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Date:	Mon, 16 Nov 2009 12:12:53 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Larry Woodman <lwoodman@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Mel Gorman <mel@....ul.ie>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Adam Litke <agl@...ibm.com>
Subject: Re: [PATCH] prevent deadlock in __unmap_hugepage_range() when
 alloc_huge_page() fails.

On Tue, 10 Nov 2009 12:00:56 -0500
Larry Woodman <lwoodman@...hat.com> wrote:

> 
> hugetlb_fault() takes the mm->page_table_lock spinlock then calls
> hugetlb_cow().  If the alloc_huge_page() in hugetlb_cow() fails due to
> an insufficient huge page pool it calls unmap_ref_private() with the
> mm->page_table_lock held.  unmap_ref_private() then calls
> unmap_hugepage_range() which tries to acquire the mm->page_table_lock.
> 
> 
> [<ffffffff810928c3>] print_circular_bug_tail+0x80/0x9f 
>  [<ffffffff8109280b>] ? check_noncircular+0xb0/0xe8
>  [<ffffffff810935e0>] __lock_acquire+0x956/0xc0e
>  [<ffffffff81093986>] lock_acquire+0xee/0x12e
>  [<ffffffff8111a7a6>] ? unmap_hugepage_range+0x3e/0x84
>  [<ffffffff8111a7a6>] ? unmap_hugepage_range+0x3e/0x84
>  [<ffffffff814c348d>] _spin_lock+0x40/0x89
>  [<ffffffff8111a7a6>] ? unmap_hugepage_range+0x3e/0x84
>  [<ffffffff8111afee>] ? alloc_huge_page+0x218/0x318
>  [<ffffffff8111a7a6>] unmap_hugepage_range+0x3e/0x84
>  [<ffffffff8111b2d0>] hugetlb_cow+0x1e2/0x3f4
>  [<ffffffff8111b935>] ? hugetlb_fault+0x453/0x4f6
>  [<ffffffff8111b962>] hugetlb_fault+0x480/0x4f6
>  [<ffffffff8111baee>] follow_hugetlb_page+0x116/0x2d9
>  [<ffffffff814c31a7>] ? _spin_unlock_irq+0x3a/0x5c
>  [<ffffffff81107b4d>] __get_user_pages+0x2a3/0x427
>  [<ffffffff81107d0f>] get_user_pages+0x3e/0x54
>  [<ffffffff81040b8b>] get_user_pages_fast+0x170/0x1b5
>  [<ffffffff81160352>] dio_get_page+0x64/0x14a
>  [<ffffffff8116112a>] __blockdev_direct_IO+0x4b7/0xb31
>  [<ffffffff8115ef91>] blkdev_direct_IO+0x58/0x6e
>  [<ffffffff8115e0a4>] ? blkdev_get_blocks+0x0/0xb8
>  [<ffffffff810ed2c5>] generic_file_aio_read+0xdd/0x528
>  [<ffffffff81219da3>] ? avc_has_perm+0x66/0x8c
>  [<ffffffff81132842>] do_sync_read+0xf5/0x146
>  [<ffffffff8107da00>] ? autoremove_wake_function+0x0/0x5a
>  [<ffffffff81211857>] ? security_file_permission+0x24/0x3a
>  [<ffffffff81132fd8>] vfs_read+0xb5/0x126
>  [<ffffffff81133f6b>] ? fget_light+0x5e/0xf8
>  [<ffffffff81133131>] sys_read+0x54/0x8c
>  [<ffffffff81011e42>] system_call_fastpath+0x16/0x1b

Confused.

That code is very old, alloc_huge_page() failures are surely common and
afaict the bug will lead to an instantly dead box.  So why haven't
people hit this bug before now?


> This can be fixed by dropping the mm->page_table_lock around the call 
> to unmap_ref_private() if alloc_huge_page() fails, its dropped right below
> in the normal path anyway:
> 

Why is that safe?  What is the page_table_lock protecting in here?

> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 5d7601b..f4daef4 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1973,12 +1973,15 @@ retry_avoidcopy:
>                  */
>                 if (outside_reserve) {
>                         BUG_ON(huge_pte_none(pte));
> +                       spin_unlock(&mm->page_table_lock);
>                         if (unmap_ref_private(mm, vma, old_page, address)) {
> +                               spin_lock(&mm->page_table_lock);
>                                 BUG_ON(page_count(old_page) != 1);
>                                 BUG_ON(huge_pte_none(pte));
>                                 goto retry_avoidcopy;
>                         }
>                         WARN_ON_ONCE(1);
> +                       spin_lock(&mm->page_table_lock);
>                 }
> 
>                 return -PTR_ERR(new_page);
> 
> 
> Signed-off-by: Larry Woodman <lwooman@...hat.com>

Patch is somewhat mucked up.  The s-o-b goes at the end of the
changelog.  And please don't include two copies of the same patch in
the one email.

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