linux-ext4 - Re: [PATCH v5 02/12] ext4: iomap that extends beyond EOF should be marked dirty

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Message-ID: <20191021132818.GB25184@quack2.suse.cz>
Date:   Mon, 21 Oct 2019 15:28:18 +0200
From:   Jan Kara <jack@...e.cz>
To:     Matthew Bobrowski <mbobrowski@...browski.org>
Cc:     tytso@....edu, jack@...e.cz, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        hch@...radead.org, david@...morbit.com, darrick.wong@...cle.com
Subject: Re: [PATCH v5 02/12] ext4: iomap that extends beyond EOF should be
 marked dirty

On Mon 21-10-19 20:17:46, Matthew Bobrowski wrote:
> This patch is effectively addressed what Dave Chinner had found and
> fixed within this commit: 8a23414ee345. Justification for needing this
> modification has been provided below:
> 
> When doing a direct IO that spans the current EOF, and there are
> written blocks beyond EOF that extend beyond the current write, the
> only metadata update that needs to be done is a file size extension.
> 
> However, we don't mark such iomaps as IOMAP_F_DIRTY to indicate that
> there is IO completion metadata updates required, and hence we may
> fail to correctly sync file size extensions made in IO completion when
> O_DSYNC writes are being used and the hardware supports FUA.
> 
> Hence when setting IOMAP_F_DIRTY, we need to also take into account
> whether the iomap spans the current EOF. If it does, then we need to
> mark it dirty so that IO completion will call generic_write_sync() to
> flush the inode size update to stable storage correctly.
> 
> Signed-off-by: Matthew Bobrowski <mbobrowski@...browski.org>

Looks good to me. You could possibly also comment in the changelog that
currently, this change doesn't have user visible impact for ext4 as none of
current users of ext4_iomap_begin() that extend files depend of
IOMAP_F_DIRTY.

Also this patch would make slightly more sense to be before 1/12 so that
you don't have there those two strange unused arguments. But these are just
small nits.

Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

							Honza

> ---
>  fs/ext4/inode.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 158eea9a1944..0dd29ae5cc8c 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3412,8 +3412,14 @@ static void ext4_set_iomap(struct inode *inode, struct iomap *iomap,
>  {
>  	u8 blkbits = inode->i_blkbits;
>  
> +	/*
> +	 * Writes that span EOF might trigger an I/O size update on completion,
> +	 * so consider them to be dirty for the purposes of O_DSYNC, even if
> +	 * there is no other metadata changes being made or are pending here.
> +	 */
>  	iomap->flags = 0;
> -	if (ext4_inode_datasync_dirty(inode))
> +	if (ext4_inode_datasync_dirty(inode) ||
> +	    offset + length > i_size_read(inode))
>  		iomap->flags |= IOMAP_F_DIRTY;
>  
>  	if (map->m_flags & EXT4_MAP_NEW)
> -- 
> 2.20.1
> 
> --<M>--
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR