linux-kernel - Re: [PATCH v2] x86/mm/ident_map: Use gbpages only where full GB page should be mapped.

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Date: Mon, 8 Jan 2024 15:21:06 -0600
From: Steve Wahl <steve.wahl@....com>
To: Steve Wahl <steve.wahl@....com>, 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>, linux-kernel@...r.kernel.org
Cc: Russ Anderson <rja@....com>, Dimitri Sivanich <sivanich@....com>
Subject: Re: [PATCH v2] x86/mm/ident_map: Use gbpages only where full GB page
 should be mapped.

Gentle ping.  

We'd like to see this get in, we're finding the most likely time to
hit the problem seems to be when kexec'ing a kdump kernel, and we
don't get the dump we need to diagnose some other problem.

Thanks,

--> Steve Wahl

On Fri, Nov 03, 2023 at 02:01:08PM -0500, Steve Wahl wrote:
> Instead of using gbpages for all memory regions, which can include
> vast areas outside what's actually been requested, use them only when
> map creation requests include the full GB page of space; descend to
> using smaller 2M pages when only portions of a GB page are included in
> the request.
> 
> No attempt is made to coalesce mapping requests. If a request requires
> a map entry at the 2M (pmd) level, subsequent mapping requests within
> the same 1G region will also be at the pmd level, even if adjacent or
> overlapping such requests could theoretically have been combined to
> map a full gbpage.  Existing usage starts with larger regions and then
> adds smaller regions, so this should not have any great consequence.
> 
> When gbpages are used exclusively to create identity maps, large
> ranges of addresses not actually requested can be included in the
> resulting table.  On UV systems, this ends up including regions that
> will cause hardware to halt the system if accessed (these are marked
> "reserved" by BIOS).  Even though code does not actually make
> references to these addresses, including them in an active map allows
> processor speculation into this region, which is enough to trigger the
> system halt.
> 
> The kernel option "nogbpages" will disallow use of gbpages entirely
> and avoid this problem, but uses a lot of extra memory for page tables
> that are not really needed.
> 
> Signed-off-by: Steve Wahl <steve.wahl@....com>
> ---
> v2: per Dave Hanson review: Additional changelog info,
>     moved pud_large() check earlier in the code, and
>     improved the comment describing the conditions
>     that restrict gbpage usage.
>    
>  arch/x86/mm/ident_map.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/mm/ident_map.c b/arch/x86/mm/ident_map.c
> index 968d7005f4a7..5c88c3a7d12a 100644
> --- a/arch/x86/mm/ident_map.c
> +++ b/arch/x86/mm/ident_map.c
> @@ -31,13 +31,23 @@ static int ident_pud_init(struct x86_mapping_info *info, pud_t *pud_page,
>  		if (next > end)
>  			next = end;
>  
> -		if (info->direct_gbpages) {
> -			pud_t pudval;
> +		/* if this is already a gbpage, this portion is already mapped */
> +		if (pud_large(*pud))
> +			continue;
>  
> -			if (pud_present(*pud))
> -				continue;
> +		/*
> +		 * To be eligible to use a gbpage:
> +		 *   - gbpages must be enabled
> +		 *   - addr must be gb aligned (start of region)
> +		 *   - next must be gb aligned (end of region)
> +		 *   - PUD must be empty (nothing already mapped in this region)
> +		 */
> +		if (info->direct_gbpages
> +		    && !(addr & ~PUD_MASK)
> +		    && !(next & ~PUD_MASK)
> +		    && !pud_present(*pud)) {
> +			pud_t pudval;
>  
> -			addr &= PUD_MASK;
>  			pudval = __pud((addr - info->offset) | info->page_flag);
>  			set_pud(pud, pudval);
>  			continue;
> -- 
> 2.26.2
>