linux-kernel - Re: [RFC PATCH] mm,memory_hotplug: Unlock 1GB-hugetlb on x86

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Date:   Wed, 27 Feb 2019 14:00:57 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Oscar Salvador <osalvador@...e.de>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, mhocko@...e.com, david@...hat.com
Subject: Re: [RFC PATCH] mm,memory_hotplug: Unlock 1GB-hugetlb on x86_64

On 2/27/19 1:51 PM, Oscar Salvador wrote:
> On Thu, Feb 21, 2019 at 10:42:12AM +0100, Oscar Salvador wrote:
>> [1] https://lore.kernel.org/patchwork/patch/998796/
>>
>> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> 
> Any further comments on this?
> I do have a "concern" I would like to sort out before dropping the RFC:
> 
> It is the fact that unless we have spare gigantic pages in other notes, the
> offlining operation will loop forever (until the customer cancels the operation).
> While I do not really like that, I do think that memory offlining should be done
> with some sanity, and the administrator should know in advance if the system is going
> to be able to keep up with the memory pressure, aka: make sure we got what we need in
> order to make the offlining operation to succeed.
> That translates to be sure that we have spare gigantic pages and other nodes
> can take them.
> 
> Given said that, another thing I thought about is that we could check if we have
> spare gigantic pages at has_unmovable_pages() time.
> Something like checking "h->free_huge_pages - h->resv_huge_pages > 0", and if it
> turns out that we do not have gigantic pages anywhere, just return as we have
> non-movable pages.

Of course, that check would be racy.  Even if there is an available gigantic
page at has_unmovable_pages() time there is no guarantee it will be there when
we want to allocate/use it.  But, you would at least catch 'most' cases of
looping forever.

> But I would rather not convulate has_unmovable_pages() with such checks and "trust"
> the administrator.

Agree
-- 
Mike Kravetz