Re: [PATCHv2] mm: Don't offset memmap for flatmem

[Laura Abbott <lauraa@codeaurora.org>]

Reviving this thread because I don't think it ever got resolved.

On 2/3/2015 6:25 PM, Laura Abbott wrote:
> On 1/29/2015 5:13 AM, Vlastimil Babka wrote:
>> On 01/26/2015 04:56 PM, Mel Gorman wrote:
>>> On Fri, Jan 23, 2015 at 10:05:48AM +0100, Vlastimil Babka wrote:
>>>> On 01/23/2015 01:33 AM, Laura Abbott wrote:
>>>>> On 1/22/2015 4:20 PM, Andrew Morton wrote:
>>>>>>
>>>>>> I don't think v2 addressed Vlastimil's review comment?
>>>>>>
>>>>>
>>>>> We're still adding the offset to node_mem_map and then subtracting it from
>>>>> just mem_map. Did I miss another comment somewhere?
>>>>
>>>> Yes that was addressed, thanks. But I don't feel comfortable acking
>>>> it yet, as I have no idea if we are doing the right thing for
>>>> CONFIG_HAVE_MEMBLOCK_NODE_MAP && CONFIG_FLATMEM case here.
>>>>
>>>> Also putting the CONFIG_FLATMEM && !CONFIG_HAVE_MEMBLOCK_NODE_MAP
>>>> under the "if (page_to_pfn(mem_map) != pgdat->node_start_pfn)" will
>>>> probably do the right thing, but looks like a weird test for this
>>>> case here.
>>>>
>>>> I have no good suggestion though, so let's CC Mel who apparently
>>>> wrote the ARCH_PFN_OFFSET correction?
>>>>
>>>
>>> I don't recall introducing ARCH_PFN_OFFSET, are you sure it was me?  I'm just
>>> back today after been offline a week so didn't review the patch but IIRC,
>>> ARCH_PFN_OFFSET deals with the case where physical memory does not start
>>> at 0. Without the offset, virtual _PAGE_OFFSET would not physical page 0.
>>> I don't recall it being related to the alignment of node 0 so if there
>>> are crashes due to misalignment of node 0 and the fix is ARCH_PFN_OFFSET
>>> related then I'm surprised.
>>
>> You're right that ARCH_PFN_OFFSET wasn't added by you, but by commit
>> 467bc461d2 which was a bugfix to your commit c713216dee, which did
>>  introduce the mem_map correction code, and after which the code looked like:
>>
>> mem_map = NODE_DATA(0)->node_mem_map;
>> #ifdef CONFIG_ARCH_POPULATES_NODE_MAP
>>                 if (page_to_pfn(mem_map) != pgdat->node_start_pfn)
>>                         mem_map -= pgdat->node_start_pfn;
>> #endif /* CONFIG_ARCH_POPULATES_NODE_MAP */
>>
>>
>> It's from 2006 so I can't expect you remember the details, but I had some
>>  trouble finding out what this does. I assume it makes sure that mem_map points
>>  to struct page corresponding to pfn 0, because that's what translations using
>>  mem_map expect.
>> But pgdat->node_mem_map points to struct page corresponding to
>>  pgdat->node_start_pfn, which might not be 0. So it subtracts node_start_pfn
>>  to fix that. This is OK, as the node_mem_map is allocated (in this very
>>  function) with padding so that it covers a MAX_ORDER_NR_PAGES aligned area
>>  where node_mem_map may point to the middle of it.
>>
>> Commit 467bc461d2 fixed this in case the first pfn is not 0, but ARCH_PFN_OFFSET.
>>  So mem_map points to struct page corresponding to pfn=ARCH_PFN_OFFSET, which
>>  is OK. But I still have few doubts:
>>
>> 1) The "if (page_to_pfn(mem_map) != pgdat->node_start_pfn)" sort of silently
>>  assumes that mem_map is allocated at the beginning of the node, i.e. at
>>  pgdat->node_start_pfn. And the only reason for this if-condition to be true,
>>  is that we haven't corrected the page_to_pfn translation, which uses mem_map.
>>  Is this assumption always OK to do? Shouldn't the if-condition be instead about
>>  pgdat->node_start_pfn not being aligned?
>>
>> 2) The #ifdef guard is about CONFIG_ARCH_POPULATES_NODE_MAP, which is nowadays  called  > CONFIG_HAVE_MEMBLOCK_NODE_MAP. But shouldn't it be #ifdef FLATMEM instead?
>>  After all, we are correcting value of mem_map based on page_to_pfn code
>> variant used on FLATMEM. arm doesn't define
>> CONFIG_ARCH_POPULATES_NODE_MAP but apparently needs this correction.
>>
>
> Just doing #ifdef FLATMEM doesn't work because ARCH_PFN_OFFSET doesn't
> seem to be picked up properly for NOMMU arches properly. Probably just
> missing a header somewhere.
>
>> 3) The node_mem_map allocation code aligns the allocation to MAX_ORDER_NR_PAGES,
>>  so the offset between the start of the allocated map and where node_mem_map
>>  points to will be up to MAX_ORDER_NR_PAGES.
>> However, here we subtract (in current kernel) (pgdat->node_start_pfn - ARCH_PFN_OFFSET).
>>  That looks like another silent assumption, that pgdat->node_start_pfn is always
>>  between ARCH_PFN_OFFSET and ARCH_PFN_OFFSET + MAX_ORDER_NR_PAGES. If it were
>>  larger, the mem_map correction would subtract too much and end up below what
>>  was allocated for node_mem_map, no? The bug report behind this patch said that
>>  first 2MB of memory was reserved using "no-map flag using DT". Unless this somehow
>>  translates to ARCH_PFN_OFFSET at build time, we would underflow mem_map, right?
>>  Maybe I'm just overly paranoid here and of course ARCH_PFN_OFFSET is determined
>>  properly on arm...
>>
>> If anyone can confirm my doubts or point me to what I'm missing, thanks.
>
> ARCH_PFN_OFFSET should always be the lowest PFN in the system, otherwise
> I think plenty of other things are broken given how many architectures
> make this assumption. That said, I don't think subtracting ARCH_PFN_OFFSET
> makes it obvious why the adjustment is being made.
>
> Thanks,
> Laura
>

I was incorrect before: it isn't just NOMMU but architectures that don't use
asm-generic/memory_model.h which failed to compile. I could respin with
more ifdefery around the ARCH_PFN_OFFSET if that sounds reasonable.

Thanks,
Laura

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