linux-ext4 - Re: [PATCH v3 02/26] ext4: convert to exclusive lock while inserting delalloc extents

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Date: Sat, 3 Feb 2024 12:56:25 -0500
From: "Theodore Ts'o" <tytso@....edu>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        adilger.kernel@...ger.ca, jack@...e.cz, ritesh.list@...il.com,
        hch@...radead.org, djwong@...nel.org, willy@...radead.org,
        zokeefe@...gle.com, yi.zhang@...wei.com, chengzhihao1@...wei.com,
        yukuai3@...wei.com, wangkefeng.wang@...wei.com
Subject: Re: [PATCH v3 02/26] ext4: convert to exclusive lock while inserting
 delalloc extents

On Sat, Jan 27, 2024 at 09:58:01AM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@...wei.com>
> 
> ext4_da_map_blocks() only hold i_data_sem in shared mode and i_rwsem
> when inserting delalloc extents, it could be raced by another querying
> path of ext4_map_blocks() without i_rwsem, .e.g buffered read path.
> Suppose we buffered read a file containing just a hole, and without any
> cached extents tree, then it is raced by another delayed buffered write
> to the same area or the near area belongs to the same hole, and the new
> delalloc extent could be overwritten to a hole extent.
> 
>  pread()                           pwrite()
>   filemap_read_folio()
>    ext4_mpage_readpages()
>     ext4_map_blocks()
>      down_read(i_data_sem)
>      ext4_ext_determine_hole()
>      //find hole
>      ext4_ext_put_gap_in_cache()
>       ext4_es_find_extent_range()
>       //no delalloc extent
>                                     ext4_da_map_blocks()
>                                      down_read(i_data_sem)
>                                      ext4_insert_delayed_block()
>                                      //insert delalloc extent
>       ext4_es_insert_extent()
>       //overwrite delalloc extent to hole
> 
> This race could lead to inconsistent delalloc extents tree and
> incorrect reserved space counter. Fix this by converting to hold
> i_data_sem in exclusive mode when adding a new delalloc extent in
> ext4_da_map_blocks().
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> Suggested-by: Jan Kara <jack@...e.cz>
> Reviewed-by: Jan Kara <jack@...e.cz>

Thanks, applied.

					- Ted