linux-kernel - Re: [RFC][PATCH 1/7] defer clearing of page

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Date:	Thu, 9 May 2013 17:07:39 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, mgorman@...e.de,
	tim.c.chen@...ux.intel.com
Subject: Re: [RFC][PATCH 1/7] defer clearing of page_private() for swap cache
 pages

On Tue, May 07, 2013 at 02:19:55PM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> There are only two callers of swapcache_free() which actually
> pass in a non-NULL 'struct page'.  Both of them
> (__remove_mapping and delete_from_swap_cache())  create a
> temporary on-stack 'swp_entry_t' and set entry.val to
> page_private().
> 
> They need to do this since __delete_from_swap_cache() does
> set_page_private(page, 0) and destroys the information.
> 
> However, I'd like to batch a few of these operations on several
> pages together in a new version of __remove_mapping(), and I
> would prefer not to have to allocate temporary storage for
> each page.  The code is pretty ugly, and it's a bit silly
> to create these on-stack 'swp_entry_t's when it is so easy to
> just keep the information around in 'struct page'.
> 
> There should not be any danger in doing this since we are
> absolutely on the path of freeing these page.  There is no
> turning back, and no other rerferences can be obtained
> after it comes out of the radix tree.

I get a BUG on this one:

[   26.114818] ------------[ cut here ]------------
[   26.115282] kernel BUG at mm/memcontrol.c:4111!
[   26.115282] invalid opcode: 0000 [#1] PREEMPT SMP 
[   26.115282] Modules linked in:
[   26.115282] CPU: 3 PID: 5026 Comm: cc1 Not tainted 3.9.0+ #8
[   26.115282] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
[   26.115282] task: ffff88007c1cdca0 ti: ffff88001b442000 task.ti: ffff88001b442000
[   26.115282] RIP: 0010:[<ffffffff810ed425>]  [<ffffffff810ed425>] __mem_cgroup_uncharge_common+0x255/0x2e0
[   26.115282] RSP: 0000:ffff88001b443708  EFLAGS: 00010206
[   26.115282] RAX: 4000000000090009 RBX: 0000000000000000 RCX: ffffc90000014001
[   26.115282] RDX: 0000000000000000 RSI: 0000000000000002 RDI: ffffea00006e5b40
[   26.115282] RBP: ffff88001b443738 R08: 0000000000000000 R09: 0000000000000000
[   26.115282] R10: 0000000000000000 R11: 0000000000000000 R12: ffffea00006e5b40
[   26.115282] R13: 0000000000000000 R14: ffffea00006e5b40 R15: 0000000000000002
[   26.115282] FS:  00007fabd08ee700(0000) GS:ffff88007fd80000(0000) knlGS:0000000000000000
[   26.115282] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   26.115282] CR2: 00007fabce27a000 CR3: 000000001985f000 CR4: 00000000000006a0
[   26.115282] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   26.115282] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[   26.115282] Stack:
[   26.115282]  ffffffff810dcbae ffff880064a0a500 0000000000000001 ffffea00006e5b40
[   26.115282]  ffffea00006e5b40 0000000000000001 ffff88001b443748 ffffffff810f0d05
[   26.115282]  ffff88001b443778 ffffffff810ddb3e ffff88001b443778 ffffea00006e5b40
[   26.115282] Call Trace:
[   26.115282]  [<ffffffff810dcbae>] ? swap_info_get+0x5e/0xe0
[   26.115282]  [<ffffffff810f0d05>] mem_cgroup_uncharge_swapcache+0x15/0x20
[   26.115282]  [<ffffffff810ddb3e>] swapcache_free+0x4e/0x70
[   26.115282]  [<ffffffff810b6e67>] __remove_mapping+0xd7/0x120
[   26.115282]  [<ffffffff810b8682>] shrink_page_list+0x5c2/0x920
[   26.115282]  [<ffffffff810b780e>] ? isolate_lru_pages.isra.37+0xae/0x120
[   26.115282]  [<ffffffff810b8ecf>] shrink_inactive_list+0x13f/0x380
[   26.115282]  [<ffffffff810b9350>] shrink_lruvec+0x240/0x4e0
[   26.115282]  [<ffffffff810b9656>] shrink_zone+0x66/0x1a0
[   26.115282]  [<ffffffff810ba1fb>] do_try_to_free_pages+0xeb/0x570
[   26.115282]  [<ffffffff810eb7d9>] ? lookup_page_cgroup_used+0x9/0x20
[   26.115282]  [<ffffffff810ba7af>] try_to_free_pages+0x9f/0xc0
[   26.115282]  [<ffffffff810b1357>] __alloc_pages_nodemask+0x5a7/0x970
[   26.115282]  [<ffffffff810cb2be>] handle_pte_fault+0x65e/0x880
[   26.115282]  [<ffffffff810cc7d9>] handle_mm_fault+0x139/0x1e0
[   26.115282]  [<ffffffff81027920>] __do_page_fault+0x160/0x460
[   26.115282]  [<ffffffff810d176c>] ? do_brk+0x1fc/0x360
[   26.115282]  [<ffffffff81212979>] ? lockdep_sys_exit_thunk+0x35/0x67
[   26.115282]  [<ffffffff81027c49>] do_page_fault+0x9/0x10
[   26.115282]  [<ffffffff813b4a72>] page_fault+0x22/0x30
[   26.115282] Code: a9 00 00 08 00 0f 85 43 fe ff ff e9 b8 fe ff ff 66 0f 1f 44 00 00 41 8b 44 24 18 85 c0 0f 89 2b fe ff ff 0f 1f 00 e9 9d fe ff ff <0f> 0b 66 0f 1f 84 00 00 00 00 00 49 89 9c 24 48 0f 00 00 e9 0a 
[   26.115282] RIP  [<ffffffff810ed425>] __mem_cgroup_uncharge_common+0x255/0x2e0
[   26.115282]  RSP <ffff88001b443708>
[   26.171597] ---[ end trace 5e49a21e51452c24 ]---


mm/memcontrol:4111
VM_BUG_ON(PageSwapCache(page));

Seems that mem_cgroup_uncharge_swapcache, somewhat ironically expects the
SwapCache flag to be unset already.

Fix might be a simple as removing that VM_BUG_ON() but there might be more to
it.  There usually is :)

Seth

> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> ---
> 
>  linux.git-davehans/mm/swap_state.c |    4 ++--
>  linux.git-davehans/mm/vmscan.c     |    2 ++
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff -puN mm/swap_state.c~__delete_from_swap_cache-dont-clear-page-private mm/swap_state.c
> --- linux.git/mm/swap_state.c~__delete_from_swap_cache-dont-clear-page-private	2013-05-07 13:48:13.698044473 -0700
> +++ linux.git-davehans/mm/swap_state.c	2013-05-07 13:48:13.703044693 -0700
> @@ -146,8 +146,6 @@ void __delete_from_swap_cache(struct pag
>  	entry.val = page_private(page);
>  	address_space = swap_address_space(entry);
>  	radix_tree_delete(&address_space->page_tree, page_private(page));
> -	set_page_private(page, 0);
> -	ClearPageSwapCache(page);
>  	address_space->nrpages--;
>  	__dec_zone_page_state(page, NR_FILE_PAGES);
>  	INC_CACHE_INFO(del_total);
> @@ -224,6 +222,8 @@ void delete_from_swap_cache(struct page
>  	spin_unlock_irq(&address_space->tree_lock);
> 
>  	swapcache_free(entry, page);
> +	set_page_private(page, 0);
> +	ClearPageSwapCache(page);
>  	page_cache_release(page);
>  }
> 
> diff -puN mm/vmscan.c~__delete_from_swap_cache-dont-clear-page-private mm/vmscan.c
> --- linux.git/mm/vmscan.c~__delete_from_swap_cache-dont-clear-page-private	2013-05-07 13:48:13.700044561 -0700
> +++ linux.git-davehans/mm/vmscan.c	2013-05-07 13:48:13.705044783 -0700
> @@ -494,6 +494,8 @@ static int __remove_mapping(struct addre
>  		__delete_from_swap_cache(page);
>  		spin_unlock_irq(&mapping->tree_lock);
>  		swapcache_free(swap, page);
> +		set_page_private(page, 0);
> +		ClearPageSwapCache(page);
>  	} else {
>  		void (*freepage)(struct page *);
> 
> _
> 
> --
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