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

[Greg Kroah-Hartman]

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?

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

thanks,

greg k-h
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