Re: [RFC PATCH v2] mm: initialize struct pages reserved by ZONE_DEVICE driver.

[David Hildenbrand]

On 06.09.19 12:02, Toshiki Fukasawa wrote:
> Thank you for your feedback.
> 
> On 2019/09/06 17:45, David Hildenbrand wrote:
>> On 06.09.19 10:09, Toshiki Fukasawa wrote:
>>> A kernel panic is observed during reading
>>> /proc/kpage{cgroup,count,flags} for first few pfns allocated by
>>> pmem namespace:
>>>
>>> BUG: unable to handle page fault for address: fffffffffffffffe
>>> [  114.495280] #PF: supervisor read access in kernel mode
>>> [  114.495738] #PF: error_code(0x0000) - not-present page
>>> [  114.496203] PGD 17120e067 P4D 17120e067 PUD 171210067 PMD 0
>>> [  114.496713] Oops: 0000 [#1] SMP PTI
>>> [  114.497037] CPU: 9 PID: 1202 Comm: page-types Not tainted 5.3.0-rc1
>>> [  114.497621] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
>>> [  114.498706] RIP: 0010:stable_page_flags+0x27/0x3f0
>>> [  114.499142] Code: 82 66 90 66 66 66 66 90 48 85 ff 0f 84 d1 03 00 00 41 54 55 48 89 fd 53 48 8b 57 08 48 8b 1f 48 8d 42 ff 83 e2 01 48 0f 44 c7 <48> 8b 00 f6 c4 02 0f 84 57 03 00 00 45 31 e4 48 8b 55 08 48 89 ef
>>> [  114.500788] RSP: 0018:ffffa5e601a0fe60 EFLAGS: 00010202
>>> [  114.501373] RAX: fffffffffffffffe RBX: ffffffffffffffff RCX: 0000000000000000
>>> [  114.502009] RDX: 0000000000000001 RSI: 00007ffca13a7310 RDI: ffffd07489000000
>>> [  114.502637] RBP: ffffd07489000000 R08: 0000000000000001 R09: 0000000000000000
>>> [  114.503270] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000240000
>>> [  114.503896] R13: 0000000000080000 R14: 00007ffca13a7310 R15: ffffa5e601a0ff08
>>> [  114.504530] FS:  00007f0266c7f540(0000) GS:ffff962dbbac0000(0000) knlGS:0000000000000000
>>> [  114.505245] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> [  114.505754] CR2: fffffffffffffffe CR3: 000000023a204000 CR4: 00000000000006e0
>>> [  114.506401] Call Trace:
>>> [  114.506660]  kpageflags_read+0xb1/0x130
>>> [  114.507051]  proc_reg_read+0x39/0x60
>>> [  114.507387]  vfs_read+0x8a/0x140
>>> [  114.507686]  ksys_pread64+0x61/0xa0
>>> [  114.508021]  do_syscall_64+0x5f/0x1a0
>>> [  114.508372]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>> [  114.508844] RIP: 0033:0x7f0266ba426b
>>>
>>> The first few pages of ZONE_DEVICE expressed as the range
>>> (altmap->base_pfn) to (altmap->base_pfn + altmap->reserve) are
>>> skipped by struct page initialization. Some pfn walkers like
>>> /proc/kpage{cgroup, count, flags} can't handle these uninitialized
>>> struct pages, which causes the error.
>>>
>>> In previous discussion, Dan seemed to have concern that the struct
>>> page area of some pages indicated by vmem_altmap->reserve may not
>>> be allocated. (See https://lore.kernel.org/lkml/CAPcyv4i5FjTOnPbXNcTzvt+e6RQYow0JRQwSFuxaa62LSuvzHQ@xxxxxxxxxxxxxx/)
>>> However, arch_add_memory() called by devm_memremap_pages() allocates
>>> struct page area for pages containing addresses in the range
>>> (res.start) to (res.start + resource_size(res)), which include the
>>> pages indicated by vmem_altmap->reserve. If I read correctly, it is
>>> allocated as requested at least on x86_64. Also, memmap_init_zone()
>>> initializes struct pages in the same range.
>>> So I think the struct pages should be initialized.>
>>
>> For !ZONE_DEVICE memory, the memmap is valid with SECTION_IS_ONLINE -
>> for the whole section. For ZONE_DEVICE memory we have no such
>> indication. In any section that is !SECTION_IS_ONLINE and
>> SECTION_MARKED_PRESENT, we could have any subsections initialized. >
>> The only indication I am aware of is pfn_zone_device_reserved() - which
>> seems to check exactly what you are trying to skip here.
>>
>> Can't you somehow use pfn_zone_device_reserved() ? Or if you considered
>> that already, why did you decide against it?
> 
> No, in current approach this function is no longer needed.
> The reason why we change the approach is that all pfn walkers
> have to be aware of the uninitialized struct pages.

We should use the same strategy for all pfn walkers then (effectively
get rid of pfn_zone_device_reserved() if that's what we want).

> 
> As for SECTION_IS_ONLINE, I'm not sure now.
> I will look into it next week.

SECTION_IS_ONLINE does currently not apply to ZONE_DEVICE and due to
sub-section support for ZONE_DEVICE, it cannot easily be reused.

> 
> Thanks,
> Toshiki Fukasawa
> 
>>
>>> Signed-off-by: Toshiki Fukasawa <t-fukasawa@xxxxxxxxxxxxx>
>>> Cc: stable@xxxxxxxxxxxxxxx
>>> ---
>>> Changes since rev 1:
>>>   Instead of avoiding uninitialized pages on the pfn walker side,
>>>   we initialize struct pages.
>>>
>>> mm/page_alloc.c | 5 +----
>>>   1 file changed, 1 insertion(+), 4 deletions(-)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 9c91949..6d180ae 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -5846,8 +5846,7 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>>>   
>>>   #ifdef CONFIG_ZONE_DEVICE
>>>   	/*
>>> -	 * Honor reservation requested by the driver for this ZONE_DEVICE
>>> -	 * memory. We limit the total number of pages to initialize to just
>>> +	 * We limit the total number of pages to initialize to just
>>>   	 * those that might contain the memory mapping. We will defer the
>>>   	 * ZONE_DEVICE page initialization until after we have released
>>>   	 * the hotplug lock.
>>> @@ -5856,8 +5855,6 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>>>   		if (!altmap)
>>>   			return;
>>>   
>>> -		if (start_pfn == altmap->base_pfn)
>>> -			start_pfn += altmap->reserve;
>>>   		end_pfn = altmap->base_pfn + vmem_altmap_offset(altmap);
>>>   	}
>>>   #endif
>>>
>>
>>


-- 

Thanks,

David / dhildenb