linux-kernel - Re: [PATCH] trim memory not covered by WB MTRRs

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Date:	Wed, 6 Jun 2007 17:53:04 -0400 (EDT)
From:	Justin Piszcz <jpiszcz@...idpixels.com>
To:	Jesse Barnes <jesse.barnes@...el.com>
cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] trim memory not covered by WB MTRRs



On Wed, 6 Jun 2007, Jesse Barnes wrote:

> On some machines, buggy BIOSes don't properly setup WB MTRRs to
> cover all available RAM, meaning the last few megs (or even gigs)
> of memory will be marked uncached.  Since Linux tends to allocate
> from high memory addresses first, this causes the machine to be
> unusably slow as soon as the kernel starts really using memory
> (i.e. right around init time).
>
> This patch works around the problem by scanning the MTRRs at
> boot and figuring out whether the current end_pfn value (setup
> by early e820 code) goes beyond the highest WB MTRR range, and
> if so, trimming it to match.  A fairly obnoxious KERN_WARNING
> is printed too, letting the user know that not all of their
> memory is available due to a likely BIOS bug.
>
> Something similar could be done on i386 if needed, but the boot
> ordering would be slightly different, since the MTRR code on i386
> depends on the boot_cpu_data structure being setup.
>
> Justin, can you please test and make sure this patch works for
> you too?  It'll only work around the problem, but it's better
> than having to do mem= by hand or waiting for a fix from your
> BIOS vendor.
>
> Thanks,
> Jesse

Jesse, it worked.

With mem=8832M (without your patch): 2.6.22-rc4:

top - 17:39:02 up 1 day,  8:07, 25 users,  load average: 2.33, 0.76, 0.30
Tasks: 325 total,  11 running, 314 sleeping,   0 stopped,   0 zombie
Cpu(s): 80.0%us, 20.0%sy,  0.0%ni,  0.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   8039620k total,  7936472k used,   103148k free,      708k buffers
Swap: 16787768k total,      128k used, 16787640k free,  6646248k cached

With no mem= in append line (with your patch): 2.6.22-rc4:

top - 17:44:01 up 1 min,  1 user,  load average: 0.97, 0.25, 0.08
Tasks: 145 total,   1 running, 144 sleeping,   0 stopped,   0 zombie
Cpu(s):  5.2%us,  3.0%sy,  1.2%ni, 86.8%id,  3.8%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   8039608k total,   969380k used,  7070228k free,     1232k buffers
Swap: 16787768k total,        0k used, 16787768k free,   109448k cached

Odd, remote netconsole did not capture the dmesg the E820 memory map.

Jun  6 17:43:03 p34 [   53.598611] ahci 0000:00:1f.2: AHCI 0001.0100 32 slots 6 ports 3 Gbps 0x3f impl SATA mode 
Jun  6 17:43:03 p34 [   53.598683] ahci 0000:00:1f.2: flags: 64bit ncq led clo pio slum part 
Jun  6 17:43:03 p34 [   53.598986] scsi0 : ahci 
Jun  6 17:43:03 p34 [   53.599131] scsi1 : ahci 
Jun  6 17:43:03 p34 [   53.599239] scsi2 : ahci 
Jun  6 17:43:03 p34 [   53.599340] scsi3 : ahci 
Jun  6 17:43:03 p34 [   53.599438] scsi4 : ahci

I will run with this patch for a while but so far, no issues, everything 
looks great.

Will it make it into 2.6.22-rc5? :)

Justin.
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