linux-kernel - Re: kernel 3.0: BUG: soft lockup: find_get

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Date:	Fri, 21 Oct 2011 14:44:15 +0200
From:	Mel Gorman <mgorman@...e.de>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Pawel Sikora <pluto@...k.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	linux-mm@...r.kernel.org, jpiszcz@...idpixels.com,
	arekm@...-linux.org, linux-kernel@...r.kernel.org
Subject: Re: kernel 3.0: BUG: soft lockup: find_get_pages+0x51/0x110

On Wed, Oct 19, 2011 at 09:30:36AM +0200, Mel Gorman wrote:
> >  RIP: 0010:[<ffffffff81127b76>]  [<ffffffff81127b76>]
> >                        migration_entry_wait+0x156/0x160
> >   [<ffffffff811016a1>] handle_pte_fault+0xae1/0xaf0
> >   [<ffffffff810feee2>] ? __pte_alloc+0x42/0x120
> >   [<ffffffff8112c26b>] ? do_huge_pmd_anonymous_page+0xab/0x310
> >   [<ffffffff81102a31>] handle_mm_fault+0x181/0x310
> >   [<ffffffff81106097>] ? vma_adjust+0x537/0x570
> >   [<ffffffff81424bed>] do_page_fault+0x11d/0x4e0
> >   [<ffffffff81109a05>] ? do_mremap+0x2d5/0x570
> >   [<ffffffff81421d5f>] page_fault+0x1f/0x30
> > 
> > mremap's down_write of mmap_sem, together with i_mmap_mutex/lock,
> > and pagetable locks, were good enough before page migration (with its
> > requirement that every migration entry be found) came in; and enough
> > while migration always held mmap_sem.  But not enough nowadays, when
> > there's memory hotremove and compaction: anon_vma lock is also needed,
> > to make sure a migration entry is not dodging around behind our back.
> > 
> 
> migration holds the anon_vma lock while it unmaps the pages and keeps holding
> it until after remove_migration_ptes is called. 

I reread this today and realised I was sloppy with my writing. migration
holds the anon_vma lock while it unmaps the pages. It also holds the
anon_vma lock during remove_migration_ptes. For the migration operation,
a reference count is held on anon_vma but not the lock itself.

> There are two anon vmas
> that should exist during mremap that were created for the move. They
> should not be able to disappear while migration runs and right now,

And what is preventing them disappearing is not the lock but the
reference count.
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