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

From: David Hildenbrand <david@redhat.com>
To: Oscar Salvador <osalvador@suse.de>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
	Michal Hocko <mhocko@kernel.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/3] x86/vmemmap: Drop handling of 1GB vmemmap ranges
Date: Wed, 3 Feb 2021 14:33:56 +0100	[thread overview]
Message-ID: <ec2f6a46-b2a8-8131-a2ac-48a02a1ea201@redhat.com> (raw)
In-Reply-To: <20210203104750.23405-3-osalvador@suse.de>

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@suse.de>
> Suggested-by: David Hildenbrand <david@redhat.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