Image caching/swap bugs

Image caching/swap bugs

[Whitney, John]

Given the following situation, I am seeing my embedded linux system on an
MPC860 crash and was
wondering if anyone else has experienced the same problem (and possibly have
a fix):

I have a 2.2.15 kernel running on a proprietary board with 32MB of RAM.  The
image includes a 16MB
ramdisk distribution I have created from scratch.  Periodically, after
uploading a new 4MB image to the
running system (via FTP), and burning into onboard flash, the system would
crash.

After looking into it, I found then when I started copying the image from
the ramdisk to flash, the kernel was caching
it in memory ("MemFree" went down, "Cached" went up).  When the kernel hit
the low freepages mark, kswapd was
kicked off to recover memory, and immediately thereafter the kernel crashed
(in various tasks, usually with
*NIP being 0x00000000).  (This is easily reproduced by copying the image to
/dev/null, eliminating the flash
driver as a suspect).

I eventually found out that something in try_to_swap_out() (mm/vmscan.c) was
causing this.  If I put an
immediate "return 0;" in this routine kswapd recovered nicely.  When the
routine was allowed to execute,
the kernel crashed.  I don't know if something is being incorrectly
overwritten in memory, or the kernel is
just flushing parts of different tasks' images and this isn't being handled
correctly.

I've noticed that if I ifdef out the "if (!pte_dirty(pte)) { ... }" section
of code in try_to_swap_out(), I again don't
crash.  Unfortunately, I am not (yet) a VM expert, so I am not sure what
this code is doing.

In the short term, I can prevent kswapd from attempting to swap out
processes (since I have no swap space
anyway) and run just fine, but it would be nice to have a "correct"
solution.  I just wanted to know if anyone
else had experienced this problem before I go delving into it (whenever I
get the time).

John Whitney
johnw@aiinet.com

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Re: Image caching/swap bugs

[Dan Malek]

Whitney, John wrote:

> I have a 2.2.15 kernel running on a proprietary board


You shouldn't be using 2.2.15 with 8xx.  I am not going to discuss
the history right now, but the only reliable verion of 2.2.xx Linux
is the 2.2.13 version on the MontaVista Web site (www.mvista.com).
This is the only one I am maintaining, have patched and verified it
works.  I am updating this to 2.2.14, only to match the rest of
the MontaVista packages so it can be built and tested along with
other platforms.  If there are some specific features of later
releases you think you need (and I don't know of any that affect 8xx),
please let me know.



	-- Dan

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Query: kernel version for MPC750/MPC7400

[Tom Roberts]

Dan Malek wrote:
> You shouldn't be using 2.2.15 with 8xx.  I am not going to discuss
> the history right now, but the only reliable verion of 2.2.xx Linux
> is the 2.2.13 version on the MontaVista Web site (www.mvista.com).

How about on an embedded MPC750 or MPC7400? Before joining this
list I discovered Kernel version 2.2.15 on Yellowdog and have been
using that. I have made a dozen or so small mods to the kernel to
get it to run on our boards, and have seen no problems, so far....
There are zillions of references to CONFIG_8xx throughout the source.

	These boards are essentially embedded supercomputers;
	the only devices they have are the drivers I wrote, which
	use memory buffers to/from the host PCI as a 'serial'
	console and a network interface.


Tom Roberts	tjroberts@lucent.com

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Re: Query: kernel version for MPC750/MPC7400

[Dan Malek]

Tom Roberts wrote:

> How about on an embedded MPC750 or MPC7400? Before joining this
> list I discovered Kernel version 2.2.15 on Yellowdog and have been
> using that.

That should work.  The question was directed toward 8xx processors,
so I thought I would answer that.

> There are zillions of references to CONFIG_8xx throughout the source.

All old, broken, out of date.  All of the updates in the "stand-alone"
8xx kernels were too much to be allowed into the "official" kernel
sources.  With 2.4.xx coming, I think I will just make that one
work and not fight to get the changes into 2.2.


	-- Dan

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Re: Query: kernel version for MPC750/MPC7400

[Thomas Poole]

Tom Roberts wrote:

> Dan Malek wrote:
> > You shouldn't be using 2.2.15 with 8xx.  I am not going to discuss
> > the history right now, but the only reliable verion of 2.2.xx Linux
> > is the 2.2.13 version on the MontaVista Web site (www.mvista.com).
>
> How about on an embedded MPC750 or MPC7400?

Debian works out-of-the-box for the MPC750. I didn't do the install so I'm not
sure what kernel it's using.

>
> Tom Roberts     tjroberts@lucent.com

Lucnet? I'm not sure I should have told you that. :)


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