+kernel@...linux.org.uk>,
        <wangkefeng.wang@...wei.com>, <yj.chiang@...iatek.com>
Subject: Re: [PATCH] arm: remove [_text, _stext) from kernel code resource

> On Wed, Dec 08, 2021 at 10:26:23AM +0800, Mark-PK Tsai wrote:
> > > On Fri, Dec 03, 2021 at 10:14:16PM +0800, Mark-PK Tsai wrote:
> > > > Remove the [_text, _stext) from Kernel Code.
> > > > Although there are some startup code in head.text, they
> > > > are freed to the buddy system after kernel boot.
> > >
> > > Hmm, I don't see it is being freed anywhere. Can you elaborate when and how
> > > the range [_text, _stext) is freed?
> > 
> > arm_memblock_init() reserve [KERNEL_START, KERNEL_END) which are defined as following.
> > 
> > #define KERNEL_START            _stext
> > #define KERNEL_END              _end
> > 
> > free_low_memory_core_early() free all the non-reserved range in lowmem,
> > so the range [_text, _stext) is also freed here.
> 
> Right, I've misread KERNEL_START as if it was _text...
>  
> > >
> > > > And we have memory protection mechanism use this
> > > > which have false alarm when some other IPs doing dma
> > > > if the dma page frame is in the [_text, _stext).
> > > >
> > > > Below are my iomem resource and reserved memory information:
> > > > console:/ # grep Kernel /proc/iomem
> > > >   20208000-219fffff : Kernel code
> > > >   21b00000-21c2e76f : Kernel data
> > > >
> > > > console:/ # cat /sys/kernel/debug/memblock/reserved
> > > > 0: 0x20201000..0x20207fff
> > > > 1: 0x20300000..0x21c2e76f
> > >
> > > What are the addresses of _text and _stext in your configuration?
> > 
> > va	 pa
> > c0008000 20208000 _text
> > c0100000 20300000 _stext
> > 
> > >
> > > What these dumps are supposed to show here?
> > >
> > 
> > Below is the dump info after applied this patch.
> > 
> > console:/ # grep Kernel /proc/iomem
> >   20300000-219fffff : Kernel code
> >   21b00000-21c2e76f : Kernel data
> > 
> > console:/ # cat /sys/kernel/debug/memblock/reserved
> > 0: 0x20201000..0x20207fff
> > 1: 0x20300000..0x21c2e76f
> > 
> > The difference is that Kernel Code resource match the reserved memblock 1
> > which is reserved in arm_memblock_init().
> 
> For that I'd extend the reservation in arm_memblock_init() to include
> [_text, _stext). 
> 
> Even if the code there is not needed after init, at least we'll keep this
> consistent with other architectures.
>  

Extending the reservation in arm_memblock_init() will consume up to SECTION_SIZE
memory for SECTION_SIZE alignment when CONFIG_STRICT_KERNEL_RWX=y.
So we really don't want to extend the reservation because it affect
the low RAM devices.

And arm64_memblock_init() also reserve [_stext, _end) in the recent kernel.