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

[Christoffer Dall]

Hi Andrea,

On Mon, May 25, 2015 at 04:15:25PM +0200, Andrea Arcangeli wrote:
> Hello Christoffer,
> 
> 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.
> 
> As further confirmation, could you try:
> 
> echo 0 > /sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan

this returns -EINVAL.

But I'm trying now with:

echo never > /sys/kernel/mm/transparent_hugepage/enabled

> 
> and verify the problem goes away without having to revert the patch?

will let you know, so far so good...

> 
> Accordingly you should reproduce much eaiser this way (setting
> $largevalue to 8192 or something, it doesn't matter).
> 
> echo $largevalue > /sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan
> echo 0 > /sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs
> echo 0 > /sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs
> 
> Then push the system into swap with some memhog -r1000 xG.

what is memhog?  I couldn't find the utility in Google...

I did try with the above settings and just push a bunch of data into
ramfs and tmpfs and indeed the sytem died very quickly (on v4.0-rc4).

> 
> The patch just allows readonly anon pages to be collapsed along with
> read-write ones, the vma permissions allows it, so they have to be
> swapcache pages, this is why swap shall be required.
> 
> Perhaps there's some arch detail that needs fixing but it'll be easier
> to track it down once you have a way to reproduce fast.
> 
Yes, would be great to be able to reproduce quickly.

Thanks,
-Christoffer