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SubjectRe: [Ocfs2-devel] [PATCH] ocfs2: try a blocking lock before return AOP_TRUNCATED_PAGE
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Hi Gang,

This patch looks good to me.

thanks,
Jun

On 2017/12/28 10:58, Gang He wrote:
>
>
>
>>>>
>> Hi Gang,
>>
>> You cleared my doubt. Should we handle the errno of ocfs2_inode_lock()
>> or just use mlog_errno()?
> Hi Jun, I think it is not necessary, since we just want to hold a while before get the DLM lock,
> we do not care about the result, since we will unlock immediately here.
> In fact, this patch does NOT add new code, just revert the old patch 1cce4df04f37, and add
> more clear comments in the front of these two lines code.
>
> Thanks
> Gang
>
>>
>> thanks,
>> Jun
>>
>> On 2017/12/28 10:11, Gang He wrote:
>>> Hi Jun,
>>>
>>>
>>>>>>
>>>> Hi Gang,
>>>>
>>>> Thanks for your explaination, and I just have one more question. Could
>>>> we use 'ocfs2_inode_lock' instead of 'ocfs2_inode_lock_full' to avoid
>>>> -EAGAIN circularly?
>>> No, please see the comments above the function
>> ocfs2_inode_lock_with_page(),
>>> there will be probably a deadlock between tasks acquiring DLM
>>> locks while holding a page lock and the downconvert thread which
>>> blocks dlm lock acquiry while acquiring page locks.
>>> Then, the OCFS2_LOCK_NONBLOCK flag was introduced as a workaround to
>>> avoid this case.
>>>
>>> Thanks
>>> Gang
>>>
>>>>
>>>> thanks,
>>>> Jun
>>>>
>>>> On 2017/12/27 18:37, Gang He wrote:
>>>>> Hi Jun,
>>>>>
>>>>>
>>>>>>>>
>>>>>> Hi Gang,
>>>>>>
>>>>>> Do you mean that too many retrys in loop cast losts of CPU-time and
>>>>>> block page-fault interrupt? We should not add any delay in
>>>>>> ocfs2_fault(), right? And I still feel a little confused why your
>>>>>> method can solve this problem.
>>>>> You can see the related code in function filemap_fault(), if ocfs2 fails to
>>>> read a page since
>>>>> it can not get a inode lock with non-block mode, the VFS layer code will
>>>> invoke ocfs2
>>>>> read page call back function circularly, this will lead to a softlockup
>>>> problem (like the below back trace).
>>>>> So, we should get a blocking lock to let the dlm lock to this node and also
>>>> can avoid CPU loop,
>>>>> second, base on my testing, the patch also can improve the efficiency in
>>>> case modifying the same
>>>>> file frequently from multiple nodes, since the lock acquisition chance is
>>>> more fair.
>>>>> In fact, the code was modified by a patch 1cce4df04f37 ("ocfs2: do not
>>>> lock/unlock() inode DLM lock"),
>>>>> before that patch, the code is the same, this patch can be considered to
>>>> revert that patch, except adding more
>>>>> clear comments.
>>>>>
>>>>> Thanks
>>>>> Gang
>>>>>
>>>>>
>>>>>>
>>>>>> thanks,
>>>>>> Jun
>>>>>>
>>>>>> On 2017/12/27 17:29, Gang He wrote:
>>>>>>> If we can't get inode lock immediately in the function
>>>>>>> ocfs2_inode_lock_with_page() when reading a page, we should not
>>>>>>> return directly here, since this will lead to a softlockup problem.
>>>>>>> The method is to get a blocking lock and immediately unlock before
>>>>>>> returning, this can avoid CPU resource waste due to lots of retries,
>>>>>>> and benefits fairness in getting lock among multiple nodes, increase
>>>>>>> efficiency in case modifying the same file frequently from multiple
>>>>>>> nodes.
>>>>>>> The softlockup problem looks like,
>>>>>>> Kernel panic - not syncing: softlockup: hung tasks
>>>>>>> CPU: 0 PID: 885 Comm: multi_mmap Tainted: G L 4.12.14-6.1-default #1
>>>>>>> Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
>>>>>>> Call Trace:
>>>>>>> <IRQ>
>>>>>>> dump_stack+0x5c/0x82
>>>>>>> panic+0xd5/0x21e
>>>>>>> watchdog_timer_fn+0x208/0x210
>>>>>>> ? watchdog_park_threads+0x70/0x70
>>>>>>> __hrtimer_run_queues+0xcc/0x200
>>>>>>> hrtimer_interrupt+0xa6/0x1f0
>>>>>>> smp_apic_timer_interrupt+0x34/0x50
>>>>>>> apic_timer_interrupt+0x96/0xa0
>>>>>>> </IRQ>
>>>>>>> RIP: 0010:unlock_page+0x17/0x30
>>>>>>> RSP: 0000:ffffaf154080bc88 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff10
>>>>>>> RAX: dead000000000100 RBX: fffff21e009f5300 RCX: 0000000000000004
>>>>>>> RDX: dead0000000000ff RSI: 0000000000000202 RDI: fffff21e009f5300
>>>>>>> RBP: 0000000000000000 R08: 0000000000000000 R09: ffffaf154080bb00
>>>>>>> R10: ffffaf154080bc30 R11: 0000000000000040 R12: ffff993749a39518
>>>>>>> R13: 0000000000000000 R14: fffff21e009f5300 R15: fffff21e009f5300
>>>>>>> ocfs2_inode_lock_with_page+0x25/0x30 [ocfs2]
>>>>>>> ocfs2_readpage+0x41/0x2d0 [ocfs2]
>>>>>>> ? pagecache_get_page+0x30/0x200
>>>>>>> filemap_fault+0x12b/0x5c0
>>>>>>> ? recalc_sigpending+0x17/0x50
>>>>>>> ? __set_task_blocked+0x28/0x70
>>>>>>> ? __set_current_blocked+0x3d/0x60
>>>>>>> ocfs2_fault+0x29/0xb0 [ocfs2]
>>>>>>> __do_fault+0x1a/0xa0
>>>>>>> __handle_mm_fault+0xbe8/0x1090
>>>>>>> handle_mm_fault+0xaa/0x1f0
>>>>>>> __do_page_fault+0x235/0x4b0
>>>>>>> trace_do_page_fault+0x3c/0x110
>>>>>>> async_page_fault+0x28/0x30
>>>>>>> RIP: 0033:0x7fa75ded638e
>>>>>>> RSP: 002b:00007ffd6657db18 EFLAGS: 00010287
>>>>>>> RAX: 000055c7662fb700 RBX: 0000000000000001 RCX: 000055c7662fb700
>>>>>>> RDX: 0000000000001770 RSI: 00007fa75e909000 RDI: 000055c7662fb700
>>>>>>> RBP: 0000000000000003 R08: 000000000000000e R09: 0000000000000000
>>>>>>> R10: 0000000000000483 R11: 00007fa75ded61b0 R12: 00007fa75e90a770
>>>>>>> R13: 000000000000000e R14: 0000000000001770 R15: 0000000000000000
>>>>>>>
>>>>>>> Fixes: 1cce4df04f37 ("ocfs2: do not lock/unlock() inode DLM lock")
>>>>>>> Signed-off-by: Gang He <ghe@suse.com>
Reviewed-by: Jun Piao <piaojun@huawei.com>
>>>>>>> ---
>>>>>>> fs/ocfs2/dlmglue.c | 9 +++++++++
>>>>>>> 1 file changed, 9 insertions(+)
>>>>>>>
>>>>>>> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
>>>>>>> index 4689940..5193218 100644
>>>>>>> --- a/fs/ocfs2/dlmglue.c
>>>>>>> +++ b/fs/ocfs2/dlmglue.c
>>>>>>> @@ -2486,6 +2486,15 @@ int ocfs2_inode_lock_with_page(struct inode *inode,
>>>>>>> ret = ocfs2_inode_lock_full(inode, ret_bh, ex, OCFS2_LOCK_NONBLOCK);
>>>>>>> if (ret == -EAGAIN) {
>>>>>>> unlock_page(page);
>>>>>>> + /*
>>>>>>> + * If we can't get inode lock immediately, we should not return
>>>>>>> + * directly here, since this will lead to a softlockup problem.
>>>>>>> + * The method is to get a blocking lock and immediately unlock
>>>>>>> + * before returning, this can avoid CPU resource waste due to
>>>>>>> + * lots of retries, and benefits fairness in getting lock.
>>>>>>> + */
>>>>>>> + if (ocfs2_inode_lock(inode, ret_bh, ex) == 0)
>>>>>>> + ocfs2_inode_unlock(inode, ex);
>>>>>>> ret = AOP_TRUNCATED_PAGE;
>>>>>>> }
>>>>>>>
>>>>>>>
>>>>> .
>>>>>
>>> .
>>>
> .
>

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