Re: [BUG] at include/linux/page-flags.h:415 (PageTransHuge)

[Thomas Hellstrom]

On 01/31/2014 03:40 PM, Vlastimil Babka wrote:
> On 01/15/2014 05:06 PM, Daniel Borkmann wrote:
>> [keeping netdev in loop as well]
>>
>> On 01/15/2014 03:27 PM, Vlastimil Babka wrote:
>>> On 01/13/2014 12:39 PM, Daniel Borkmann wrote:
>>>> On 01/13/2014 11:16 AM, Vlastimil Babka wrote:
>>>>> On 01/11/2014 02:32 PM, Daniel Borkmann wrote:
>>>>>> On 01/11/2014 07:22 AM, Andrew Morton wrote:
>>>>>>> On Fri, 10 Jan 2014 19:23:26 +0100 Daniel Borkmann <borkmann@xxxxxxxxxxxxx> wrote:
>>>>>>>
>>>>>>>> This is being reliably triggered for each mmaped() packet(7)
>>>>>>>> socket from user space, basically during unmapping resp.
>>>>>>>> closing the TX socket.
>>>>>>>>
>>>>>>>> I believe due to some change in transparent hugepages code ?
>>>>>>>>
>>>>>>>> When I disable transparent hugepages, everything works fine,
>>>>>>>> no BUG triggered.
>>>>>>>>
>>>>>>>> I'd be happy to test patches.
>>>>>>> Did the inclusion of c424be1cbbf852e46acc8 ("mm: munlock: fix a bug
>>>>>>> where THP tail page is encountered") in current mainline fix this?
>>>>>> Thanks for your answer Andrew!
>>>>>>
>>>>>> Hm, I just cherry-picked that onto current net-next as I have some work
>>>>>> there, and this time I got ...
>>>>>>
>>>>>> (User space uses packet mmap() and mlockall(MCL_CURRENT | MCL_FUTURE)
>>>>>>        and on shutdown munlockall() ...)
>>>>>>
>>>>>> [   63.863672] ------------[ cut here ]------------
>>>>>> [   63.863702] kernel BUG at mm/mlock.c:507!
>>>>>> [   63.863721] invalid opcode: 0000 [#1] SMP
>>>>>> [   63.863743] Modules linked in: fuse ebtable_nat xt_CHECKSUM nf_conntrack_netbios_ns nf_conntrack_broadcast ipt_MASQUERADE ip6table_nat nf_nat_ipv6 ip6table_mangle ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 iptable_nat nf_nat_ipv4 nf_nat iptable_mangle nf_conntrack_ipv4 nf_defrag_ipv4 xt_conntrack nf_conntrack bridge ebtable_filter ebtables stp llc ip6table_filter ip6_tables rfcomm bnep snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_intel snd_hda_codec iwlwifi cfg80211 snd_hwdep btusb snd_seq bluetooth sdhci_pci snd_seq_device e1000e tpm_tis snd_pcm thinkpad_acpi sdhci ptp tpm uvcvideo pps_core snd_page_alloc snd_timer snd rfkill mmc_core iTCO_wdt iTCO_vendor_support lpc_ich mfd_core soundcore joydev wmi videobuf2_vmalloc videobuf2_memops videobuf2_core i2c_i801 pcspkr videodev media uinput i915
>>>>>> [   63.864152]  i2c_algo_bit drm_kms_helper drm i2c_core video
>>>>>> [   63.864181] CPU: 1 PID: 1617 Comm: trafgen Not tainted 3.13.0-rc6+ #15
>>>>>> [   63.864209] Hardware name: LENOVO 2429BP3/2429BP3, BIOS G4ET37WW (1.12 ) 05/29/2012
>>>>>> [   63.864242] task: ffff8801ee060000 ti: ffff8800b5954000 task.ti: ffff8800b5954000
>>>>>> [   63.864274] RIP: 0010:[<ffffffff8116fa9a>]  [<ffffffff8116fa9a>] munlock_vma_pages_range+0x2ea/0x2f0
>>>>>> [   63.864318] RSP: 0018:ffff8800b5955e08  EFLAGS: 00010202
>>>>>> [   63.864341] RAX: 00000000000001ff RBX: ffff8800b58f7508 RCX: 0000000000000034
>>>>>> [   63.864372] RDX: 00000007f0708992 RSI: ffffea0002c3e700 RDI: ffffea0002c3e700
>>>>>> [   63.864402] RBP: ffff8800b5955ee0 R08: 3800000000000000 R09: a8000b0f9c000000
>>>>>> [   63.864432] R10: 57ffdef066c3e700 R11: ffffff5cfb00c14a R12: ffffea0002c3e700
>>>>>> [   63.864462] R13: ffff8800b5955f48 R14: 00007f0708992000 R15: 00007f0708992000
>>>>>> [   63.864492] FS:  00007f0708b92740(0000) GS:ffff88021e240000(0000) knlGS:0000000000000000
>>>>>> [   63.864526] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>>> [   63.864551] CR2: 00007f33bb373000 CR3: 00000000b2a2c000 CR4: 00000000001407e0
>>>>>> [   63.864581] Stack:
>>>>>> [   63.864593]  ffff8800b5955ed0 00007f0708b91fff 00007f0708b92000 ffff8800b5955e48
>>>>>> [   63.864632]  000001ff810c864b ffff8801ee060000 0000000000000000 0000000000000000
>>>>>> [   63.864669]  ffff8800b5955e58 ffff8801ee060000 0000000700000086 ffff8801ee060000
>>>>>> [   63.864708] Call Trace:
>>>>>> [   63.864724]  [<ffffffff816956bc>] ? _raw_spin_unlock_irq+0x2c/0x30
>>>>>> [   63.864754]  [<ffffffff81171b52>] ? vma_merge+0xc2/0x330
>>>>>> [   63.864786]  [<ffffffff8116fb9c>] mlock_fixup+0xfc/0x190
>>>>>> [   63.864812]  [<ffffffff8116fde7>] do_mlockall+0x87/0xc0
>>>>>> [   63.864836]  [<ffffffff811702df>] sys_munlockall+0x2f/0x50
>>>>>> [   63.864873]  [<ffffffff8169e192>] system_call_fastpath+0x16/0x1b
>>>>>> [   63.864898] Code: d7 48 89 95 28 ff ff ff e8 a4 04 fe ff 84 c0 48 8b 95 28 ff ff ff 0f 85 5a ff ff ff e9 46 ff ff ff e8 3f ac 51 00 e8 34 ac 51 00 <0f> 0b 0f 1f 40 00 0f 1f 44 00 00 55 48 89 e5 41 57 41 56 41 55
>>>>>> [   63.865114] RIP  [<ffffffff8116fa9a>] munlock_vma_pages_range+0x2ea/0x2f0
>>>>>> [   63.865148]  RSP <ffff8800b5955e08>
>>>>>> [   63.874968] ------------[ cut here ]------------
>>>>>>
>>>>>> ... when I find some time, I'll try with normal torvalds' tree, maybe some
>>>>>> other patches are missing as well, not sure right now.
>>>>> Uh so the triggered assertion is the one added by this very patch, and there are no more changes wrt this in mainline.
>>>>>
>>>>> If you can still try debug patches, please try this. Thanks.
>>>> Yes, thanks, I'll come back to you some time by today.
>>> Daniel sent me (off-list) instructions to reproduce:
>>>
>>>> Then in the kernel source tree, you'll find:
>>>>
>>>>      tools/testing/selftests/net/
>>>>
>>>> There, just do a 'make' and run ./psock_tpacket
>>> It reproduces deterministically in mainline since 3.12, i.e. my munlock
>>> performance series. Based on the initial debug output, I've expanded the
>>> debug patch below a bit:
>>>
>>>>> From: Vlastimil Babka <vbabka@xxxxxxx>
>>>>> Date: Mon, 13 Jan 2014 11:13:53 +0100
>>>>> Subject: [PATCH] debug munlock_vma_pages_range
>>>>>
>>>>> ---
>>>>>      mm/mlock.c | 22 ++++++++++++++++++++--
>>>>>      1 file changed, 20 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/mm/mlock.c b/mm/mlock.c
>>>>> index c59c420..7d0e29a 100644
>>>>> --- a/mm/mlock.c
>>>>> +++ b/mm/mlock.c
>>>>> @@ -448,12 +448,14 @@ static unsigned long __munlock_pagevec_fill(struct pagevec *pvec,
>>>>>      void munlock_vma_pages_range(struct vm_area_struct *vma,
>>>>>      			     unsigned long start, unsigned long end)
>>>>>      {
>>>>> +	unsigned long orig_start = start;
>>>>> +	unsigned long page_increm = 0;
>>>>> +
>>>>>      	vma->vm_flags &= ~VM_LOCKED;
>>>>>
>>>>>      	while (start < end) {
>>>>>      		struct page *page = NULL;
>>>>>      		unsigned int page_mask;
>>>>> -		unsigned long page_increm;
>>>>>      		struct pagevec pvec;
>>>>>      		struct zone *zone;
>>>>>      		int zoneid;
>>>>> @@ -504,7 +506,23 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
>>>>>      			}
>>>>>      		}
>>>>>      		/* It's a bug to munlock in the middle of a THP page */
>>>>> -		VM_BUG_ON((start >> PAGE_SHIFT) & page_mask);
>>>>> +		if ((start >> PAGE_SHIFT) & page_mask) {
>>>>> +			dump_page(page);
>>>>> +			printk("start=%lu pfn=%lu orig_start=%lu "
>>>>> +			       "prev_page_increm=%lu page_mask=%u "
>>>>> +			       "vm_start=%lu vm_end=%lu vm_flags=%lu\n",
>>>>> +				start, page_to_pfn(page), orig_start,
>>>>> +				page_increm, page_mask,
>>>>> +				vma->vm_start, vma->vm_end,
>>>>> +				vma->vm_flags);
>>> +                        printk("vm_ops=%pF, open=%pF, fault=%pF, remap_pages=%pF\n", vma->vm_ops,
>>> +                                vma->vm_ops->open, vma->vm_ops->fault, vma->vm_ops->remap_pages);
>>> +                        if (PageCompound(page)) {
>>> +                                printk("page is compound with order=%d\n", compound_order(page));
>>> +                        }
>>>>> +			if (PageTail(page)) {
>>>>> +				struct page *first_page = page->first_page;
>>>>> +				printk("first_page pfn=%lu\n",
>>>>> +						page_to_pfn(first_page));
>>>>> +				dump_page(first_page);
>>>>> +			}
>>>>> +			VM_BUG_ON(true);
>>>>> +		}
>>>>>      		page_increm = 1 + page_mask;
>>>>>      		start += page_increm * PAGE_SIZE;
>>>>>      next:
>>>>>
>>> And got output like this:
>>>
>>> page:ffffea0002474a40 count:5 mapcount:1 mapping:          (null) index:0x0
>>> page flags: 0x100000000004004(referenced|head)
>>> start=140242647736320 pfn=682616 orig_start=140242647736320 prev_page_increm=0 page_mask=511 vm_start=140242647736320 vm_end=140242651930624 vm_flags=268435707
>>> vm_ops=packet_mmap_ops+0x0/0xfffffffffffff8e0 [af_packet], open=packet_mm_open+0x0/0x30 [af_packet], fault=          (null), remap_pages=          (null)
>>> page is compound with order=2
>>>
>>> Observations:
>>> - address 140242647736320 is where the vma starts, and is not aligned to 512 pages
>>>     (so it cannot be a THP head which the munlock expects). Yet there is a head page
>>>     that triggers the PageTransHuge() and consequently hpage_nr_pages() in munlock_vma_page()
>>>     That's why page_mask is determined to be 511 and the code thinks it's in the
>>>     middle of a THP page.
>>> - in fact, the page is a compound page with order=2
>>> - the VM flags (except (may)read/write) are VM_SHARED and VM_MIXEDMAP
>>> - the vma was mmapped by packet_mmap() (net/packet/af_packet.c) which uses
>>>     vm_insert_page(), which adds the VM_MIXEDMAP flag
>>> - the buffers that are mapped were allocated by alloc_one_pg_vec_page()
>>>     where flags indeed include __GFP_COMP
>>>
>>> So clearly there is a way to have mlock/munlock operate on a vma that contains
>>> compound pages and confuse the checks for PageTransHuge().
>>>
>>> The checks for THP in munlock came with commit ff6a6da60b89 ("mm: accelerate munlock()
>>> treatment of THP pages"), i.e. since 3.9, but did not trigger a bug. It however
>>> makes munlock_vma_pages_range() skip pages until the next 512-pages-aligned page,
>>> when it encounters a head page. If the head page is of smaller order and is followed
>>> by normal LRU pages (theoretically, I'm not sure if that's possible, or done anywhere),
>>> they wouldn't get munlocked.
>>>
>>> My commit 7225522bb429 ("mm: munlock: batch non-THP page isolation and
>>> munlock+putback using pagevec") (since 3.12) has added a new PageTransHuge() check
>>> that can trigger on tail pages of the compound page here. Commit c424be1cbbf852e46acc8
>>> ("mm: munlock: fix a bug where THP tail page is encountered") in current rc's removes
>>> one class of bugs here, but still non-THP compound pages are not expected in mlock/munlock,
>>> which leads to this assertion failing.
>>>
>>> The question is what is the correct fix, and I'm not that familiar with VM_MIXEDMAP
>>> to decide.
>>>
>>> Option 1: mlocking VM_MIXEDMAP vma's has no sense. They should be treated like VM_PFNMAP
>>>             and added to VM_SPECIAL, which makes m(un)lock skip them completely.
>>>
>>> Option 2: if indeed VM_MIXEDMAP can contain PageLRU pages for which mlocking is useful,
>>>             VM_NO_THP should be checked in munlock before attempting PageTransHuge() and
>>>             friends. VM_NO_THP already contains VM_MIXEDMAP, so knowing that there can be
>>>             no THP means we don't try optimize for it and no unexpected head pages trip us.
>>>
>>> Thoughts?
> OK, here's a RFC patch to hopefully help get us somewhere. I went for
> Option1, as I didn't see anyone using VM_MIXEDMAP also for LRU pages,
> and Option2 was ugly to implement and also seemed quite arbitrary. I'm
> not sure if making VM_MIXEDMAP also non-mergeable this way is an issue
> though.
>
>

Hi!

Forgive an ignorant question, but are anonymous COW'd pages LRU pages?
The reason I'm asking is that TTM VM_MIXEDMAP vmas may contain such pages.

/Thomas
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