linux-kernel - Re: [PATCH v20 09/32] tty, proc, kernfs, random: Use direct_splice

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Message-ID: <CAHk-=wgzSLXt38J_B2=QWCDi7A1c5B0_cJ3XYRj9rYn+YXbjQA@mail.gmail.com>
Date:   Fri, 19 May 2023 09:22:22 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     David Howells <dhowells@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>,
        Christian Brauner <brauner@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v20 09/32] tty, proc, kernfs, random: Use direct_splice_read()

On Fri, May 19, 2023 at 1:12 AM Christoph Hellwig <hch@...radead.org> wrote:
>
> Pinging Al (and maybe Linus): is there any good reason to not simply
> default to direct_splice_read if ->read_iter is implemented and
> ->splice_read is not once you remove ITER_PIPE?

For me, the reason isn't so much technical as "historical pain".

We've had *so* many problems with splice on random file descriptors
that I at some point decided "no splice by default".

Now, admittedly most of the problems were due to the whole set_fs()
ambiguity, which you fixed and no longer exists. So maybe we could go
back to "implement splice by default".

I agree that as long as the default implementation is obviously safe,
it should be ok, and maybe direct_splice_read is that obvious..

           Linus