linux-kernel - Re: [PATCH v2] fs/splice: don't block splice_direct_to

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Message-ID: <CAKPOu+9VYJeZbc6xLJzJY=mtmDm+Of9DEKk0kQwnn0nvVzN_4A@mail.gmail.com>
Date:   Tue, 26 Sep 2023 12:41:42 +0200
From:   Max Kellermann <max.kellermann@...os.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] fs/splice: don't block splice_direct_to_actor() after
 data was read

On Tue, Sep 26, 2023 at 12:21 PM Christian Brauner <brauner@...nel.org> wrote:
> Hm, so the thing that is worrysome about this change is that this may
> cause regressions afaict as this is a pretty significant change from
> current behavior.

Would you prefer a new flag for explicitly selecting "wait until at
least one byte was transferred, but don't wait further"? Because many
applications need this behavior, and some (like nginx) have already
worked around the problem by limiting the maximum transaction size,
which I consider a bad workaround, because it leads to unnecessary
system calls and still doesn't really solve the latency problem.

On the other hand, what exactly would the absence of this flag mean...
the old behavior, without my patch, can lead to partial transfers, and
the absence of the flag doesn't mean it can't happen; my patch tackles
just one corner case, but one that is important for me.

We have been running this patch in production for nearly a year (and
will continue to do so until upstream kernels have a proper solution)
and never observed a problem, and I consider it safe, but I
acknowledge the risk that this may reveal obscure application bugs if
applied globally to all Linux kernels, so I understand your worries.

Max