Re: Guaranteed allocation of huge pages (1G) using movablecore=N doesn't seem to work at all

[Michal Hocko <mhocko@kernel.org>]

On Mon 06-11-17 19:36:38, Maxim Levitsky wrote:
> Isn't this a non backward compatible change? Why to remove an optional non
> default option for use cases like mine.

Well, strictly speaking it is. The reality is that with the current
implementation the option breaks the hotplug usecase. I can see your
argument about the opt in and we might need to hold on this patch for
merging uut it seems that http://lkml.kernel.org/r/20171003072619.8654-1-mhocko@kernel.org
is not the problem you are seeing.

> I won't argue with you on this, but my question was different, and was why
> the kernel can't move other pages from moveable zone in my case.

OK, I have re-read your original report where you say
: This was tested on 4.14.0-rc5 (my custom compiled) and on several
: older kernels (4.10,4.12,4.13) from ubuntu repositories.

Does that mean that this a new regression in 4.14-rc5 or you see the
problem in other kernels too?

If this a new rc5 thing then 79b63f12abcb ("mm, hugetlb: do not
allocate non-migrateable gigantic pages from movable zones") might be
related. Although it shouldn't if hugepages_treat_as_movable is enabled.

I wouldn't be all that surprised if this was an older issue, though. If
I look at pfn_range_valid_gigantic it seems that the page count check
makes it just too easy to fail even on migratable memory. To be honest
I consider the giga pages runtime support rather fragile and that is
why I wasn't very much afraid to remove hacks that allow breaking other
usecases.
-- 
Michal Hocko
SUSE Labs

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