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

From: David Hildenbrand <[email-masked]>
Date: Fri, 4 Oct 2019 11:33:00 +0200

On 04.10.19 11:03, David Hildenbrand wrote:
> On 04.10.19 11:00, David Hildenbrand wrote:
>> On 03.10.19 18:48, Aneesh Kumar K.V wrote:
>>> 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?
>>
>> Okay, let's recap. We should call add_pages()/__remove_pages()
>> and arch_add_memory()/arch_remove_memory() with the exact same ranges.
>>
>> So with PHYS_PFN(res->start) and PHYS_PFN(resource_size(res)
>>
>> Now, only a subset of the pages gets actually initialized,
>> meaning the NID and the ZONE we read could be stale.
>> That, we have to fix.
>>
>> What about something like this (am I missing something?):
>>
>> From d77b5c50f86570819a437517a897cc40ed29eefb Mon Sep 17 00:00:00 2001
>> From: "Aneesh Kumar K.V" <[email-masked]>
>> Date: Fri, 27 Sep 2019 16:02:24 +0530
>> Subject: [PATCH] mm/memunmap: Don't access uninitialized memmap in
>>  memunmap_pages()
>>
>> With an altmap, the memmap falling into the reserved altmap space are
>> not initialized and, therefore, contain a garbage NID and a garbage
>> zone. Make sure to read the NID/zone from a memmap that was initialzed.
>>
>> 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]>
>> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
>> [ minimze code changes, rephrase description ]
>> Signed-off-by: David Hildenbrand <[email-masked]>
>> ---
>>  mm/memremap.c | 11 +++++++----
>>  1 file changed, 7 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/memremap.c b/mm/memremap.c
>> index 557e53c6fb46..8b11c0da345c 100644
>> --- a/mm/memremap.c
>> +++ b/mm/memremap.c
>> @@ -123,6 +123,7 @@ static void dev_pagemap_cleanup(struct dev_pagemap *pgmap)
>>  void memunmap_pages(struct dev_pagemap *pgmap)
>>  {
>>  	struct resource *res = &pgmap->res;
>> +	struct page *first_page;
>>  	unsigned long pfn;
>>  	int nid;
>>  
>> @@ -131,14 +132,16 @@ void memunmap_pages(struct dev_pagemap *pgmap)
>>  		put_page(pfn_to_page(pfn));
>>  	dev_pagemap_cleanup(pgmap);
>>  
>> +	/* make sure to access a memmap that was actually initialized */
>> +	first_page = pfn_to_page(pfn_first(pgmap));
>> +
>>  	/* pages are dead and unused, undo the arch mapping */
>> -	nid = page_to_nid(pfn_to_page(PHYS_PFN(res->start)));
>> +	nid = page_to_nid(first_page);
>>  
>>  	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(first_page), res->start,
>> +			       resource_size(res), NULL);
> 
> Keeping the PHYS_PFN() calls of course ...
> 
>>  	} else {
>>  		arch_remove_memory(nid, res->start, resource_size(res),
>>  				pgmap_altmap(pgmap));
>>
> 
> 

The current state (with the modified patch) can be found at:
	https://github.com/davidhildenbrand/linux/tree/zone_offline

@Aneesh, It would be great if you could test the namespace removal
thingy and tell me if we are still missing something :)

-- 

Thanks,

David / dhildenb

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