[PATCH] percpu, x86: don't use PMD_SIZE as embedded atom_size on32bit

[Tejun Heo]

With the embed percpu first chunk allocator, x86 uses either PAGE_SIZE
or PMD_SIZE for atom_size.  PMD_SIZE is used when CPU supports PSE so
that percpu areas are aligned to PMD mappings and possibly allow using
PMD mappings in vmalloc areas in the future.  Using larger atom_size
doesn't waste actual memory; however, it does require larger vmalloc
space allocation later on for !first chunks.

With reasonably sized vmalloc area, PMD_SIZE shouldn't be a problem
but x86_32 at this point is anything but reasonable in terms of
address space and using later atom_size reportedly leads to frequent
percpu allocation failures on certain setups.

This patch makes x86_32 always use PAGE_SIZE as atom_size for embed
first chunk allocator.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Yanmin Zhang <yanmin.zhang@xxxxxxxxx>
Reported-by: ShuoX Liu <shuox.liu@xxxxxxxxx>
LKML-Reference: <4F97BA98.6010001@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
Can you please verify this resolves the issue you guys are seeing?
Once verified, I'll push it through percpu/for-3.5-fixes.

Thanks.

 arch/x86/kernel/setup_percpu.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
index 71f4727..ed96573 100644
--- a/arch/x86/kernel/setup_percpu.c
+++ b/arch/x86/kernel/setup_percpu.c
@@ -185,10 +185,22 @@ void __init setup_per_cpu_areas(void)
 #endif
 	rc = -EINVAL;
 	if (pcpu_chosen_fc != PCPU_FC_PAGE) {
-		const size_t atom_size = cpu_has_pse ? PMD_SIZE : PAGE_SIZE;
 		const size_t dyn_size = PERCPU_MODULE_RESERVE +
 			PERCPU_DYNAMIC_RESERVE - PERCPU_FIRST_CHUNK_RESERVE;
+		size_t atom_size;
 
+		/*
+		 * If PSE is available, use PMD_SIZE for atom_size so that
+		 * embedded percpu areas are aligned to PMD.  This, in the
+		 * future, can also allow using PMD mappings in vmalloc
+		 * area.  Use PAGE_SIZE on 32bit as vmalloc space is highly
+		 * contended and large vmalloc area alloc can easily fail.
+		 */
+		atom_size = PAGE_SIZE;
+#ifdef CONFIG_X86_64
+		if (cpu_has_pse)
+			atom_size = PMD_SIZE;
+#endif
 		rc = pcpu_embed_first_chunk(PERCPU_FIRST_CHUNK_RESERVE,
 					    dyn_size, atom_size,
 					    pcpu_cpu_distance,
--
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