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Subject: Re: [PATCH] xfs: fix BUG_ON in xfs_getbmap()
To:     "Darrick J. Wong" <djwong@kernel.org>,
        Ye Bin <yebin@huaweicloud.com>
References: <20230327140218.4154709-1-yebin@huaweicloud.com>
 <20230327151524.GC16180@frogsfrogsfrogs>
CC:     <linux-xfs@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        Christoph Hellwig <hch@infradead.org>
From:   "yebin (H)" <yebin10@huawei.com>
Message-ID: <64224406.5090106@huawei.com>
Date:   Tue, 28 Mar 2023 09:33:58 +0800
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On 2023/3/27 23:15, Darrick J. Wong wrote:
> [add Christoph to cc since he added/last touched this assert, I think]
>
> On Mon, Mar 27, 2023 at 10:02:18PM +0800, Ye Bin wrote:
>> From: Ye Bin <yebin10@huawei.com>
>>
>> There's issue as follows:
>> XFS: Assertion failed: (bmv->bmv_iflags & BMV_IF_DELALLOC) != 0, file: fs/xfs/xfs_bmap_util.c, line: 329
> Why not get rid of the assertion?  It's not like it changes the course
> of the code flow -- userspace still gets told there's a delalloc extent.
Thank you for your reply.
I think it's incorrect to return the delalloc extent to the user in this 
case. Because
users expect to obtain none delalloc  extent information. If there is a 
delalloc  extent
found at this time, there is a problem with the functionality. I even 
think that here
we should return an error to the userspace instead of return an 
incorrect result to
the userspace .
> Or, if the assert does serve some purpose, then do we need to take
> the mmaplock for cow fork reporting too?
Let me analyze whether it is necessary to take the mmaplock for cow fork 
reporting.
> --D
>
>> ------------[ cut here ]------------
>> kernel BUG at fs/xfs/xfs_message.c:102!
>> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
>> CPU: 1 PID: 14612 Comm: xfs_io Not tainted 6.3.0-rc2-next-20230315-00006-g2729d23ddb3b-dirty #422
>> RIP: 0010:assfail+0x96/0xa0
>> RSP: 0018:ffffc9000fa178c0 EFLAGS: 00010246
>> RAX: 0000000000000000 RBX: 0000000000000001 RCX: ffff888179a18000
>> RDX: 0000000000000000 RSI: ffff888179a18000 RDI: 0000000000000002
>> RBP: 0000000000000000 R08: ffffffff8321aab6 R09: 0000000000000000
>> R10: 0000000000000001 R11: ffffed1105f85139 R12: ffffffff8aacc4c0
>> R13: 0000000000000149 R14: ffff888269f58000 R15: 000000000000000c
>> FS:  00007f42f27a4740(0000) GS:ffff88882fc00000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 0000000000b92388 CR3: 000000024f006000 CR4: 00000000000006e0
>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> Call Trace:
>>   <TASK>
>>   xfs_getbmap+0x1a5b/0x1e40
>>   xfs_ioc_getbmap+0x1fd/0x5b0
>>   xfs_file_ioctl+0x2cb/0x1d50
>>   __x64_sys_ioctl+0x197/0x210
>>   do_syscall_64+0x39/0xb0
>>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>
>> Above issue may happen as follows:
>>           ThreadA                       ThreadB
>> do_shared_fault
>>   __do_fault
>>    xfs_filemap_fault
>>     __xfs_filemap_fault
>>      filemap_fault
>>                               xfs_ioc_getbmap -> Without BMV_IF_DELALLOC flag
>> 			      xfs_getbmap
>> 			       xfs_ilock(ip, XFS_IOLOCK_SHARED);
>> 			       filemap_write_and_wait
>>   do_page_mkwrite
>>    xfs_filemap_page_mkwrite
>>     __xfs_filemap_fault
>>      xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
>>      iomap_page_mkwrite
>>       ...
>>       xfs_buffered_write_iomap_begin
>>        xfs_bmapi_reserve_delalloc -> Allocate delay extent
>>                                xfs_ilock_data_map_shared(ip)
>> 	                      xfs_getbmap_report_one
>> 			       ASSERT((bmv->bmv_iflags & BMV_IF_DELALLOC) != 0)
>> 	                        -> trigger BUG_ON
>>
>> As xfs_filemap_page_mkwrite() only hold XFS_MMAPLOCK_SHARED lock, there's
>> small window mkwrite can produce delay extent after file write in xfs_getbmap().
>> To solve above issue, hold XFS_MMAPLOCK_EXCL lock when do xfs_getbmap(),
>> to prevent write operations by do_page_mkwrite().
>> During doing __xfs_filemap_fault() we can't hold IOLOCK lock, as it's may lead
>> to ABBA dealock with xfs_file_write_iter().It's very easy to reproduce when
>> do fsstress, lockdep will detect deadlock.
>>
>> Signed-off-by: Ye Bin <yebin10@huawei.com>
>> ---
>>   fs/xfs/xfs_bmap_util.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
>> index a09dd2606479..f23771a0cc8d 100644
>> --- a/fs/xfs/xfs_bmap_util.c
>> +++ b/fs/xfs/xfs_bmap_util.c
>> @@ -463,11 +463,13 @@ xfs_getbmap(
>>   			max_len = XFS_ISIZE(ip);
>>   		break;
>>   	case XFS_DATA_FORK:
>> +		lock = XFS_MMAPLOCK_EXCL;
>> +		xfs_ilock(ip, lock);
>>   		if (!(iflags & BMV_IF_DELALLOC) &&
>>   		    (ip->i_delayed_blks || XFS_ISIZE(ip) > ip->i_disk_size)) {
>>   			error = filemap_write_and_wait(VFS_I(ip)->i_mapping);
>>   			if (error)
>> -				goto out_unlock_iolock;
>> +				goto out_unlock_ilock;
>>   
>>   			/*
>>   			 * Even after flushing the inode, there can still be
>> @@ -486,7 +488,7 @@ xfs_getbmap(
>>   		else
>>   			max_len = XFS_ISIZE(ip);
>>   
>> -		lock = xfs_ilock_data_map_shared(ip);
>> +		lock |= xfs_ilock_data_map_shared(ip);
>>   		break;
>>   	}
>>   
>> -- 
>> 2.31.1
>>
> .
>