Re: [Ocfs2-devel] [PATCH] ocfs2: try a blocking lock before return AOP_TRUNCATED_PAGE

[piaojun]

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?

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@xxxxxxxx>
>>> ---
>>>  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;
>>>  	}
>>>  
>>>
> .
>