linux-ext4 - Re: [PATCH v2 1/2] ext4: only update i_reserved_data

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Date:   Tue, 11 Apr 2023 11:08:00 +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 v2 1/2] ext4: only update i_reserved_data_blocks on
 successful block allocation

On Thu 06-04-23 21:28:33, 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.
> 
> For non-extent based file, we update the number of reserved blocks after
> ext4_ind_map_blocks() is executed, which causes a problem that when we call
> ext4_ind_map_blocks() to create a block, it doesn't always create a block,
> but we always reduce the number of reserved blocks. So we move the logic
> for updating reserved blocks to ext4_ind_map_blocks() to ensure that the
> number of reserved blocks is updated only after we do succeed in allocating
> some new blocks.
> 
> Fixes: 5f634d064c70 ("ext4: Fix quota accounting error with fallocate")
> Signed-off-by: Baokun Li <libaokun1@...wei.com>

Looks good, just one nit below.

> diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
> index c68bebe7ff4b..9acab70ddf5e 100644
> --- a/fs/ext4/indirect.c
> +++ b/fs/ext4/indirect.c
> @@ -651,6 +651,14 @@ int ext4_ind_map_blocks(handle_t *handle, struct inode *inode,
>  
>  	ext4_update_inode_fsync_trans(handle, inode, 1);
>  	count = ar.len;
> +
> +	/*
> +	 * Update reserved blocks/metadata blocks after successful block
> +	 * allocation which had been deferred till now.
> +	 */
> +	if ((count > 0) && (flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE))

You don't need the count > 0 condition here. It should be always true.

								Honza

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