On Fri, 23 Apr 2010 16:53:44 +0900
Minchan Kim <minchan.kim@gmail.com> wrote:

quoted text
> On Fri, Apr 23, 2010 at 4:17 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > On Fri, 23 Apr 2010 16:00:31 +0900
> > Minchan Kim <minchan.kim@gmail.com> wrote:
> >
> >> On Fri, Apr 23, 2010 at 2:27 PM, KAMEZAWA Hiroyuki
> >> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> >> > On Fri, 23 Apr 2010 14:11:37 +0900
> >> > Minchan Kim <minchan.kim@gmail.com> wrote:
> >> >
> >> >> On Fri, Apr 23, 2010 at 12:01 PM, KAMEZAWA Hiroyuki
> >> >> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> >> >> >
> >> >> > This patch itself is for -mm ..but may need to go -stable tree for memory
> >> >> > hotplug. (but we've got no report to hit this race...)
> >> >> >
> >> >> > This one is the simplest, I think and works well on my test set.
> >> >> > ==
> >> >> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> >> >> >
> >> >> > In rmap.c, at checking rmap in vma chain in page->mapping, anon_vma->lock
> >> >> > or mapping->i_mmap_lock is held and enter following loop.
> >> >> >
> >> >> >        for_each_vma_in_this_rmap_link(list from page->mapping) {
> >> >> >                unsigned long address = vma_address(page, vma);
> >> >> >                if (address == -EFAULT)
> >> >> >                        continue;
> >> >> >                ....
> >> >> >        }
> >> >> >
> >> >> > vma_address is checking [start, end, pgoff] v.s. page->index.
> >> >> >
> >> >> > But vma's [start, end, pgoff] is updated without locks. vma_address()
> >> >> > can hit a race and may return wrong result.
> >> >> >
> >> >> > This bahavior is no problem in usual routine as try_to_unmap() etc...
> >> >> > But for page migration, rmap_walk() has to find all migration_ptes
> >> >> > which migration code overwritten valid ptes. This race is critical and cause
> >> >> > BUG that a migration_pte is sometimes not removed.
> >> >> >
> >> >> > pr 21 17:27:47 localhost kernel: ------------[ cut here ]------------
> >> >> > Apr 21 17:27:47 localhost kernel: kernel BUG at include/linux/swapops.h:105!
> >> >> > Apr 21 17:27:47 localhost kernel: invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC
> >> >> > Apr 21 17:27:47 localhost kernel: last sysfs file: /sys/devices/virtual/net/br0/statistics/collisions
> >> >> > Apr 21 17:27:47 localhost kernel: CPU 3
> >> >> > Apr 21 17:27:47 localhost kernel: Modules linked in: fuse sit tunnel4 ipt_MASQUERADE iptable_nat nf_nat bridge stp llc sunrpc cpufreq_ondemand acpi_cpufreq freq_table mperf xt_physdev ip6t_REJECT nf_conntrack_ipv6 ip6table_filter ip6_tables ipv6 dm_multipath uinput ioatdma ppdev parport_pc i5000_edac bnx2 iTCO_wdt edac_core iTCO_vendor_support shpchp parport e1000e kvm_intel dca kvm i2c_i801 i2c_core i5k_amb pcspkr megaraid_sas [last unloaded: microcode]
> >> >> > Apr 21 17:27:47 localhost kernel:
> >> >> > Apr 21 17:27:47 localhost kernel: Pid: 27892, comm: cc1 Tainted: G        W   2.6.34-rc4-mm1+ #4 D2519/PRIMERGY
> >> >> > Apr 21 17:27:47 localhost kernel: RIP: 0010:[<ffffffff8114e9cf>]  [<ffffffff8114e9cf>] migration_entry_wait+0x16f/0x180
> >> >> > Apr 21 17:27:47 localhost kernel: RSP: 0000:ffff88008d9efe08  EFLAGS: 00010246
> >> >> > Apr 21 17:27:47 localhost kernel: RAX: ffffea0000000000 RBX: ffffea0000241100 RCX: 0000000000000001
> >> >> > Apr 21 17:27:47 localhost kernel: RDX: 000000000000a4e0 RSI: ffff880621a4ab00 RDI: 000000000149c03e
> >> >> > Apr 21 17:27:47 localhost kernel: RBP: ffff88008d9efe38 R08: 0000000000000000 R09: 0000000000000000
> >> >> > Apr 21 17:27:47 localhost kernel: R10: 0000000000000000 R11: 0000000000000001 R12: ffff880621a4aae8
> >> >> > Apr 21 17:27:47 localhost kernel: R13: 00000000bf811000 R14: 000000000149c03e R15: 0000000000000000
> >> >> > Apr 21 17:27:47 localhost kernel: FS:  00007fe6abc90700(0000) GS:ffff880005a00000(0000) knlGS:0000000000000000
> >> >> > Apr 21 17:27:47 localhost kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >> >> > Apr 21 17:27:47 localhost kernel: CR2: 00007fe6a37279a0 CR3: 000000008d942000 CR4: 00000000000006e0
> >> >> > Apr 21 17:27:47 localhost kernel: DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> >> >> > Apr 21 17:27:47 localhost kernel: DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> >> >> > Apr 21 17:27:47 localhost kernel: Process cc1 (pid: 27892, threadinfo ffff88008d9ee000, task ffff8800b23ec820)
> >> >> > Apr 21 17:27:47 localhost kernel: Stack:
> >> >> > Apr 21 17:27:47 localhost kernel: ffffea000101aee8 ffff880621a4aae8 ffff88008d9efe38 00007fe6a37279a0
> >> >> > Apr 21 17:27:47 localhost kernel: <0> ffff8805d9706d90 ffff880621a4aa00 ffff88008d9efef8 ffffffff81126d05
> >> >> > Apr 21 17:27:47 localhost kernel: <0> ffff88008d9efec8 0000000000000246 0000000000000000 ffffffff81586533
> >> >> > Apr 21 17:27:47 localhost kernel: Call Trace:
> >> >> > Apr 21 17:27:47 localhost kernel: [<ffffffff81126d05>] handle_mm_fault+0x995/0x9b0
> >> >> > Apr 21 17:27:47 localhost kernel: [<ffffffff81586533>] ? do_page_fault+0x103/0x330
> >> >> > Apr 21 17:27:47 localhost kernel: [<ffffffff8104bf40>] ? finish_task_switch+0x0/0xf0
> >> >> > Apr 21 17:27:47 localhost kernel: [<ffffffff8158659e>] do_page_fault+0x16e/0x330
> >> >> > Apr 21 17:27:47 localhost kernel: [<ffffffff81582f35>] page_fault+0x25/0x30
> >> >> > Apr 21 17:27:47 localhost kernel: Code: 53 08 85 c9 0f 84 32 ff ff ff 8d 41 01 89 4d d8 89 45 d4 8b 75 d4 8b 45 d8 f0 0f b1 32 89 45 dc 8b 45 dc 39 c8 74 aa 89 c1 eb d7 <0f> 0b eb fe 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 e5
> >> >> > Apr 21 17:27:47 localhost kernel: RIP  [<ffffffff8114e9cf>] migration_entry_wait+0x16f/0x180
> >> >> > Apr 21 17:27:47 localhost kernel: RSP <ffff88008d9efe08>
> >> >> > Apr 21 17:27:47 localhost kernel: ---[ end trace 4860ab585c1fcddb ]---
> >> >> >
> >> >> >
> >> >> >
> >> >> > This patch adds vma_address_safe(). And update [start, end, pgoff]
> >> >> > under seq counter.
> >> >> >
> >> >> > Cc: Mel Gorman <mel@csn.ul.ie>
> >> >> > Cc: Minchan Kim <minchan.kim@gmail.com>
> >> >> > Cc: Christoph Lameter <cl@linux-foundation.org>
> >> >> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> >> >>
> >> >> That's exactly same what I have in mind. :)
> >> >> But I am hesitating. That's because AFAIR, we try to remove seqlock. Right?
> >> >
> >> > Ah,..."don't use seqlock" is trend ?
> >> >
> >> >> But in this case, seqlock is good, I think. :)
> >> >>
> >> > BTW, this isn't seqlock but seq_counter :)
> >> >
> >> > I'm still testing. What I doubt other than vma_address() is fork().
> >> > at fork(), followings _may_ happen. (but I'm not sure).
> >> >
> >> >        chain vma.
> >> >        copy page table.
> >> >           -> migration entry is copied, too.
> >> >
> >> > At remap,
> >> >        for each vma
> >> >            look into page table and replace.
> >> >
> >> > Then,
> >> >                                                rmap_walk().
> >> >        fork(parent, child)
> >> >                                                look into child's page table.
> >> >                                                => we fond nothing.
> >> >        spin_lock(child's pagetable);
> >> >        spin_lock(parant's page table);
> >> >        copy migration entry
> >> >        spin_unlock(paranet's page table)
> >> >        spin_unlock(child's page table)
> >> >                                                update parent's paga table
> >> >
> >> > If we always find parant's page table before child's , there is no race.
> >> > But I can't get prit_tree's list order as clear image. Hmm.
> >> >
> >> > Thanks,
> >> > -Kame
> >> >
> >>
> >> That's good point, Kame.
> >> I looked into prio_tree quickly.
> >> If I understand it right, list order is backward.
> >>
> >> dup_mmap calls vma_prio_tree_add.
> >>
> >>  * prio_tree_root
> >>  *      |
> >>  *      A       vm_set.head
> >>  *     / \      /
> >>  *    L   R -> H-I-J-K-M-N-O-P-Q-S
> >>  *    ^   ^    <-- vm_set.list -->
> >>  *  tree nodes
> >>  *
> >>
> >> Maybe, parent and childs's vma are H~S.
> >> Then, comment said.
> >>
> >> "vma->shared.vm_set.parent != NULL    ==> a tree node"
> >> So vma_prio_tree_add call not list_add_tail but list_add.
> >>
> > Ah, thank you for explanation.
> >
> >> Anyway, I think order isn't mixed.
> >> So, could we traverse it by backward in rmap?
> >>
> > Doesn't it make prio-tree code dirty ?
> >
> > Here is another idea....but ..hmm. Does this make fork() slow in some cases ?
> 
> Yes. I think this idea is good to me. :)
> Great, Kame.
> 
> But as you said, migration is rare.
> so we wouldn't lost much performance in many case.
> 
> Actually, If I understand prio_tree right, I think backward walking of
> prio_tree is nod bad.
> I don't think it make code dirty. :)
> I admit it's different per people.
> 
it's okay to me. My concern was difficulty of maintainance.

quoted text
> I like both ideas.
> I passes decision to others.  :)
> 
me, too.

maybe a problem is we need tese case to hit this race.

Thanks,
-Kame

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