linux-kernel - Re: [PATCH v5 3/4] x86/vmemmap: Handle unpopulated sub-pmd ranges

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Message-ID: <3471fe0e-d8f4-c8fe-2096-8d9c8b1ab5bc@redhat.com>
Date:   Tue, 9 Mar 2021 18:52:38 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/4] x86/vmemmap: Handle unpopulated sub-pmd ranges

On 09.03.21 18:41, Oscar Salvador wrote:
> When sizeof(struct page) is not a power of 2, sections do not span
> a PMD anymore and so when populating them some parts of the PMD will
> remain unused.
> Because of this, PMDs will be left behind when depopulating sections
> since remove_pmd_table() thinks that those unused parts are still in
> use.
> 
> Fix this by marking the unused parts with PAGE_UNUSED, so memchr_inv()
> will do the right thing and will let us free the PMD when the last user
> of it is gone.
> 
> This patch is based on a similar patch by David Hildenbrand:
> 
> https://lore.kernel.org/linux-mm/20200722094558.9828-9-david@redhat.com/
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> Reviewed-by: David Hildenbrand <david@...hat.com>
> ---
>   arch/x86/mm/init_64.c | 63 ++++++++++++++++++++++++++++++++++++++++++---------
>   1 file changed, 52 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 9ecb3c488ac8..3bb3988c7681 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -871,7 +871,50 @@ int arch_add_memory(int nid, u64 start, u64 size,
>   	return add_pages(nid, start_pfn, nr_pages, params);
>   }
>   
> -#define PAGE_INUSE 0xFD
> +#ifdef CONFIG_SPARSEMEM_VMEMMAP
> +#define PAGE_UNUSED 0xFD
> +
> +/* Returns true if the PMD is completely unused and thus it can be freed */
> +static bool __meminit vmemmap_pmd_is_unused(unsigned long addr, unsigned long end)
> +{

I don't think the new name is any better. It implies that all it does is 
a check - yet it actually clears the given range. (I prefer the old 
name, but well, I came up with that, so what do I know :D )


-- 
Thanks,

David / dhildenb