Re: boot warnings due to swap: make each swap partition have one address_space

[Hugh Dickins <hughd@google.com>]

On Fri, 25 Jan 2013, Shaohua Li wrote:
> On Thu, Jan 24, 2013 at 10:45:57PM -0500, Sasha Levin wrote:
> > Hi folks,
> > 
> > Commit "swap: make each swap partition have one address_space" is triggering
> > a series of warnings on boot:
> > 
> > [    3.446071] ------------[ cut here ]------------
> > [    3.446664] WARNING: at lib/debugobjects.c:261 debug_print_object+0x8e/0xb0()
> > [    3.447715] ODEBUG: init active (active state 0) object type: percpu_counter hint:           (null)
> > [    3.450360] Modules linked in:
> > [    3.451593] Pid: 1, comm: swapper/0 Tainted: G        W    3.8.0-rc4-next-20130124-sasha-00004-g838a1b4 #266
> > [    3.454508] Call Trace:
> > [    3.455248]  [<ffffffff8110d1bc>] warn_slowpath_common+0x8c/0xc0
> > [    3.455248]  [<ffffffff8110d291>] warn_slowpath_fmt+0x41/0x50
> > [    3.455248]  [<ffffffff81a2bb5e>] debug_print_object+0x8e/0xb0
> > [    3.455248]  [<ffffffff81a2c26b>] __debug_object_init+0x20b/0x290
> > [    3.455248]  [<ffffffff81a2c305>] debug_object_init+0x15/0x20
> > [    3.455248]  [<ffffffff81a3fbed>] __percpu_counter_init+0x6d/0xe0
> > [    3.455248]  [<ffffffff81231bdc>] bdi_init+0x1ac/0x270
> > [    3.455248]  [<ffffffff8618f20b>] swap_setup+0x3b/0x87
> > [    3.455248]  [<ffffffff8618f257>] ? swap_setup+0x87/0x87
> > [    3.455248]  [<ffffffff8618f268>] kswapd_init+0x11/0x7c
> > [    3.455248]  [<ffffffff810020ca>] do_one_initcall+0x8a/0x180
> > [    3.455248]  [<ffffffff86168cfd>] do_basic_setup+0x96/0xb4
> > [    3.455248]  [<ffffffff861685ae>] ? loglevel+0x31/0x31
> > [    3.455248]  [<ffffffff861885cd>] ? sched_init_smp+0x150/0x157
> > [    3.455248]  [<ffffffff86168ded>] kernel_init_freeable+0xd2/0x14c
> > [    3.455248]  [<ffffffff83cade10>] ? rest_init+0x140/0x140
> > [    3.455248]  [<ffffffff83cade19>] kernel_init+0x9/0xf0
> > [    3.455248]  [<ffffffff83d5727c>] ret_from_fork+0x7c/0xb0
> > [    3.455248]  [<ffffffff83cade10>] ? rest_init+0x140/0x140
> > [    3.455248] ---[ end trace 0b176d5c0f21bffb ]---
> > 
> > I haven't looked deeper into it yet, and will do so tomorrow, unless this
> > spew is obvious to anyone.
> 
> Does this one help?
> 
> Subject: give-each-swapper-space-separate-backing_dev_info
> 
> The backing_dev_info can't be shared by all swapper address space.

Whyever not?  It's perfectly normal for different inodes/address_spaces
to share a single backing_dev!  Sasha's trace says that it's wrong to
initialize it MAX_SWAPFILES times: fair enough.  But why should I now
want to spend 32kB (not even counting their __percpu counters) on all
these pseudo-backing_devs?

Hugh

p.s. a grand little change would be to move page_cluster and swap_setup()
from mm/swap.c to mm/swap_state.c: they have nothing to do with the other
contents of swap.c, and everything to do with the contents of swap_state.c.
Why swap.c is called swap.c is rather a mystery.

> 
> Reported-by: Sasha Levin <sasha.levin@oracle.com>
> Signed-off-by: Shaohua Li <shli@fusionio.com>
> ---
>  mm/swap.c       |    1 +
>  mm/swap_state.c |   11 +++++++----
>  2 files changed, 8 insertions(+), 4 deletions(-)
> 
> Index: linux/mm/swap.c
> ===================================================================
> --- linux.orig/mm/swap.c	2013-01-22 10:11:58.310933234 +0800
> +++ linux/mm/swap.c	2013-01-25 12:14:49.524863610 +0800
> @@ -859,6 +859,7 @@ void __init swap_setup(void)
>  	int i;
>  
>  	for (i = 0; i < MAX_SWAPFILES; i++) {
> +		swapper_spaces[i].backing_dev_info += i;
>  		bdi_init(swapper_spaces[i].backing_dev_info);
>  		spin_lock_init(&swapper_spaces[i].tree_lock);
>  		INIT_LIST_HEAD(&swapper_spaces[i].i_mmap_nonlinear);
> Index: linux/mm/swap_state.c
> ===================================================================
> --- linux.orig/mm/swap_state.c	2013-01-24 18:08:05.149390977 +0800
> +++ linux/mm/swap_state.c	2013-01-25 12:14:12.849323671 +0800
> @@ -31,16 +31,19 @@ static const struct address_space_operat
>  	.migratepage	= migrate_page,
>  };
>  
> -static struct backing_dev_info swap_backing_dev_info = {
> -	.name		= "swap",
> -	.capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
> +static struct backing_dev_info swap_backing_dev_info[MAX_SWAPFILES] = {
> +	[0 ... MAX_SWAPFILES - 1] = {
> +		.name		= "swap",
> +		.capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK |
> +			BDI_CAP_SWAP_BACKED,
> +	}
>  };
>  
>  struct address_space swapper_spaces[MAX_SWAPFILES] = {
>  	[0 ... MAX_SWAPFILES - 1] = {
>  		.page_tree	= RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
>  		.a_ops		= &swap_aops,
> -		.backing_dev_info = &swap_backing_dev_info,
> +		.backing_dev_info = &swap_backing_dev_info[0],
>  	}
>  };
>  
> 

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