Re: [PATCH] sparsemem/bootmem: catch greater than section size allocations

On Tue, Feb 28, 2012 at 12:11:51PM -0800, Nishanth Aravamudan wrote:

On 28.02.2012 [14:53:26 +0100], Johannes Weiner wrote:

quoted

On Fri, Feb 24, 2012 at 11:33:58AM -0800, Nishanth Aravamudan wrote:

quoted

While testing AMS (Active Memory Sharing) / CMO (Cooperative Memory
Overcommit) on powerpc, we tripped the following:

kernel BUG at mm/bootmem.c:483!
cpu 0x0: Vector: 700 (Program Check) at [c000000000c03940]
    pc: c000000000a62bd8: .alloc_bootmem_core+0x90/0x39c
    lr: c000000000a64bcc: .sparse_early_usemaps_alloc_node+0x84/0x29c
    sp: c000000000c03bc0
   msr: 8000000000021032
  current = 0xc000000000b0cce0
  paca    = 0xc000000001d80000
    pid   = 0, comm = swapper
kernel BUG at mm/bootmem.c:483!
enter ? for help
[c000000000c03c80] c000000000a64bcc
.sparse_early_usemaps_alloc_node+0x84/0x29c
[c000000000c03d50] c000000000a64f10 .sparse_init+0x12c/0x28c
[c000000000c03e20] c000000000a474f4 .setup_arch+0x20c/0x294
[c000000000c03ee0] c000000000a4079c .start_kernel+0xb4/0x460
[c000000000c03f90] c000000000009670 .start_here_common+0x1c/0x2c

This is

        BUG_ON(limit && goal + size > limit);

and after some debugging, it seems that

	goal = 0x7ffff000000
	limit = 0x80000000000

and sparse_early_usemaps_alloc_node ->
sparse_early_usemaps_alloc_pgdat_section -> alloc_bootmem_section calls

	return alloc_bootmem_section(usemap_size() * count, section_nr);

This is on a system with 8TB available via the AMS pool, and as a quirk
of AMS in firmware, all of that memory shows up in node 0. So, we end up
with an allocation that will fail the goal/limit constraints. In theory,
we could "fall-back" to alloc_bootmem_node() in
sparse_early_usemaps_alloc_node(), but since we actually have HOTREMOVE
defined, we'll BUG_ON() instead. A simple solution appears to be to
disable the limit check if the size of the allocation in
alloc_bootmem_secition exceeds the section size.

It makes sense to allow the usemaps to spill over to subsequent
sections instead of panicking, so FWIW:

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

That being said, it would be good if check_usemap_section_nr() printed
the cross-dependencies between pgdats and sections when the usemaps of
a node spilled over to other sections than the ones holding the pgdat.

How about this?

---
From: Johannes Weiner <hannes@cmpxchg.org>
Subject: sparsemem/bootmem: catch greater than section size allocations fix

If alloc_bootmem_section() no longer guarantees section-locality, we
need check_usemap_section_nr() to print possible cross-dependencies
between node descriptors and the usemaps allocated through it.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---

diff --git a/mm/sparse.c b/mm/sparse.c
index 61d7cde..9e032dc 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c

@@ -359,6 +359,7 @@ static void __init sparse_early_usemaps_alloc_node(unsigned long**usemap_map,
 				continue;
 			usemap_map[pnum] = usemap;
 			usemap += size;
+			check_usemap_section_nr(nodeid, usemap_map[pnum]);
 		}
 		return;
 	}

This makes sense to me -- ok if I fold it into the re-worked patch
(based upon Mel's comments)?

Sure thing!

quoted

Furthermore, I wonder if we can remove the sparse-specific stuff from
bootmem.c as well, as now even more so than before, calculating the
desired area is really none of bootmem's business.

Would something like this be okay?

---
From: Johannes Weiner <hannes@cmpxchg.org>
Subject: [patch] mm: remove sparsemem allocation details from the bootmem allocator

alloc_bootmem_section() derives allocation area constraints from the
specified sparsemem section.  This is a bit specific for a generic
memory allocator like bootmem, though, so move it over to sparsemem.

Since __alloc_bootmem_node() already retries failed allocations with
relaxed area constraints, the fallback code in sparsemem.c can be
removed and the code becomes a bit more compact overall.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

I've not tested it, but the intention seems sensible. I think it should
remain a separate change.

Yes, I agree.  I'll resend it in a bit as stand-alone patch.