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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