linux-ext4 - Re: [PATCH] ext2: discard block reservation on last writable file release

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Message-ID: <20210104140151.GD4018@quack2.suse.cz>
Date:   Mon, 4 Jan 2021 15:01:51 +0100
From:   Jan Kara <jack@...e.cz>
To:     Chengguang Xu <cgxu519@...ernel.net>
Cc:     jack@...e.com, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext2: discard block reservation on last writable file
 release

On Sat 02-01-21 18:18:05, Chengguang Xu wrote:
> Currently reserved blocks are discarded on every writable
> file release, it's not efficient for multiple writer case.
> 
> Signed-off-by: Chengguang Xu <cgxu519@...ernel.net>

Thanks for the patch. I agree that in principle something like this is
desirable but there's a small catch. i_writecount is also elevated from
vfs_truncate() which does not have inode open. So it can happen that
->release() gets called, sees inode->i_writecount > 1, but never gets
called again (and thus reservation is not properly released). So I prefer
to leave ext2 as is until this gets resolved - especially since ext2 fs
driver isn't really used on any performance sensitive multi-writer
workloads AFAIK (ext4 driver is usually used in such cases).

								Honza

> ---
>  fs/ext2/file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ext2/file.c b/fs/ext2/file.c
> index 96044f5dbc0e..9a19d8fe7ffd 100644
> --- a/fs/ext2/file.c
> +++ b/fs/ext2/file.c
> @@ -141,7 +141,7 @@ static int ext2_file_mmap(struct file *file, struct vm_area_struct *vma)
>   */
>  static int ext2_release_file (struct inode * inode, struct file * filp)
>  {
> -	if (filp->f_mode & FMODE_WRITE) {
> +	if (filp->f_mode & FMODE_WRITE && (atomic_read(&inode->i_writecount) == 1)) {
>  		mutex_lock(&EXT2_I(inode)->truncate_mutex);
>  		ext2_discard_reservation(inode);
>  		mutex_unlock(&EXT2_I(inode)->truncate_mutex);
> -- 
> 2.18.4
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR