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

[Laura Abbott]

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

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