linux-ext4 - Re: [PATCH -next] ext4: fix bug_on in ext4_iomap

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Date:   Thu, 16 Jun 2022 09:44:49 +0800
From:   yebin <yebin10@...wei.com>
To:     Ritesh Harjani <ritesh.list@...il.com>
CC:     <tytso@....edu>, <adilger.kernel@...ger.ca>,
        <linux-ext4@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <jack@...e.cz>
Subject: Re: [PATCH -next] ext4: fix bug_on in ext4_iomap_begin as race
 between bmap and write



On 2022/6/15 23:21, Ritesh Harjani wrote:
> On 22/06/15 09:58PM, Ye Bin wrote:
>> We got issue as follows:
>> ------------[ cut here ]------------
>> WARNING: CPU: 3 PID: 9310 at fs/ext4/inode.c:3441 ext4_iomap_begin+0x182/0x5d0
>> RIP: 0010:ext4_iomap_begin+0x182/0x5d0
>> RSP: 0018:ffff88812460fa08 EFLAGS: 00010293
>> RAX: ffff88811f168000 RBX: 0000000000000000 RCX: ffffffff97793c12
>> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
>> RBP: ffff88812c669160 R08: ffff88811f168000 R09: ffffed10258cd20f
>> R10: ffff88812c669077 R11: ffffed10258cd20e R12: 0000000000000001
>> R13: 00000000000000a4 R14: 000000000000000c R15: ffff88812c6691ee
>> FS:  00007fd0d6ff3740(0000) GS:ffff8883af180000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00007fd0d6dda290 CR3: 0000000104a62000 CR4: 00000000000006e0
>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> Call Trace:
>>   iomap_apply+0x119/0x570
>>   iomap_bmap+0x124/0x150
>>   ext4_bmap+0x14f/0x250
>>   bmap+0x55/0x80
>>   do_vfs_ioctl+0x952/0xbd0
>>   __x64_sys_ioctl+0xc6/0x170
>>   do_syscall_64+0x33/0x40
>>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> Above issue may happen as follows:
>>            bmap                    write
>> bmap
>>    ext4_bmap
>>      iomap_bmap
>>        ext4_iomap_begin
>>                              ext4_file_write_iter
>> 			      ext4_buffered_write_iter
>> 			        generic_perform_write
>> 				  ext4_da_write_begin
>> 				    ext4_da_write_inline_data_begin
>> 				      ext4_prepare_inline_data
>> 				        ext4_create_inline_data
>> 					  ext4_set_inode_flag(inode,
>> 						EXT4_INODE_INLINE_DATA);
>>        if (WARN_ON_ONCE(ext4_has_inline_data(inode))) ->trigger bug_on
>>
>> To solved above issue hold inode lock in ext4_bamp.
> 											^^^ ext4_bmap()
>
> I checked the paths where bmap() kernel api can be called i.e. from jbd2/fc and
> generic_swapfile_activate() (apart from ioctl())
> For jbd2, it will be called with j_inode within bmap(), hence taking a inode lock
> of the inode passed within ext4_bmap() (j_inode in this case) should be safe here.
> Same goes with swapfile path as well.
>
> However I feel maybe we should hold inode_lock_shared() since there is no
> block/extent map layout changes that can happen via ext4_bmap().
> Hence read lock is what IMO should be used here.
>
> -ritesh
Thank you for your advice.
>
>> Signed-off-by: Ye Bin <yebin10@...wei.com>
>> ---
>>   fs/ext4/inode.c | 12 +++++++++---
>>   1 file changed, 9 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index 53877ffe3c41..f4a95c80f644 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -3142,13 +3142,15 @@ static sector_t ext4_bmap(struct address_space *mapping, sector_t block)
>>   {
>>   	struct inode *inode = mapping->host;
>>   	journal_t *journal;
>> +	sector_t ret = 0;
>>   	int err;
>>
>> +	inode_lock(inode);
>>   	/*
>>   	 * We can get here for an inline file via the FIBMAP ioctl
>>   	 */
>>   	if (ext4_has_inline_data(inode))
>> -		return 0;
>> +		goto out;
>>
>>   	if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY) &&
>>   			test_opt(inode->i_sb, DELALLOC)) {
>> @@ -3187,10 +3189,14 @@ static sector_t ext4_bmap(struct address_space *mapping, sector_t block)
>>   		jbd2_journal_unlock_updates(journal);
>>
>>   		if (err)
>> -			return 0;
>> +			goto out;
>>   	}
>>
>> -	return iomap_bmap(mapping, block, &ext4_iomap_ops);
>> +	ret = iomap_bmap(mapping, block, &ext4_iomap_ops);
>> +
>> +out:
>> +	inode_unlock(inode);
>> +	return ret;
>>   }
>>
>>   static int ext4_read_folio(struct file *file, struct folio *folio)
>> --
>> 2.31.1
>>
> .
>