linux-kernel - Re: copy on write for splice() from file to pipe?

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Message-ID: <824fa356-7d6e-6733-8848-ab84d850c27a@kernel.dk>
Date:   Fri, 10 Feb 2023 15:25:56 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ming Lei <ming.lei@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        Matthew Wilcox <willy@...radead.org>,
        Stefan Metzmacher <metze@...ba.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API Mailing List <linux-api@...r.kernel.org>,
        io-uring <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Samba Technical <samba-technical@...ts.samba.org>
Subject: Re: copy on write for splice() from file to pipe?

On 2/10/23 3:17?PM, Linus Torvalds wrote:
> On Fri, Feb 10, 2023 at 2:08 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>>  (a) the first one is to protect from endless loops
> 
> Just to clarify: they're not "endless loops" per se, but we have
> splice sources and destinations that always succeed, like /dev/zero
> and /dev/null.
> 
> So things like "sendfile()" that are happy to just repeat until done
> do need to have some kind of signal handling even for the case when
> we're not actually waiting for data. That's what that whole
> 
>         /*
>          * Check for signal early to make process killable when there are
>          * always buffers available
>          */
> 
> this is all about. See commit c725bfce7968 ("vfs: Make sendfile(2)
> killable even better") for a less obvious example than that
> "zero->null" kind of thing.
> 
> (I actually suspect that /dev/zero no longer works as a splice source,
> since we disabled the whole "fall back to regular IO" that Christoph
> did in 36e2c7421f02 "fs: don't allow splice read/write without
> explicit ops").

Yet another one... Since it has a read_iter, should be fixable with just
adding the generic splice_read.

-- 
Jens Axboe