Re: [PATCH] mm/mmap.c: Remove redundant vma looping

[Richard Weinberger <richard.weinberger@gmail.com>]

On Sat, Oct 3, 2015 at 9:38 PM, Chen Gang <xili_gchen_5257@hotmail.com> wrote:
> From 36dbcc145819655682f80efd49e72b01515b4e9a Mon Sep 17 00:00:00 2001
> From: Chen Gang <gang.chen.5i5j@gmail.com>
> Date: Sun, 4 Oct 2015 03:22:41 +0800
> Subject: [PATCH] mm/mmap.c: Remove redundant vma looping
>
> vma->vm_file->f_mapping and vma->anon_vma are shared with the same vma
> looping, so merge them.
>
> Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
> ---
>  mm/mmap.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 8393580..f7c1631 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3201,9 +3201,7 @@ int mm_take_all_locks(struct mm_struct *mm)
>                         goto out_unlock;
>                 if (vma->vm_file && vma->vm_file->f_mapping)
>                         vm_lock_mapping(mm, vma->vm_file->f_mapping);
> -       }
>
> -       for (vma = mm->mmap; vma; vma = vma->vm_next) {
>                 if (signal_pending(current))
>                         goto out_unlock;
>                 if (vma->anon_vma)

With that change you're reintroducing an issue.
Please see:
commit 7cd5a02f54f4c9d16cf7fdffa2122bc73bb09b43
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date:   Mon Aug 11 09:30:25 2008 +0200

    mm: fix mm_take_all_locks() locking order

    Lockdep spotted:

    =======================================================
    [ INFO: possible circular locking dependency detected ]
    2.6.27-rc1 #270
    -------------------------------------------------------
    qemu-kvm/2033 is trying to acquire lock:
     (&inode->i_data.i_mmap_lock){----}, at: [<ffffffff802996cc>]
mm_take_all_locks+0xc2/0xea

    but task is already holding lock:
     (&anon_vma->lock){----}, at: [<ffffffff8029967a>]
mm_take_all_locks+0x70/0xea

    which lock already depends on the new lock.


git blame often explains funky code. :-)

-- 
Thanks,
//richard

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