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

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Date:   Wed, 27 Jan 2021 12:29:27 +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] x86/vmemmap: Handle unpopulated sub-pmd ranges

On 27.01.21 12:23, Oscar Salvador wrote:
> When the size of a struct page is not multiple of 2MB, 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_INUSE, 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/
> https://lore.kernel.org/linux-mm/20200722094558.9828-10-david@redhat.com/
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> ---
>   arch/x86/mm/init_64.c | 81 +++++++++++++++++++++++++++++++++++++++----
>   1 file changed, 74 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index b5a3fa4033d3..9b6d45ea353b 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -873,6 +873,71 @@ int arch_add_memory(int nid, u64 start, u64 size,
>   
>   #define PAGE_INUSE 0xFD

Note that I called it "PAGE_UNUSED", using "PAGE_INUSE" might make it a 
little harder to digest.

-- 
Thanks,

David / dhildenb