Date: Tue, 31 Oct 2000 06:07:58 -0800 (PST) From: carp@world.std.com To: freebsd-gnats-submit@FreeBSD.org Subject: i386/22441: pmap_growkernel() is not effective at kernel vm limit of 0xffc00000 Message-ID: <20001031140758.4943437B4C5@hub.freebsd.org>
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>Number: 22441 >Category: i386 >Synopsis: pmap_growkernel() is not effective at kernel vm limit of 0xffc00000 >Confidential: no >Severity: serious >Priority: low >Responsible: freebsd-bugs >State: open >Quarter: >Keywords: >Date-Required: >Class: sw-bug >Submitter-Id: current-users >Arrival-Date: Tue Oct 31 06:10:01 PST 2000 >Closed-Date: >Last-Modified: >Originator: Bill Carpenter >Release: FreeBSD 4.1.1-RELEASE >Organization: >Environment: FreeBSD buffoon.eng.infolibria.com 4.1.1-RELEASE FreeBSD 4.1.1-RELEASE #0: Tue Sep 26 00:46:59 GMT 2000 jkh@narf.osd.bsdi.com:/usr/src/sys/compile/GENERIC i386 >Description: When called with a kernel vm limit of 0xffc00000, pamp_growkernel() does not set-up the page mapping hardware because of an overflow. In this line: addr = (addr + PAGE_SIZE * NPTEPG) & ~(PAGE_SIZE * NPTEPG - 1); ( Line 1403 in $FreeBSD: src/sys/i386/i386/pmap.c,v 1.250.2.5 2000/08/05 00:39:08 peter Exp $) addr + PAGE_SIZE * NPTEPG overflows to zero when addr is 0xffc00000. When addr is zero, the conditional in the following while loop is never satisfied and the hardware page mapping data structures are never initialized to cover the extended kernel vm. I would also like to suggest that test of kernel_vm_end in vm_map_findspace() (src/sys/vm/vm_map.c) is a minor software layering violation. kernel_vm_end is properly a pmap layer variable and the test against kernel_vm_end should occur in pmap_growkernel(). I realize that making this change might appear to impede performance, but pmap_growkernel() only gets called during kernel initialization when the kernel is setting up sub-maps of the kernel map. (kernel_map) >How-To-Repeat: I am writing a driver that creates kernel vm up against the limit of kernel vm and therefore requests that the kernel vm grow to 0xffc00000. I could submit a sample of the driver, but paper and pencil should convince the reader that when PAGE_SIZE is 4096 and NPTEPG is 1024, 0xffc00000 + (4096 * 1024) overflows to zero. >Fix: In line 1403, I used a standard rounding macro and all appears well. For instance, I have compiled and tested the kernel with: addr = roundup(addr, (PAGE_SIZE*NPTEPG)); Now that I got all of this typed, it appears that the kernel is being extended an extra 4Mbytes whenever the limit submitted to pmap_growkernel() falls on a 4Mbyte (i386 page directory) boundary. The kernel, correctly, rounds up its vm limit to the next page and in most cases, the boundary submitted to pmap_growkernel() falls on a page boundary. >Release-Note: >Audit-Trail: >Unformatted: To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-bugs" in the body of the messagehome | help
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