linux-kernel - Re: [PATCH v3 7/7] gfs2: Fix mmap + page fault deadlocks for direct I/O

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Message-ID: <20210726170250.GL20621@quack2.suse.cz>
Date:   Mon, 26 Jul 2021 19:02:50 +0200
From:   Jan Kara <jack@...e.cz>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>, cluster-devel@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        ocfs2-devel@....oracle.com
Subject: Re: [PATCH v3 7/7] gfs2: Fix mmap + page fault deadlocks for direct
 I/O

On Fri 23-07-21 22:58:40, Andreas Gruenbacher wrote:
> Also disable page faults during direct I/O requests and implement the same kind
> of retry logic as in the buffered I/O case.
> 
> Direct I/O requests differ from buffered I/O requests in that they use
> bio_iov_iter_get_pages for grabbing page references and faulting in pages
> instead of triggering real page faults.  Those manual page faults can be
> disabled with the iocb->noio flag.
> 
> Signed-off-by: Andreas Gruenbacher <agruenba@...hat.com>
> ---
>  fs/gfs2/file.c | 34 +++++++++++++++++++++++++++++++++-
>  1 file changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/gfs2/file.c b/fs/gfs2/file.c
> index f66ac7f56f6d..7986f3be69d2 100644
> --- a/fs/gfs2/file.c
> +++ b/fs/gfs2/file.c
> @@ -782,21 +782,41 @@ static ssize_t gfs2_file_direct_read(struct kiocb *iocb, struct iov_iter *to,
>  	struct file *file = iocb->ki_filp;
>  	struct gfs2_inode *ip = GFS2_I(file->f_mapping->host);
>  	size_t count = iov_iter_count(to);
> +	size_t written = 0;
>  	ssize_t ret;
>  
> +	/*
> +	 * In this function, we disable page faults when we're holding the
> +	 * inode glock while doing I/O.  If a page fault occurs, we drop the
> +	 * inode glock, fault in the pages manually, and then we retry.  Other
> +	 * than in gfs2_file_read_iter, iomap_dio_rw can trigger implicit as
> +	 * well as manual page faults, and we need to disable both kinds
> +	 * separately.
> +	 */
> +
>  	if (!count)
>  		return 0; /* skip atime */
>  
>  	gfs2_holder_init(ip->i_gl, LM_ST_DEFERRED, 0, gh);
> +retry:
>  	ret = gfs2_glock_nq(gh);
>  	if (ret)
>  		goto out_uninit;
>  
> +	pagefault_disable();

Is there any use in pagefault_disable() here? iomap_dio_rw() should not
trigger any page faults anyway, should it?

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