linux-kernel - Re: [PATCH 8/9] memory-hotplug: enable memory hotplug to handle hugepage

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Date:	Mon, 25 Feb 2013 11:57:56 -0500
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Hillf Danton <dhillf@...il.com>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH 8/9] memory-hotplug: enable memory hotplug to handle
 hugepage

Hi Hillf,

On Sat, Feb 23, 2013 at 03:05:30PM +0800, Hillf Danton wrote:
> Hello Naoya
> 
> [add Michal in cc list]
> 
> On Fri, Feb 22, 2013 at 3:41 AM, Naoya Horiguchi
> <n-horiguchi@...jp.nec.com> wrote:
> >
> > +/* Returns true for head pages of in-use hugepages, otherwise returns false. */
> > +int is_hugepage_movable(struct page *hpage)
> s/int/bool/  can we?

Yes, we can. I'll do this.

> > +{
> > +       struct page *page;
> > +       struct page *tmp;
> > +       struct hstate *h = page_hstate(hpage);
> Make sense to compute hstate for a tail page?

No need to do this here.
It's better to put it after PageTail check.

> > +       int ret = 0;
> > +
> > +       VM_BUG_ON(!PageHuge(hpage));
> > +       if (PageTail(hpage))
> > +               return 0;
> VM_BUG_ON(!PageHuge(hpage) || PageTail(hpage)), can we?

I think that firing BUG_ON() for tail pages is overkill.
Pfn range over which scan_movable_pages() runs could start
at the pfn inside the hugepage when we try to hot-remove
the memory block used by 1GB hugepage. In that case,
is_hugepage_movable() can be called for tail pages as a
normal behavior.

But anyway, I'll add the comment for this corner case.

> > +       spin_lock(&hugetlb_lock);
> > +       list_for_each_entry_safe(page, tmp, &h->hugepage_activelist, lru)
> s/_safe//  can we?

OK.

> > +               if (page == hpage)
> > +                       ret = 1;
> Can we bail out with ret set to be true?

Yes, inserting break is good for performance.

> > +       spin_unlock(&hugetlb_lock);
> > +       return ret;
> > +}

Thank you!
Naoya
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