linux-kernel - CONFIG_PHYSICAL_ALIGN limited to 4M?

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Date:	Tue, 02 Jan 2007 12:05:18 +0100
From:	Rene Herman <rene.herman@...il.com>
To:	Vivek Goyal <vgoyal@...ibm.com>
CC:	Andi Kleen <ak@...e.de>, Linus Torvalds <torvalds@...l.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: CONFIG_PHYSICAL_ALIGN limited to 4M?

Good day.

A while ago it was remarked on list here that keeping the kernel 4M 
aligned physically might be a performance win if the added 1M (it 
normally loads at 1M) meant it would fit on one 4M aligned hugepage 
instead of 2 and since that time I've been doing such.

In fact, while I was at it, I ran the kernel at 16M; while admittedly a 
bit of a non-issue, having never experienced ZONE_DMA shortage, I am an 
ISA user on a >16M machine so this seemed to make sense -- no kernel 
eating up "precious" ISA-DMAable memory.

Recently CONFIG_PHYSICAL_START was replaced by CONFIG_PHYSICAL_ALIGN 
(commit e69f202d0a1419219198566e1c22218a5c71a9a6) and while 4M alignment 
is still possible, that's also the strictest alignment allowed meaning I 
can't load my (non-relocatable) kernel at 16M anymore.

If I just apply the following and set it to 16M, things seem to be 
working for me. Was there an important reason to limit the alignment to 
4M, and if so, even on non relocatable kernels?

Rene.

View attachment "config_physical_align_16m.diff" of type "text/plain" (462 bytes)