On Tue, Mar 07, 2023 at 08:10:32PM -0700, Jens Axboe wrote:
> In preparation for enabling FMODE_NOWAIT for pipes, ensure that the read
> and write path handle it correctly. This includes the pipe locking,
> page allocation for writes, and confirming pipe buffers.
>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> ---
> fs/pipe.c | 34 ++++++++++++++++++++++++++--------
> 1 file changed, 26 insertions(+), 8 deletions(-)
>
> diff --git a/fs/pipe.c b/fs/pipe.c
> index 340f253913a2..10366a6cb5b6 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -108,6 +108,16 @@ static inline void __pipe_unlock(struct pipe_inode_info *pipe)
> mutex_unlock(&pipe->mutex);
> }
>
> +static inline bool __pipe_trylock(struct pipe_inode_info *pipe, bool nonblock)
> +{
> + if (!nonblock) {
> + __pipe_lock(pipe);
> + return true;
> + }
> +
> + return mutex_trylock(&pipe->mutex);
> +}
> +
> void pipe_double_lock(struct pipe_inode_info *pipe1,
> struct pipe_inode_info *pipe2)
> {
> @@ -234,6 +244,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
> struct file *filp = iocb->ki_filp;
> struct pipe_inode_info *pipe = filp->private_data;
> bool was_full, wake_next_reader = false;
> + const bool nonblock = iocb->ki_flags & IOCB_NOWAIT;
> ssize_t ret;
>
> /* Null read succeeds. */
> @@ -241,7 +252,8 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
> return 0;
>
> ret = 0;
> - __pipe_lock(pipe);
> + if (!__pipe_trylock(pipe, nonblock))
> + return -EAGAIN;
>
> /*
> * We only wake up writers if the pipe was full when we started
> @@ -297,7 +309,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
> chars = total_len;
> }
>
> - error = pipe_buf_confirm(pipe, buf, false);
> + error = pipe_buf_confirm(pipe, buf, nonblock);
> if (error) {
> if (!ret)
> ret = error;
> @@ -342,7 +354,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
> break;
> if (ret)
> break;
> - if (filp->f_flags & O_NONBLOCK) {
> + if (filp->f_flags & O_NONBLOCK || nonblock) {
> ret = -EAGAIN;
> break;
> }
> @@ -423,12 +435,14 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
> ssize_t chars;
> bool was_empty = false;
> bool wake_next_writer = false;
> + const bool nonblock = iocb->ki_flags & IOCB_NOWAIT;
>
> /* Null write succeeds. */
> if (unlikely(total_len == 0))
> return 0;
>
> - __pipe_lock(pipe);
> + if (!__pipe_trylock(pipe, nonblock))
> + return -EAGAIN;
>
> if (!pipe->readers) {
> send_sig(SIGPIPE, current, 0);
> @@ -461,7 +475,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
>
> if ((buf->flags & PIPE_BUF_FLAG_CAN_MERGE) &&
> offset + chars <= PAGE_SIZE) {
> - ret = pipe_buf_confirm(pipe, buf, false);
> + ret = pipe_buf_confirm(pipe, buf, nonblock);
> if (ret)
> goto out;
>
> @@ -493,9 +507,13 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
> int copied;
>
> if (!page) {
> - page = alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT);
> + gfp_t gfp = __GFP_HIGHMEM | __GFP_ACCOUNT;
> +
> + if (!nonblock)
> + gfp |= GFP_USER;
Just for my education: Does this encode the assumpation that the
non-blocking code can only be reached from io_uring and thus GFP_USER
can be dropped for that case? IOW, if there's other code that could in
the future reach the non blocking condition would this still be correct?
> + page = alloc_page(gfp);
> if (unlikely(!page)) {
> - ret = ret ? : -ENOMEM;
> + ret = ret ? : nonblock ? -EAGAIN : -ENOMEM;
Hm, could we try and avoid the nested "?:?:" please. Imho, that's easy
to misparse. Idk, doesn't need to be exactly that code but sm like:
if (!nonblock) {
gfp |= GFP_USER;
ret = -EAGAIN;
} else {
ret = -ENOMEM;
}
page = alloc_page(gfp);
if (unlikely(!page))
break;
else
ret = 0;
pipe->tmp_page = page;
or sm else.
