linux-kernel - Re: [PATCH] ext4: only update i_reserved_data

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Message-ID: <20230327124700.mnldh4sosp3ptbls@quack3>
Date:   Mon, 27 Mar 2023 14:47:00 +0200
From:   Jan Kara <jack@...e.cz>
To:     Baokun Li <libaokun1@...wei.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, jack@...e.cz, ritesh.list@...il.com,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        yangerkun@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH] ext4: only update i_reserved_data_blocks on successful
 block allocation

On Sat 25-03-23 14:34:43, Baokun Li wrote:
> In our fault injection test, we create an ext4 file, migrate it to
> non-extent based file, then punch a hole and finally trigger a WARN_ON
> in the ext4_da_update_reserve_space():
> 
> EXT4-fs warning (device sda): ext4_da_update_reserve_space:369:
> ino 14, used 11 with only 10 reserved data blocks
> 
> When writing back a non-extent based file, if we enable delalloc, the
> number of reserved blocks will be subtracted from the number of blocks
> mapped by ext4_ind_map_blocks(), and the extent status tree will be
> updated. We update the extent status tree by first removing the old
> extent_status and then inserting the new extent_status. If the block range
> we remove happens to be in an extent, then we need to allocate another
> extent_status with ext4_es_alloc_extent().
> 
>        use old    to remove   to add new
>     |----------|------------|------------|
>               old extent_status
> 
> The problem is that the allocation of a new extent_status failed due to a
> fault injection, and __es_shrink() did not get free memory, resulting in
> a return of -ENOMEM. Then do_writepages() retries after receiving -ENOMEM,
> we map to the same extent again, and the number of reserved blocks is again
> subtracted from the number of blocks in that extent. Since the blocks in
> the same extent are subtracted twice, we end up triggering WARN_ON at
> ext4_da_update_reserve_space() because used > ei->i_reserved_data_blocks.

Hum, but this second call to ext4_map_blocks() should find already allocated
blocks in the indirect block and thus should not be subtracting
ei->i_reserved_data_blocks for the second time. What am I missing?

								Honza

> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR