Re: [PATCH v2] ext4: skip eof zeroing when the zero range covers the eof block

[Baokun Li]

On 2026/10/23 17:23, Zhang Yi wrote:
> On 10/22/2026 8:26 PM, Baokun Li wrote:
>> ext4_zero_range() drops the page cache of the range before allocating,
>> to avoid exposing stale cached data.  However, the dropped page can be
>> read back by the eof zeroing:
>>
>>   fallocate(fd, FALLOC_FL_ZERO_RANGE, 0, 4096)
>>     ext4_zero_range()
>>       ext4_truncate_page_cache_block_range(0, 4096)
>>         truncate_pagecache_range(0, 4095)
>>           truncate_inode_pages_range()
>>             delete_from_page_cache_batch()  # drops the page
>>       ext4_alloc_file_blocks(..., 4096)
>>         ext4_block_zero_eof(512, LLONG_MAX) # not block aligned
>>           ext4_load_tail_bh()
>>              __filemap_get_folio(FGP_WRITEBEGIN | FGP_ACCESSED)
>>              # FGP_CREAT reads the dropped page back
>>              # zeroes [512,1024), marks the buffer dirty
>>         ext4_map_blocks(...)  # converts the extent to unwritten
>>         ext4_update_inode_size(4096)
>>
>> The bytes below the old EOF keep the stale disk content.  The page is
>> left dirty, so read() returns old data while O_DIRECT reads zeroes, and
>> a write into the same block is not converted and is lost until the page
>> is dropped:
>>
>>   page [0, 4096), 1k block size, file ends at 512
>>    0            512          1024                      4096
>>    |--- data ----|-- tail ---|---- zero range ----------|
>>    | old data    |           |                          |
>>
>> For example:
>>
>>   $ xfs_io -f -c "pwrite -S 1 0 512" f
>>   $ sync
>>   $ fallocate -z -o 0 -l 4096 f
>>   $ xfs_io -c "pread -v 0 16" f  # old data, not zeroes
>>
>> FALLOC_FL_WRITE_ZEROES hits the same path.  ext4_issue_zeroout() zeroes
>> the block on disk without updating the page cache, and the block stays
>> written.  Therefore, writeback of the dirty page overwrites the zeroes
>> with old data.
>>
>> Since the whole eof block is converted to unwritten or zeroed out,
>> zeroing it here is redundant.  Therefore, skip the zeroing when this
>> allocation covers the eof block entirely.
>>
>> Fixes: c4602a1d09ec ("ext4: move zero partial block range functions out of active handle")
>> Fixes: f4265b8d32c4 ("ext4: add FALLOC_FL_WRITE_ZEROES support")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Baokun Li <libaokun@xxxxxxxxxxxxxxxxx>
> 
> Hi Baokun,

Hi Yi,

> 
> Thank you for the fix, this is a nice catch. This makes sense to me.
> Reviewed-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> 

Thank you for your review!

> One small point, the condition you added looks a bit confusing. I'm
> wondering whether, in the future, it would be more straightforward to
> extract ext4_block_zero_eof() from ext4_alloc_file_blocks() into
> ext4_zero_range() and ext4_do_fallocate()?

That sounds great. By writing zeros before truncating the page,
we won’t have to worry about the page cache being read back,
which means we won’t need to check the position of the EOF block
anymore.

I’ll send this out as a separate refactoring patch.

Thanks for the suggestion.


Cheers,
Baokun