linux-kernel - Re: [PATCH v2 2/3] x86/vmemmap: Drop handling of 1GB vmemmap ranges

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Date:   Wed, 3 Feb 2021 14:33:56 +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 v2 2/3] x86/vmemmap: Drop handling of 1GB vmemmap ranges

On 03.02.21 11:47, Oscar Salvador wrote:
> We never get to allocate 1GB pages when mapping the vmemmap range.
> Drop the dead code both for the aligned and unaligned cases and leave
> only the direct map handling.
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> Suggested-by: David Hildenbrand <david@...hat.com>
> ---
>   arch/x86/mm/init_64.c | 31 ++++---------------------------
>   1 file changed, 4 insertions(+), 27 deletions(-)
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index b0e1d215c83e..28729c6b9775 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1062,7 +1062,6 @@ remove_pud_table(pud_t *pud_start, unsigned long addr, unsigned long end,
>   	unsigned long next, pages = 0;
>   	pmd_t *pmd_base;
>   	pud_t *pud;
> -	void *page_addr;
>   
>   	pud = pud_start + pud_index(addr);
>   	for (; addr < end; addr = next, pud++) {
> @@ -1072,32 +1071,10 @@ remove_pud_table(pud_t *pud_start, unsigned long addr, unsigned long end,
>   			continue;
>   
>   		if (pud_large(*pud)) {
> -			if (IS_ALIGNED(addr, PUD_SIZE) &&
> -			    IS_ALIGNED(next, PUD_SIZE)) {
> -				if (!direct)
> -					free_pagetable(pud_page(*pud),
> -						       get_order(PUD_SIZE));
> -
> -				spin_lock(&init_mm.page_table_lock);
> -				pud_clear(pud);
> -				spin_unlock(&init_mm.page_table_lock);
> -				pages++;
> -			} else {
> -				/* If here, we are freeing vmemmap pages. */
> -				memset((void *)addr, PAGE_INUSE, next - addr);
> -
> -				page_addr = page_address(pud_page(*pud));
> -				if (!memchr_inv(page_addr, PAGE_INUSE,
> -						PUD_SIZE)) {
> -					free_pagetable(pud_page(*pud),
> -						       get_order(PUD_SIZE));
> -
> -					spin_lock(&init_mm.page_table_lock);
> -					pud_clear(pud);
> -					spin_unlock(&init_mm.page_table_lock);
> -				}
> -			}
> -
> +			spin_lock(&init_mm.page_table_lock);
> +			pud_clear(pud);
> +			spin_unlock(&init_mm.page_table_lock);
> +			pages++;
>   			continue;
>   		}

One problem I see with existing code / this change making more obvious 
is that when trying to remove in other granularity than we added (e.g., 
unplug a 128MB DIMM avaialble during boot), we remove the direct map of 
unrelated DIMMs.

I think we should keep the

if (IS_ALIGNED(addr, PUD_SIZE) &&
     IS_ALIGNED(next, PUD_SIZE)) {
...
}

bits. Thoguhts?

Apart from that looks good.

-- 
Thanks,

David / dhildenb