Re: [ 29/48] mm: migration: add migrate_entry_wait_huge()

[Satoru Takeuchi]

At Thu, 20 Jun 2013 10:02:13 -0700,
Greg Kroah-Hartman wrote:
> 
> On Thu, Jun 20, 2013 at 06:52:43PM +0900, Satoru Takeuchi wrote:
> > Hi Naoya,
> > 
> > At Tue, 18 Jun 2013 09:17:55 -0700,
> > Greg Kroah-Hartman wrote:
> > > 
> > > From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > 
> > > 3.9-stable review patch.  If anyone has any objections, please let me know.
> > > 
> > > ------------------
> > > 
> > > From: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> > > 
> > > 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().
> 
> Have you tried this?

Not yet. I'm now preparing the kernel to reproduce this problem.

> 
> Also, the same issue is still in 3.10-rc6, right?

Yes.

Thanks,
Satoru
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