Re: [PATCH v2] ext4: fix race condition between buffer write and page_mkwrite

Baokun Li
2023-06-06T12:19:34Z

On 2023/6/5 23:08, Jan Kara wrote:
> On Mon 05-06-23 15:55:35, Matthew Wilcox wrote:
>> On Mon, Jun 05, 2023 at 02:21:41PM +0200, Jan Kara wrote:
>>> On Mon 05-06-23 11:16:55, Jan Kara wrote:
>>>> Yeah, I agree, that is also the conclusion I have arrived at when thinking
>>>> about this problem now. We should be able to just remove the conversion
>>>> from ext4_page_mkwrite() and rely on write(2) or truncate(2) doing it when
>>>> growing i_size.
>>> OK, thinking more about this and searching through the history, I've
>>> realized why the conversion is originally in ext4_page_mkwrite(). The
>>> problem is described in commit 7b4cc9787fe35b ("ext4: evict inline data
>>> when writing to memory map") but essentially it boils down to the fact that
>>> ext4 writeback code does not expect dirty page for a file with inline data
>>> because ext4_write_inline_data_end() should have copied the data into the
>>> inode and cleared the folio's dirty flag.
>>>
>>> Indeed messing with xattrs from the writeback path to copy page contents
>>> into inline data xattr would be ... interesting. Hum, out of good ideas for
>>> now :-|.
>> Is it so bad? Now that we don't have writepage in ext4, only
>> writepages, it seems like we have a considerably more benign locking
>> environment to work in.
> Well, yes, without ->writepage() it might be *possible*. But still rather
> ugly. The problem is that in ->writepages() i_size is not stable. Thus also
> whether the inode data is inline or not is not stable. We'd need inode_lock
> for that but that is not easily doable in the writeback path - inode lock
> would then become fs_reclaim unsafe...
>
> Honza

If we try to add inode_lock to ext4_writepages to complete the
conversion, there may be a deadlock as follows:

CPU0             CPU1
writeback_single_inode
spin_lock(&inode->i_lock) ---> LOCK B
enable_verity
inode_lock(inode)  ---> LOCK A
vops->begin_enable_verity
ext4_begin_enable_verity
ext4_inode_attach_jinode
spin_lock(&inode->i_lock)   ---> try LOCK B
__writeback_single_inode          |
do_writepages                ABBA deadlock
ext4_writepages                 |
inode_lock(inode)  ---> try LOCK A

If we add inode_lock to the write back process to complete the inline
conversion,
it seems that we still have to add an ops ...

I've been going over this problem for a long time, but I can't think of
a good way
to solve it.

--
With Best Regards,
Baokun Li
.