Re: [RFC][PATCH 1/7] defer clearing of page_private() for swap cachepages

[Michal Hocko]

On Thu 09-05-13 17:07:39, Seth Jennings wrote:
> On Tue, May 07, 2013 at 02:19:55PM -0700, Dave Hansen wrote:
> > 
> > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > 
> > 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 :)

This has been already fixed in the -mm tree
(http://git.kernel.org/cgit/linux/kernel/git/mhocko/mm.git/commit/?h=since-3.9&id=b341f7ffa5fe6ae11afa87e2fecc32c6093541f8)
-- 
Michal Hocko
SUSE Labs
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