Re: [PATCH v4 02/34] ext4: check the extent status again before inserting delalloc block

Zhang Yi
2024-04-26T13:19:53Z

On 2024/4/26 20:57, Ritesh Harjani (IBM) wrote:
> Ritesh Harjani (IBM) <ritesh.list@gmail.com> writes:
>
>> Zhang Yi <yi.zhang@huaweicloud.com> writes:
>>
>>> From: Zhang Yi <yi.zhang@huawei.com>
>>>
>>> Now we lookup extent status entry without holding the i_data_sem before
>>> inserting delalloc block, it works fine in buffered write path and
>>> because it holds i_rwsem and folio lock, and the mmap path holds folio
>>> lock, so the found extent locklessly couldn't be modified concurrently.
>>> But it could be raced by fallocate since it allocate block whitout
>>> holding i_rwsem and folio lock.
>>>
>>> ext4_page_mkwrite() ext4_fallocate()
>>> block_page_mkwrite()
>>> ext4_da_map_blocks()
>>> //find hole in extent status tree
>>> ext4_alloc_file_blocks()
>>> ext4_map_blocks()
>>> //allocate block and unwritten extent
>>> ext4_insert_delayed_block()
>>> ext4_da_reserve_space()
>>> //reserve one more block
>>> ext4_es_insert_delayed_block()
>>> //drop unwritten extent and add delayed extent by mistake
>>>
>>> Then, the delalloc extent is wrong until writeback, the one more
>>> reserved block can't be release any more and trigger below warning:
>>>
>>> EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared!
>>>
>>> Hold i_data_sem in write mode directly can fix the problem, but it's
>>> expansive, we should keep the lockless check and check the extent again
>>> once we need to add an new delalloc block.
>>
>> Hi Zhang,
>>
>> It's a nice finding. I was wondering if this was caught in any of the
>> xfstests?
>>

Hi, Ritesh

I caught this issue when I tested my iomap series in generic/344 and
generic/346. It's easy to reproduce because the iomap's buffered write path
doesn't hold folio lock while inserting delalloc blocks, so it could be raced
by the mmap page fault path. But the buffer_head's buffered write path can't
trigger this problem, the race between buffered write path and fallocate path
was discovered while I was analyzing the code, so I'm not sure if it could
be caught by xfstests now, at least I haven't noticed this problem so far.

>> I have reworded some of the commit message, feel free to use it if you
>> think this version is better. The use of which path uses which locks was
>> a bit confusing in the original commit message.
>>

Thanks for the message improvement, it looks more clear then mine, I will
use it.

Thanks,
Yi.

>> <reworded from your original commit msg>
>>
>> ext4_da_map_blocks(), first looks up the extent status tree for any
>> extent entry with i_data_sem held in read mode. It then unlocks
>> i_data_sem, if it can't find an entry and take this lock in write
>> mode for inserting a new da entry.
>
> Sorry about this above paragraph. I messed this paragraph.
> Here is the correct version of this.
>
> ext4_da_map_blocks looks up for any extent entry in the extent status
> tree (w/o i_data_sem) and then the looks up for any ondisk extent
> mapping (with i_data_sem in read mode).
>
> If it finds a hole in the extent status tree or if it couldn't find any
> entry at all, it then takes the i_data_sem in write mode to add a da entry
> into the extent status tree. This can actually race with page mkwrite
> & fallocate path.
>
> Note that this is ok between... <and the rest can remain same>
>
>>
>> This is ok between -
>> 1. ext4 buffered-write path v/s ext4_page_mkwrite(), because of the
>> folio lock
>> 2. ext4 buffered write path v/s ext4 fallocate because of the inode
>> lock.
>>
>
>
>> But this can race between ext4_page_mkwrite() & ext4 fallocate path -
>>
>> ext4_page_mkwrite() ext4_fallocate()
>> block_page_mkwrite()
>> ext4_da_map_blocks()
>> //find hole in extent status tree
>> ext4_alloc_file_blocks()
>> ext4_map_blocks()
>> //allocate block and unwritten extent
>> ext4_insert_delayed_block()
>> ext4_da_reserve_space()
>> //reserve one more block
>> ext4_es_insert_delayed_block()
>> //drop unwritten extent and add delayed extent by mistake
>>
>> Then, the delalloc extent is wrong until writeback and the extra
>> reserved block can't be released any more and it triggers below warning:
>>
>> EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared!
>>
>> This patch fixes the problem by looking up extent status tree again
>> while the i_data_sem is held in write mode. If it still can't find
>> any entry, then we insert a new da entry into the extent status tree.
>>
>>>
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
>>> ---
>>> fs/ext4/inode.c | 19 +++++++++++++++++++
>>> 1 file changed, 19 insertions(+)
>>>
>>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>>> index 6a41172c06e1..118b0497a954 100644
>>> --- a/fs/ext4/inode.c
>>> +++ b/fs/ext4/inode.c
>>> @@ -1737,6 +1737,7 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
>>> if (ext4_es_is_hole(&es))
>>> goto add_delayed;
>>>
>>> +found:
>>> /*
>>> * Delayed extent could be allocated by fallocate.
>>> * So we need to check it.
>>> @@ -1781,6 +1782,24 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
>>>
>>> add_delayed:
>>> down_write(&EXT4_I(inode)->i_data_sem);
>>> + /*
>>> + * Lookup extents tree again under i_data_sem, make sure this
>>> + * inserting delalloc range haven't been delayed or allocated
>>> + * whitout holding i_rwsem and folio lock.
>>> + */
>>
>> page fault path (ext4_page_mkwrite does not take i_rwsem) and fallocate
>> path (no folio lock) can race. Make sure we lookup the extent status
>> tree here again while i_data_sem is held in write mode, before inserting
>> a new da entry in the extent status tree.
>>
>>
>
>
> -ritesh
>