Subject: Re: [PATCH v5 01/10] mm/memunmap: Use the correct start and end pfn when removing pages from zone

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Thu, 3 Oct 2019 22:18:10 +0530

On 10/1/19 8:33 PM, David Hildenbrand wrote:
> On 01.10.19 16:57, David Hildenbrand wrote:
>> On 01.10.19 16:40, David Hildenbrand wrote:
>>> From: "Aneesh Kumar K.V" <[email-masked]>
>>>
>>> With altmap, all the resource pfns are not initialized. While initializing
>>> pfn, altmap reserve space is skipped. Hence when removing pfn from zone
>>> skip pfns that were never initialized.
>>>
>>> Update memunmap_pages to calculate start and end pfn based on altmap
>>> values. This fixes a kernel crash that is observed when destroying
>>> a namespace.
>>>
>>> [   81.356173] kernel BUG at include/linux/mm.h:1107!
>>> cpu 0x1: Vector: 700 (Program Check) at [c000000274087890]
>>>      pc: c0000000004b9728: memunmap_pages+0x238/0x340
>>>      lr: c0000000004b9724: memunmap_pages+0x234/0x340
>>> ...
>>>      pid   = 3669, comm = ndctl
>>> kernel BUG at include/linux/mm.h:1107!
>>> [c000000274087ba0] c0000000009e3500 devm_action_release+0x30/0x50
>>> [c000000274087bc0] c0000000009e4758 release_nodes+0x268/0x2d0
>>> [c000000274087c30] c0000000009dd144 device_release_driver_internal+0x174/0x240
>>> [c000000274087c70] c0000000009d9dfc unbind_store+0x13c/0x190
>>> [c000000274087cb0] c0000000009d8a24 drv_attr_store+0x44/0x60
>>> [c000000274087cd0] c0000000005a7470 sysfs_kf_write+0x70/0xa0
>>> [c000000274087d10] c0000000005a5cac kernfs_fop_write+0x1ac/0x290
>>> [c000000274087d60] c0000000004be45c __vfs_write+0x3c/0x70
>>> [c000000274087d80] c0000000004c26e4 vfs_write+0xe4/0x200
>>> [c000000274087dd0] c0000000004c2a6c ksys_write+0x7c/0x140
>>> [c000000274087e20] c00000000000bbd0 system_call+0x5c/0x68
>>>
>>> Cc: Dan Williams <[email-masked]>
>>> Cc: Andrew Morton <[email-masked]>
>>> Cc: Jason Gunthorpe <[email-masked]>
>>> Cc: Logan Gunthorpe <[email-masked]>
>>> Cc: Ira Weiny <[email-masked]>
>>> Reviewed-by: Pankaj Gupta <[email-masked]>
>>> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
>>> [ move all pfn-realted declarations into a single line ]
>>> Signed-off-by: David Hildenbrand <[email-masked]>
>>> ---
>>>   mm/memremap.c | 13 ++++++++-----
>>>   1 file changed, 8 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/mm/memremap.c b/mm/memremap.c
>>> index 557e53c6fb46..026788b2ac69 100644
>>> --- a/mm/memremap.c
>>> +++ b/mm/memremap.c
>>> @@ -123,7 +123,7 @@ static void dev_pagemap_cleanup(struct dev_pagemap *pgmap)
>>>   void memunmap_pages(struct dev_pagemap *pgmap)
>>>   {
>>>   	struct resource *res = &pgmap->res;
>>> -	unsigned long pfn;
>>> +	unsigned long pfn, nr_pages, start_pfn, end_pfn;
>>>   	int nid;
>>>   
>>>   	dev_pagemap_kill(pgmap);
>>> @@ -131,14 +131,17 @@ void memunmap_pages(struct dev_pagemap *pgmap)
>>>   		put_page(pfn_to_page(pfn));
>>>   	dev_pagemap_cleanup(pgmap);
>>>   
>>> +	start_pfn = pfn_first(pgmap);
>>> +	end_pfn = pfn_end(pgmap);
>>> +	nr_pages = end_pfn - start_pfn;
>>> +
>>>   	/* pages are dead and unused, undo the arch mapping */
>>> -	nid = page_to_nid(pfn_to_page(PHYS_PFN(res->start)));
>>> +	nid = page_to_nid(pfn_to_page(start_pfn));
>>>   
>>>   	mem_hotplug_begin();
>>>   	if (pgmap->type == MEMORY_DEVICE_PRIVATE) {
>>> -		pfn = PHYS_PFN(res->start);
>>> -		__remove_pages(page_zone(pfn_to_page(pfn)), pfn,
>>> -				 PHYS_PFN(resource_size(res)), NULL);
>>> +		__remove_pages(page_zone(pfn_to_page(start_pfn)), start_pfn,
>>> +			       nr_pages, NULL);
>>>   	} else {
>>>   		arch_remove_memory(nid, res->start, resource_size(res),
>>>   				pgmap_altmap(pgmap));
>>>
>>
>> Aneesh, I was wondering why the use of "res->start" is correct (and we
>> shouldn't also witch to start_pfn/nr_pages here. It would be good if Dan
>> could review.
>>
> 
> To be more precise, I wonder if it should actually be
> 
> __remove_pages(page_zone(pfn_to_page(start_pfn)), res->start,
>                 resource_size(res))
> 

yes, that would be make it much clear.

But for MEMORY_DEVICE_PRIVATE start_pfn and pfn should be same?

> IOW, keep calling __remove_pages() with the same parameters but read
> nid/zone from the offset one.
> 
> Hope some memunmap_pages() expert can clarify.
> 

-aneesh

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