Re: [PATCH] NFS: Fix O_DIRECT read problem when another write is going on

[Trond Myklebust]

Hi Su,

On Mon, 2019-09-30 at 17:11 +0800, Su Yanjun wrote:
> In xfstests generic/465 tests failed. Because O_DIRECT r/w use
> async rpc calls, when r/w rpc calls are running concurrently we
> may read partial data which is wrong.
> 
> For example as follows.
>  user buffer
> /--------\
> >    |XXXX|
>  rpc0 rpc1
> 
> When rpc0 runs it encounters eof so return 0, then another writes
> something. When rpc1 runs it returns some data. The total data
> buffer contains wrong data.
> 
> In this patch we check eof mark for each direct request. If
> encounters
> eof then set eof mark in the request, when we meet it again report
> -EAGAIN error. In nfs_direct_complete we convert -EAGAIN as if read
> nothing. When the reader issue another read it will read ok.
> 
> Signed-off-by: Su Yanjun <suyj.fnst@xxxxxxxxxxxxxx>
> ---
>  fs/nfs/direct.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
> index 222d711..7f737a3 100644
> --- a/fs/nfs/direct.c
> +++ b/fs/nfs/direct.c
> @@ -93,6 +93,7 @@ struct nfs_direct_req {
>  				bytes_left,	/* bytes left to be
> sent */
>  				error;		/* any reported error
> */
>  	struct completion	completion;	/* wait for i/o completion */
> +	int			eof;		/* eof mark in the
> req */
>  
>  	/* commit state */
>  	struct nfs_mds_commit_info mds_cinfo;	/* Storage for cinfo
> */
> @@ -380,6 +381,12 @@ static void nfs_direct_complete(struct
> nfs_direct_req *dreq)
>  {
>  	struct inode *inode = dreq->inode;
>  
> +	/* read partial data just as read nothing */
> +	if (dreq->error == -EAGAIN) {
> +		dreq->count = 0;
> +		dreq->error = 0;
> +	}
> +
>  	inode_dio_end(inode);
>  
>  	if (dreq->iocb) {
> @@ -413,8 +420,13 @@ static void nfs_direct_read_completion(struct
> nfs_pgio_header *hdr)
>  	if (hdr->good_bytes != 0)
>  		nfs_direct_good_bytes(dreq, hdr);
>  
> -	if (test_bit(NFS_IOHDR_EOF, &hdr->flags))
> +	if (dreq->eof)
> +		dreq->error = -EAGAIN;
> +
> +	if (test_bit(NFS_IOHDR_EOF, &hdr->flags)) {
>  		dreq->error = 0;
> +		dreq->eof = 1;
> +	}
>  
>  	spin_unlock(&dreq->lock);
>  

Thanks for looking into this issue. I agree with your analysis of what
is going wrong in generic/465.

However, I think the problem is greater than just EOF. I think we also
need to look at the generic error handling, and ensure that it handles
a truncated RPC call in the middle of a series of calls correctly.

Please see the two patches I sent you just now and check if they fix
the problem for you.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@xxxxxxxxxxxxxxx