Re: [PATCHv4 00/25] THP-enabled tmpfs/shmem

[Kirill A. Shutemov]

On Thu, Mar 24, 2016 at 12:08:55PM -0700, Hugh Dickins wrote:
> On Thu, 24 Mar 2016, Kirill A. Shutemov wrote:
> > On Wed, Mar 23, 2016 at 01:09:05PM -0700, Hugh Dickins wrote:
> > > The small files thing formed my first impression.  My second
> > > impression was similar, when I tried mmap(NULL, size_of_RAM,
> > > PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_SHARED, -1, 0) and
> > > cycled around the arena touching all the pages (which of
> > > course has to push a little into swap): that soon OOMed.
> > > 
> > > But there I think you probably just have some minor bug to be fixed:
> > > I spent a little while trying to debug it, but then decided I'd
> > > better get back to writing to you.  I didn't really understand what
> > > I was seeing, but when I hacked some stats into shrink_page_list(),
> > > converting !is_page_cache_freeable(page) to page_cache_references(page)
> > > to return the difference instead of the bool, a large proportion of
> > > huge tmpfs pages seemed to have count 1 too high to be freeable at
> > > that point (and one huge tmpfs page had a count of 3477).
> > 
> > I'll reply to your other points later, but first I wanted to address this
> > obvious bug.
> 
> Thanks.  That works better, but is not yet right: memory isn't freed
> as it should be, so when I exit then try to run a second time, the
> mmap() just gets ENOMEM (with /proc/sys/vm/overcommit_memory 0):
> MemFree is low.  No rush to fix, I've other stuff to do.
> 
> I don't get as far as that on the laptop, since the first run is OOM
> killed while swapping; but I can't vouch for the OOM-kill-correctness
> of the base tree I'm using, and this laptop has a history of OOMing
> rather too easily if all's not right.

Hm. I don't see the issue.

I tried to reproduce it in my VM with following script:

#!/bin/sh -efu

swapon -a

ram="$(grep MemTotal /proc/meminfo | sed 's,[^0-9\]\+,,; s, kB,k,')"

usemem -w -f /dev/zero "$ram"

swapoff -a
swapon -a

usemem -w -f /dev/zero "$ram"

cat /proc/meminfo
grep thp /proc/vmstat

-----

usemem is a tool from this archive:

http://www.spinics.net/lists/linux-mm/attachments/gtarazbJaHPaAT.gtar

It works fine even if would double size of mapping.

Do you have a reproducer?

-- 
 Kirill A. Shutemov