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    SubjectRe: [PATCH RERESEND 10/11] splice: file->pipe: -EINVAL for non-regular files w/o FMODE_NOWAIT
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    On 12/14/23 11:45 AM, Ahelenia Ziemia?ska wrote:
    > We request non-blocking I/O in the generic implementation, but some
    > files ? ttys ? only check O_NONBLOCK. Refuse them here, lest we
    > risk sleeping with the pipe locked for indeterminate lengths of
    > time.

    A worthy goal here is ensuring that _everybody_ honors IOCB_NOWAIT,
    rather than just rely on O_NONBLOCK. This does involve converting to
    ->read_iter/->write_iter if the driver isn't already using it, but some
    of them already have that, yet don't check IOCB_NOWAIT or treat it the
    same as O_NONBLOCK.

    Adding special checks like this is not a good idea, imho.

    > This also masks inconsistent wake-ups (usually every second line)
    > when splicing from ttys in icanon mode.
    >
    > Regular files don't /have/ a distinct O_NONBLOCK mode,
    > because they always behave non-blockingly, and for them FMODE_NOWAIT is
    > used in the purest sense of
    > /* File is capable of returning -EAGAIN if I/O will block */
    > which is not set by the vast majority of filesystems,
    > and it's not the semantic we want here.

    The main file systems do very much set it, like btrfs, ext4, and xfs. If
    you look at total_file_systems / ones_flagging_it the ratio may be high,
    but in terms of installed userbase, the majority definitely will have
    it. Also see comment on cover letter for addressing this IOCB_NOWAIT
    confusion.

    --
    Jens Axboe


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