linux-kernel - Re: [PATCH] mm/hotplug: Only respect mem= parameter during boot stage

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Message-ID: <20191209100717.GC6156@dhcp22.suse.cz>
Date:   Mon, 9 Dec 2019 11:07:17 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org, jgross@...e.com,
        william.kucharski@...cle.com, mingo@...nel.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] mm/hotplug: Only respect mem= parameter during boot stage

On Fri 06-12-19 23:05:24, Baoquan He wrote:
> In commit 357b4da50a62 ("x86: respect memory size limiting via mem=
> parameter") a global varialbe global max_mem_size is added to store
> the value which is parsed from 'mem= '. This truly stops those
> DIMM from being added into system memory during boot.
> 
> However, it also limits the later memory hotplug functionality. Any
> memory board can't be hot added any more if its region is beyond the
> max_mem_size. System will print error like below:
> 
> [  216.387164] acpi PNP0C80:02: add_memory failed
> [  216.389301] acpi PNP0C80:02: acpi_memory_enable_device() error
> [  216.392187] acpi PNP0C80:02: Enumeration failure
> 
> >From document of 'mem =' parameter, it should be a restriction during
> boot, but not impact the system memory adding/removing after booting.
> 
>   mem=nn[KMG]     [KNL,BOOT] Force usage of a specific amount of memory
> 
> So fix it by also checking if it's during SYSTEM_BOOTING stage when
> restrict memory adding. Otherwise, skip the restriction.

Could you be more specific about why the boot vs. later hotplug makes
any difference? The documentation is explicit about the boot time but
considering this seems to be like that since ever I strongly suspect
that this is just an omission.

Btw. how have you tested the situation fixed by 357b4da50a62?

> Fixes: 357b4da50a62 ("x86: respect memory size limiting via mem= parameter")
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  mm/memory_hotplug.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 55ac23ef11c1..5466a0a00901 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -105,7 +105,7 @@ static struct resource *register_memory_resource(u64 start, u64 size)
>  	unsigned long flags =  IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
>  	char *resource_name = "System RAM";
>  
> -	if (start + size > max_mem_size)
> +	if (start + size > max_mem_size && system_state == SYSTEM_BOOTING)
>  		return ERR_PTR(-E2BIG);
>  
>  	/*
> -- 
> 2.17.2

-- 
Michal Hocko
SUSE Labs