Subject: Re: [patch 1/3] powerpc: Move 64bit VDSO to improve context switch performance

From: Andreas Schwab <[email-masked]>
Date: Sun, 04 Oct 2009 14:35:41 +0200

Benjamin Herrenschmidt <[email-masked]> writes:

> Maybe a better fix is to force alignment in the kernel by requesting
> size + 64k - 4k and aligning it.

Sure you are right.

Andreas.

>>From b9441a3d2148d439e2730def3222a7b70dccc432 Mon Sep 17 00:00:00 2001
From: Andreas Schwab <[email-masked]>
Date: Sun, 4 Oct 2009 14:29:04 +0200
Subject: [PATCH] powerpc: align vDSO base address

Signed-off-by: Andreas Schwab <[email-masked]>
---
 arch/powerpc/kernel/vdso.c |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/arch/powerpc/kernel/vdso.c b/arch/powerpc/kernel/vdso.c
index 94e2df3..137dc22 100644
--- a/arch/powerpc/kernel/vdso.c
+++ b/arch/powerpc/kernel/vdso.c
@@ -50,6 +50,9 @@
 /* Max supported size for symbol names */
 #define MAX_SYMNAME	64
 
+/* The alignment of the vDSO */
+#define VDSO_ALIGNMENT	(1 << 16)
+
 extern char vdso32_start, vdso32_end;
 static void *vdso32_kbase = &vdso32_start;
 static unsigned int vdso32_pages;
@@ -231,15 +234,21 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 	 * pick a base address for the vDSO in process space. We try to put it
 	 * at vdso_base which is the "natural" base for it, but we might fail
 	 * and end up putting it elsewhere.
+	 * Add enough to the size so that the result can be aligned.
 	 */
 	down_write(&mm->mmap_sem);
 	vdso_base = get_unmapped_area(NULL, vdso_base,
-				      vdso_pages << PAGE_SHIFT, 0, 0);
+				      (vdso_pages << PAGE_SHIFT) +
+				      ((VDSO_ALIGNMENT - 1) & PAGE_MASK),
+				      0, 0);
 	if (IS_ERR_VALUE(vdso_base)) {
 		rc = vdso_base;
 		goto fail_mmapsem;
 	}
 
+	/* Add required alignment. */
+	vdso_base = ALIGN(vdso_base, VDSO_ALIGNMENT);
+
 	/*
 	 * Put vDSO base into mm struct. We need to do this before calling
 	 * install_special_mapping or the perf counter mmap tracking code
-- 
1.6.4.4

-- 
Andreas Schwab, [email-masked]
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"And now for something completely different."