linux-kernel - Re: [PATCH v3 07/17] arm64, hibernate: move page handling function to new trans

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Date:   Fri, 6 Sep 2019 16:18:37 +0100
From:   James Morse <james.morse@....com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     jmorris@...ei.org, sashal@...nel.org, ebiederm@...ssion.com,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        corbet@....net, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, marc.zyngier@....com,
        vladimir.murzin@....com, matthias.bgg@...il.com,
        bhsharma@...hat.com, linux-mm@...ck.org, mark.rutland@....com
Subject: Re: [PATCH v3 07/17] arm64, hibernate: move page handling function to
 new trans_pgd.c

Hi Pavel,

On 21/08/2019 19:31, Pavel Tatashin wrote:
> Now, that we abstracted the required functions move them to a new home.
> Later, we will generalize these function in order to be useful outside
> of hibernation.

> diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
> new file mode 100644
> index 000000000000..00b62d8640c2
> --- /dev/null
> +++ b/arch/arm64/mm/trans_pgd.c
> @@ -0,0 +1,211 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * Copyright (c) 2019, Microsoft Corporation.
> + * Pavel Tatashin <patatash@...ux.microsoft.com>

Hmmm, while line-count isn't a useful metric: this file contains 41% of the code that was
in hibernate.c, but has stripped the substantial copyright-pedigree that the hibernate
code had built up over the years.
(counting lines identified by 'cloc' as code, not comments or blank)

If you are copying or moving a non trivial quantity of code, you need to preserve the
copyright. Something like 'Derived from the arm64 hibernate support which has:'....


> + */
> +
> +/*
> + * Transitional tables are used during system transferring from one world to
> + * another: such as during hibernate restore, and kexec reboots. During these
> + * phases one cannot rely on page table not being overwritten.

I think you need to mention that hibernate and kexec are rewriting memory, and may
overwrite the live page tables, therefore ...


> + *
> + */
> +
> +#include <asm/trans_pgd.h>
> +#include <asm/pgalloc.h>
> +#include <asm/pgtable.h>
> +#include <linux/suspend.h>

#include <linux/bug.h>
#include <linux/mm.h>
#include <linux/mmzone.h>


> +static void _copy_pte(pte_t *dst_ptep, pte_t *src_ptep, unsigned long addr)
> +{
> +	pte_t pte = READ_ONCE(*src_ptep);
> +

> +	if (pte_valid(pte)) {

> +		/*
> +		 * Resume will overwrite areas that may be marked
> +		 * read only (code, rodata). Clear the RDONLY bit from
> +		 * the temporary mappings we use during restore.
> +		 */
> +		set_pte(dst_ptep, pte_mkwrite(pte));

> +	} else if (debug_pagealloc_enabled() && !pte_none(pte)) {

> +		/*
> +		 * debug_pagealloc will removed the PTE_VALID bit if
> +		 * the page isn't in use by the resume kernel. It may have
> +		 * been in use by the original kernel, in which case we need
> +		 * to put it back in our copy to do the restore.
> +		 *
> +		 * Before marking this entry valid, check the pfn should
> +		 * be mapped.
> +		 */

> +		BUG_ON(!pfn_valid(pte_pfn(pte)));


Thanks,

James