[BUG] Read-Only THP causes stalls (commit 10359213d)

[Kirill A. Shutemov]

On Sun, May 24, 2015 at 09:34:04PM +0200, Christoffer Dall wrote:
> Hi all,
> 
> I noticed a regression on my arm64 APM X-Gene system a couple
> of weeks back.  I would occassionally see the system lock up and see RCU
> stalls during the caching phase of kernbench.  I then wrote a small
> script that does nothing but cache the files
> (http://paste.ubuntu.com/11324767/) and ran that in a loop.  On a known
> bad commit (v4.1-rc2), out of 25 boots, I never saw it get past 21
> iterations of the loop.  I have since tried to run a bisect from v3.19 to
> v4.0 using 100 iterations as my criteria for a good commit.
> 
> This resulted in the following first bad commit:
> 
> 10359213d05acf804558bda7cc9b8422a828d1cd
> (mm: incorporate read-only pages into transparent huge pages, 2015-02-11)
> 
> Indeed, running the workload on v4.1-rc4 still produced the behavior,
> but reverting the above commit gets me through 100 iterations of the
> loop.
> 
> I have not tried to reproduce on an x86 system.  Turning on a bunch
> of kernel debugging features *seems* to hide the problem.  My config for
> the XGene system is defconfig + CONFIG_BRIDGE and
> CONFIG_POWER_RESET_XGENE.
> 
> Please let me know if I can help test patches or other things I can
> do to help.  I'm afraid that by simply reading the patch I didn't see
> anything obviously wrong with it which would cause this behavior.

I don't see the problem on x86.

Some backtraces could help to track it down.

-- 
 Kirill A. Shutemov