Re: [PATCH 08/13] iomap: assign current->backing_dev_info in iomap_file_buffered_write

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, May 19, 2023 at 11:35:16AM +0200, Christoph Hellwig wrote:
> Move the assignment to current->backing_dev_info from the callers into
> iomap_file_buffered_write to reduce boiler plate code and reduce the
> scope to just around the page dirtying loop.
> 
> Note that zonefs was missing this assignment before.

I'm still wondering (a) what the hell current->backing_dev_info is for,
and (b) if we need it around the iomap_unshare operation.

$ git grep current..backing_dev_info
fs/btrfs/file.c:1148:   current->backing_dev_info = inode_to_bdi(inode);
fs/btrfs/file.c:1169:                   current->backing_dev_info = NULL;
fs/btrfs/file.c:1692:   current->backing_dev_info = NULL;
fs/ceph/file.c:1795:    current->backing_dev_info = inode_to_bdi(inode);
fs/ceph/file.c:1943:    current->backing_dev_info = NULL;
fs/ext4/file.c:288:     current->backing_dev_info = inode_to_bdi(inode);
fs/ext4/file.c:290:     current->backing_dev_info = NULL;
fs/f2fs/file.c:4520:    current->backing_dev_info = inode_to_bdi(inode);
fs/f2fs/file.c:4522:    current->backing_dev_info = NULL;
fs/fuse/file.c:1366:    current->backing_dev_info = inode_to_bdi(inode);
fs/fuse/file.c:1412:    current->backing_dev_info = NULL;
fs/gfs2/file.c:1044:    current->backing_dev_info = inode_to_bdi(inode);
fs/gfs2/file.c:1048:    current->backing_dev_info = NULL;
fs/nfs/file.c:652:              current->backing_dev_info = inode_to_bdi(inode);
fs/nfs/file.c:654:              current->backing_dev_info = NULL;
fs/ntfs/file.c:1914:    current->backing_dev_info = inode_to_bdi(vi);
fs/ntfs/file.c:1918:    current->backing_dev_info = NULL;
fs/ntfs3/file.c:823:    current->backing_dev_info = inode_to_bdi(inode);
fs/ntfs3/file.c:996:    current->backing_dev_info = NULL;
fs/xfs/xfs_file.c:721:  current->backing_dev_info = inode_to_bdi(inode);
fs/xfs/xfs_file.c:756:  current->backing_dev_info = NULL;
mm/filemap.c:3995:      current->backing_dev_info = inode_to_bdi(inode);
mm/filemap.c:4056:      current->backing_dev_info = NULL;

AFAICT nobody uses it at all?  Unless there's some bizarre user that
isn't extracting it from @current?

Oh, hey, new question (c) isn't this set incorrectly for xfs realtime
files?

--D

> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/gfs2/file.c         | 3 ---
>  fs/iomap/buffered-io.c | 3 +++
>  fs/xfs/xfs_file.c      | 5 -----
>  3 files changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/gfs2/file.c b/fs/gfs2/file.c
> index 499ef174dec138..261897fcfbc495 100644
> --- a/fs/gfs2/file.c
> +++ b/fs/gfs2/file.c
> @@ -25,7 +25,6 @@
>  #include <linux/dlm.h>
>  #include <linux/dlm_plock.h>
>  #include <linux/delay.h>
> -#include <linux/backing-dev.h>
>  #include <linux/fileattr.h>
>  
>  #include "gfs2.h"
> @@ -1041,11 +1040,9 @@ static ssize_t gfs2_file_buffered_write(struct kiocb *iocb,
>  			goto out_unlock;
>  	}
>  
> -	current->backing_dev_info = inode_to_bdi(inode);
>  	pagefault_disable();
>  	ret = iomap_file_buffered_write(iocb, from, &gfs2_iomap_ops);
>  	pagefault_enable();
> -	current->backing_dev_info = NULL;
>  	if (ret > 0)
>  		written += ret;
>  
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 550525a525c45c..b2779bd1f10611 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -3,6 +3,7 @@
>   * Copyright (C) 2010 Red Hat, Inc.
>   * Copyright (C) 2016-2019 Christoph Hellwig.
>   */
> +#include <linux/backing-dev.h>
>  #include <linux/module.h>
>  #include <linux/compiler.h>
>  #include <linux/fs.h>
> @@ -869,8 +870,10 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
>  	if (iocb->ki_flags & IOCB_NOWAIT)
>  		iter.flags |= IOMAP_NOWAIT;
>  
> +	current->backing_dev_info = inode_to_bdi(iter.inode);
>  	while ((ret = iomap_iter(&iter, ops)) > 0)
>  		iter.processed = iomap_write_iter(&iter, i);
> +	current->backing_dev_info = NULL;
>  
>  	if (unlikely(ret < 0))
>  		return ret;
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index bfba10e0b0f3c2..98d763cc3b114c 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -27,7 +27,6 @@
>  
>  #include <linux/dax.h>
>  #include <linux/falloc.h>
> -#include <linux/backing-dev.h>
>  #include <linux/mman.h>
>  #include <linux/fadvise.h>
>  #include <linux/mount.h>
> @@ -717,9 +716,6 @@ xfs_file_buffered_write(
>  	if (ret)
>  		goto out;
>  
> -	/* We can write back this queue in page reclaim */
> -	current->backing_dev_info = inode_to_bdi(inode);
> -
>  	trace_xfs_file_buffered_write(iocb, from);
>  	ret = iomap_file_buffered_write(iocb, from,
>  			&xfs_buffered_write_iomap_ops);
> @@ -751,7 +747,6 @@ xfs_file_buffered_write(
>  		goto write_retry;
>  	}
>  
> -	current->backing_dev_info = NULL;
>  out:
>  	if (iolock)
>  		xfs_iunlock(ip, iolock);
> -- 
> 2.39.2
>