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Date: Sat, 22 Jun 2013 07:56:26 +0900
Message-ID: <8738sb9ip1.wl%satoru.takeuchi@gmail.com>
From: Satoru Takeuchi <satoru.takeuchi@gmail.com>
To: Michal Hocko <mhocko@suse.cz>
Cc: Satoru Takeuchi <satoru.takeuchi@gmail.com>,
        Greg Kroah-Hartman <greg@kroah.com>, linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Rik van Riel <riel@redhat.com>, Wanpeng Li <liwanp@linux.vnet.ibm.com>,
        Mel Gorman <mgorman@suse.de>, Andi Kleen <andi@firstfloor.org>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [ 29/48] mm: migration: add migrate_entry_wait_huge()
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At Fri, 21 Jun 2013 14:47:27 +0200,
Michal Hocko wrote:
> 
> On Thu 20-06-13 18:52:43, Satoru Takeuchi wrote:
> > Hi Naoya,
> > 
> > At Tue, 18 Jun 2013 09:17:55 -0700,
> > Greg Kroah-Hartman wrote:
> > > 
> > > From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > 
> > > 3.9-stable review patch.  If anyone has any objections, please let me know.
> > > 
> > > ------------------
> > > 
> > > From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > > 
> > > commit 30dad30922ccc733cfdbfe232090cf674dc374dc upstream.
> > > 
> > > When we have a page fault for the address which is backed by a hugepage
> > > under migration, the kernel can't wait correctly and do busy looping on
> > > hugepage fault until the migration finishes.  As a result, users who try
> > > to kick hugepage migration (via soft offlining, for example) occasionally
> > > experience long delay or soft lockup.
> > > 
> > > This is because pte_offset_map_lock() can't get a correct migration entry
> > > or a correct page table lock for hugepage.  This patch introduces
> > > migration_entry_wait_huge() to solve this.
> > 
> > I suspect that this code doesn't work correctly on i686 box with CONFIG_HIGHPTE.
> > If we call hugetlb_fault() -> migration_entry_wait_huge() -> __migration_entry_wait(),
> > this function tries to kunmap pte, in this case pte is not-kmapped pmd, via pte_unmap_unlock().
> > If CONFIG_DEBUG_HIGHMEM is also enabled, it results in BUG_ON() at __kunmap_atomic().
> > 
> > Correct me if I'm wrong.
> 
> I haven't checked the code closer but the patch doesn't change anything
> regarding pte_unmap_unlock. The only thing that it touches is the
> _locking_. So whether there ever was a problem with kmap or not this
> patch doesn't change it.
> 
> That being said, the patch is OK for the stable tree.

OK, I see. This is a different problem from this patch. If I'll find a real problem
with kmap, I'll report it as the different problem.

Thanks,
Satoru