[PATCH] ARM: mm: enforce pageblock alignment when freeing memmap entries at init

On Wed, Jan 26, 2011 at 12:29 AM, Mel Gorman [off-list ref] wrote:

On Thu, Jan 20, 2011 at 03:47:11PM -0800, Dima Zavin wrote:

quoted

On some machines, the nodes do not always start on pageblock
boundaries. In these cases it is possible for free_unused_memmap
to free mappings for pages inside a pageblock with otherwise
valid pages. This presents problems for page migration since it
operates on whole pageblocks at a time.

This patch is not aligning on a pageblock boundary - it's aligning on
MAX_ORDER_NR_PAGES which is the boundary the buddy allocator works on.
This is a minor but important nit as different assumptions are made
about pageblocks and MAX_ORDER_NR_PAGES. Anyway;

Thank you for the nit. I'm kind of a VM noob so any insight is
appreciated. I naively equated the two in my head when writing the
description.

If the node is not starting on the MAX_ORDER boundary then node_start_pfn
should also not be on the same boundary but the patch does not imply
that this has been broken. If anything it appears at a glance that there
is memmap *not* being freed because it was allocated on the
MAX_ORDER_NR_PAGES boundary and only partially freed.

The comment then is confusing because the function is freeing memmap but
rounding down to free more of it ensures that all pages get a mapping?
It's not clear at all to me what was broken or how this patch fixes it
but bear in mind that it's rare I look at how ARM is initialised.

The function frees the mappings *between* the nodes. So when the patch
rounds down the start of the node, it preserves (i.e. frees less) the
mappings from MAX_ORDER_NR_PAGES boundary to the actual first valid
page. Otherwise, I would have to define CONFIG_HOLES_IN_ZONE and pay
the penalty. Looking through the history, a similar patch was already
taken to roundup the previous bank end to MAX_ORDER_NR_PAGES. I'm
doing the same to the start.

Am I totally off base?

--Dima

quoted

Round down bank_start to pageblock boundary so that whole
pageblocks always have valid mappings.

Signed-off-by: Dima Zavin <redacted>
---
?arch/arm/mm/init.c | ? ?6 +++++-
?1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 5164069..98de5d8 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c

@@ -418,7 +418,11 @@ static void __init free_unused_memmap(struct meminfo *mi)

? ? ? for_each_bank(i, mi) {
? ? ? ? ? ? ? struct membank *bank = &mi->bank[i];

- ? ? ? ? ? ? bank_start = bank_pfn_start(bank);
+ ? ? ? ? ? ? /* Round bank_start down to the start of a pageblock so that
+ ? ? ? ? ? ? ?* all pages in a pageblock always have a mapping.
+ ? ? ? ? ? ? ?*/
+ ? ? ? ? ? ? bank_start = round_down(bank_pfn_start(bank),
+ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? MAX_ORDER_NR_PAGES);

? ? ? ? ? ? ? /*
? ? ? ? ? ? ? ?* If we had a previous bank, and there is a space
--
1.7.3.1

--
Mel Gorman
Linux Technology Center
IBM Dublin Software Lab