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

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Date:   Tue, 10 Dec 2019 16:04:11 +0800
From:   Baoquan He <bhe@...hat.com>
To:     David Hildenbrand <david@...hat.com>,
        Jürgen Groß <jgross@...e.com>,
        Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        william.kucharski@...cle.com, mingo@...nel.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] mm/hotplug: Only respect mem= parameter during boot stage

On 12/09/19 at 12:11pm, David Hildenbrand wrote:
> On 09.12.19 12:08, Jürgen Groß wrote:
> > On 09.12.19 12:01, David Hildenbrand wrote:
> >> On 09.12.19 11:24, Jürgen Groß wrote:
> >>> On 09.12.19 11:07, Michal Hocko wrote:
> >>>> 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?
> >>>
> >>> I guess he hasn't.
> >>>
> >>> The backtrace of the problem at that time was:
> >>>
> >>> [ 8321.876844]  [<ffffffff81019ab9>] dump_trace+0x59/0x340
> >>> [ 8321.882683]  [<ffffffff81019e8a>] show_stack_log_lvl+0xea/0x170
> >>> [ 8321.889298]  [<ffffffff8101ac31>] show_stack+0x21/0x40
> >>> [ 8321.895043]  [<ffffffff81319530>] dump_stack+0x5c/0x7c
> >>> [ 8321.900779]  [<ffffffff8107fbf1>] warn_slowpath_common+0x81/0xb0
> >>> [ 8321.907482]  [<ffffffff81009f54>] xen_alloc_pte+0x1d4/0x390
> >>> [ 8321.913718]  [<ffffffff81064950>]
> >>> pmd_populate_kernel.constprop.6+0x40/0x80
> >>> [ 8321.921498]  [<ffffffff815ef0a8>] phys_pmd_init+0x210/0x255
> >>> [ 8321.927724]  [<ffffffff815ef2c7>] phys_pud_init+0x1da/0x247
> >>> [ 8321.933951]  [<ffffffff815efb81>] kernel_physical_mapping_init+0xf5/0x1d4
> >>> [ 8321.941533]  [<ffffffff815ebc7d>] init_memory_mapping+0x18d/0x380
> >>> [ 8321.948341]  [<ffffffff810647f9>] arch_add_memory+0x59/0xf0
> >>> [ 8321.954570]  [<ffffffff815eceed>] add_memory_resource+0x8d/0x160
> >>> [ 8321.961283]  [<ffffffff815ecff2>] add_memory+0x32/0xf0
> >>> [ 8321.967025]  [<ffffffff813e1c91>] acpi_memory_device_add+0x131/0x2e0
> >>> [ 8321.974128]  [<ffffffff8139f752>] acpi_bus_attach+0xe2/0x190
> >>> [ 8321.980453]  [<ffffffff8139f6ce>] acpi_bus_attach+0x5e/0x190
> >>> [ 8321.986778]  [<ffffffff8139f6ce>] acpi_bus_attach+0x5e/0x190
> >>> [ 8321.993103]  [<ffffffff8139f6ce>] acpi_bus_attach+0x5e/0x190
> >>> [ 8321.999428]  [<ffffffff813a1157>] acpi_bus_scan+0x37/0x70
> >>> [ 8322.005461]  [<ffffffff81fba955>] acpi_scan_init+0x77/0x1b4
> >>> [ 8322.011690]  [<ffffffff81fba70c>] acpi_init+0x297/0x2b3
> >>> [ 8322.017530]  [<ffffffff8100213a>] do_one_initcall+0xca/0x1f0
> >>> [ 8322.023855]  [<ffffffff81f74266>] kernel_init_freeable+0x194/0x226
> >>> [ 8322.030760]  [<ffffffff815eb1ba>] kernel_init+0xa/0xe0
> >>> [ 8322.036503]  [<ffffffff815f7bc5>] ret_from_fork+0x55/0x80
> >>>
> >>> So this patch would break it again.
> >>>
> >>> I'd recommend ...
> >>>
> >>>>
> >>>>> 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)
> >>>
> >>> ... changing this to: ... && system_state != SYSTEM_RUNNING
> >>
> >> I think we usually use system_state < SYSTEM_RUNNING
> >>
> > 
> > Works for me as well. :-)
> > 

Thanks for reviewing and suggestions, will correct it as
'system_state < SYSTEM_RUNNING'. 

> 
> As this patch has to be resent, I'd also enjoy a comment explaining why
> this special check is in place
> 
> /* Make sure memory hotplug works although mem= was specified */
> 
> or sth. like that :)

OK, will consider what is better to be placed here. Thanks.