linux-kernel - Re: [PATCH v5 1/6] arm64/mm: Introduce the init_pg

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Message-ID: <20180924140338.nu7jn3hfcwwoztnt@lakrids.cambridge.arm.com>
Date:   Mon, 24 Sep 2018 15:03:38 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Jun Yao <yaojun8558363@...il.com>
Cc:     linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        will.deacon@....com, james.morse@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/6] arm64/mm: Introduce the init_pg_dir.

On Mon, Sep 24, 2018 at 02:01:37PM +0100, Mark Rutland wrote:
> On Mon, Sep 17, 2018 at 12:43:28PM +0800, Jun Yao wrote:
> > To make the swapper_pg_dir read only, we will move it to the rodata
> > section. And force the kernel to set up the initial page table in
> > the init_pg_dir. After generating all levels page table, we copy
> > only the top level into the swapper_pg_dir during paging_init().

> > +#define INIT_PG_TABLES					\
> > +	. = ALIGN(PAGE_SIZE);				\
> > +	init_pg_dir = .;				\
> > +	. += SWAPPER_DIR_SIZE;				\
> > +	init_pg_end = .;
> 
> This will allocate a full set of PGD + PUD + PMD (+ PTE) levels, but the
> commit message says we only copy the top level table.
> 
> The memory for otjher levels of table can be re-allocated once the
> kernel init phase is over, so that seems a bit worrying.
> 
> Do we switch over to new copies of those levels, or do we continue using
> the entries from INIT_PG_TABLES?

I now see that we already allocate copies of those levels of table (and
free the originals in paging_init), so this is not a problem.

Sorry for the noise!

Mark.