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

From: Hugh Dickins
Date: Thu Mar 24 2016 - 15:09:17 EST

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,
> > 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.