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percpu allocator

PATCH New Tejun Heo 1 participant 2 messages Apr 27, 2012 1 message not in the archive

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Memory management Per-CPU memory allocator
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A message that is not in the archive (sent off-list or not yet received): [email protected]

Tejun Heo

author maintainer
#1 lore raw

Hello,

Quoted 25 lines · Pavel V. Panteleev

I have the following problem with pcpu embed allocator in kernel 2.6.33.1. pcpu_embed_first_chunk() function allocate only size_sum = (ai->static_size + ai->reserved_size + ai->dyn_size) for an unit in the group. So, for 4 groups (1 unit in each) and with memory only on the first node I have the following:

pcpu_embed_first_chunk(): ai->groups[0].base_offset=0x0 pcpu_embed_first_chunk(): ai->groups[1].base_offset=0xa000 pcpu_embed_first_chunk(): ai->groups[2].base_offset=0x14000 pcpu_embed_first_chunk(): ai->groups[3].base_offset=0x1e000

pcpu_embed_first_chunk(): ai->unit_size=0x10000

It means, that for each group memory of size_sum=0xa000 is used only. So, in the case of memory only on the first node, memory for the following group will be allocated near the memory of the previous group. Even though memory of size_sum=0xa000 is used only, but ai->unit_size=0x10000.

After filling group_offsets and group_sizes in pcpu_setup_first_chunk() function we have, that (group_offsets[i] + group_sizes[i]) can be larger than group_offsets[i+1]. But in pcpu_get_vm_areas() function there is checker, which tell us, that such situation is impossible:

Quoted 3 lines

BUG_ON(start2 >= start && start2 < end);

May be I should not use embed allocator in such situation?

Nice catch. pcpu_embed_first_chunk() allocates full unit and then free whatever is unused (for alignment, IIRC) before proceeding to the next group. What it should do is first allocate and prepare all groups and then free whatever is unused. I'll write up a patch.

Thanks.

Signature · 1 line
tejun
author maintainer
#2 lore raw

PATCH 1/1 percpu: pcpu_embed_first_chunk() should free unused parts after all allocs are complete

pcpu_embed_first_chunk() allocates memory for each node, copies percpu data and frees unused portions of it before proceeding to the next group. This assumes that allocations for different nodes doesn't overlap; however, depending on memory topology, the bootmem allocator may end up allocating memory from a different node than the requested one which may overlap with the portion freed from one of the previous percpu areas. This leads to percpu groups for different nodes overlapping which is a serious bug.

This patch separates out copy & partial free from the allocation loop such that all allocations are complete before partial frees happen.

This also fixes overlapping frees which could happen on allocation failure path - out_free_areas path frees whole groups but the groups could have portions freed at that point.

Signed-off-by Tejun Heo <[email protected]>
Cc [email protected]
Reported-by "Pavel V. Panteleev" <[email protected]>

LKML-Reference: <[email protected]>
---
Can you please verify this patch fixes the problem?

Thanks.

 mm/percpu.c |   10 ++++++++++
 1 file changed, 10 insertions(+)
mm/percpu.c +10 -0
index f47af91..7975693 100644
@@ -1650,6 +1650,16 @@ int __init pcpu_embed_first_chunk(size_t reserved_size, size_t dyn_size,
 		areas[group] = ptr;
 
 		base = min(ptr, base);
+	}
+
+	/*
+	 * Copy data and free unused parts.  This should happen after all
+	 * allocations are complete; otherwise, we may end up with
+	 * overlapping groups.
+	 */
+	for (group = 0; group < ai->nr_groups; group++) {
+		struct pcpu_group_info *gi = &ai->groups[group];
+		void *ptr = areas[group];
 
 		for (i = 0; i < gi->nr_units; i++, ptr += ai->unit_size) {
 			if (gi->cpu_map[i] == NR_CPUS) {