On Mon, 2024-01-29 at 09:49 +0000, David Howells wrote:
> Fix netfs_unbuffered_write_iter() to return immediately if
> generic_write_checks() returns 0, indicating there's nothing to write.
> Note that netfs_file_write_iter() already does this.
>
> Also, whilst we're at it, put in checks for the size being zero before we
> even take the locks. Note that generic_write_checks() can still reduce the
> size to zero, so we still need that check.
>
> Without this, a warning similar to the following is logged to dmesg:
>
> netfs: Zero-sized write [R=1b6da]
>
> and the syscall fails with EIO, e.g.:
>
> /sbin/ldconfig.real: Writing of cache extension data failed:
> Input/output error
>
> This can be reproduced on 9p by:
>
> xfs_io -f -c 'pwrite 0 0' /xfstest.test/foo
>
> Fixes: 153a9961b551 ("netfs: Implement unbuffered/DIO write support")
> Reported-by: Eric Van Hensbergen <[email protected]>
> Link: https://lore.kernel.org/r/ZbQUU6QKmIftKsmo@FV7GG9FTHL/
> Signed-off-by: David Howells <[email protected]>
> cc: Dominique Martinet <[email protected]>
> cc: Jeff Layton <[email protected]>
> cc: [email protected]
> cc: [email protected]
> cc: [email protected]
> cc: [email protected]
> ---
> fs/netfs/buffered_write.c | 3 +++
> fs/netfs/direct_write.c | 5 ++++-
> 2 files changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/fs/netfs/buffered_write.c b/fs/netfs/buffered_write.c
> index a3059b3168fd..9a0d32e4b422 100644
> --- a/fs/netfs/buffered_write.c
> +++ b/fs/netfs/buffered_write.c
> @@ -477,6 +477,9 @@ ssize_t netfs_file_write_iter(struct kiocb *iocb, struct
> iov_iter *from)
>
> _enter("%llx,%zx,%llx", iocb->ki_pos, iov_iter_count(from),
> i_size_read(inode));
>
> + if (!iov_iter_count(from))
> + return 0;
> +
> if ((iocb->ki_flags & IOCB_DIRECT) ||
> test_bit(NETFS_ICTX_UNBUFFERED, &ictx->flags))
> return netfs_unbuffered_write_iter(iocb, from);
> diff --git a/fs/netfs/direct_write.c b/fs/netfs/direct_write.c
> index 60a40d293c87..bee047e20f5d 100644
> --- a/fs/netfs/direct_write.c
> +++ b/fs/netfs/direct_write.c
> @@ -139,6 +139,9 @@ ssize_t netfs_unbuffered_write_iter(struct kiocb *iocb,
> struct iov_iter *from)
>
> _enter("%llx,%zx,%llx", iocb->ki_pos, iov_iter_count(from),
> i_size_read(inode));
>
> + if (!iov_iter_count(from))
> + return 0;
> +
> trace_netfs_write_iter(iocb, from);
> netfs_stat(&netfs_n_rh_dio_write);
>
> @@ -146,7 +149,7 @@ ssize_t netfs_unbuffered_write_iter(struct kiocb *iocb,
> struct iov_iter *from)
> if (ret < 0)
> return ret;
> ret = generic_write_checks(iocb, from);
> - if (ret < 0)
> + if (ret <= 0)
> goto out;
> ret = file_remove_privs(file);
> if (ret < 0)
>
Reviewed-by: Jeff Layton <[email protected]>
