From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>,
Colin Ian King <colin.king@canonical.com>,
Christoph Hellwig <hch@lst.de>,
Johannes Berg <johannes@sipsolutions.net>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: splice() from /dev/zero to a pipe does not work (5.9+)
Date: Fri, 7 May 2021 19:17:52 +0000 [thread overview]
Message-ID: <YJWSYDk4gAT1hkf6@zeniv-ca.linux.org.uk> (raw)
In-Reply-To: <CAHk-=whVMtMPRMMX9W_B7JhVTyRzVoH71Xw8TbtYjThaoCzJ=A@mail.gmail.com>
On Fri, May 07, 2021 at 12:06:31PM -0700, Linus Torvalds wrote:
> That said - looking at the current 'pipe_zero()', it uses
> 'push_pipe()' to actually allocation regular pages, and then clear
> them.
>
> Which is basically what a generic_file_splice_read() would do, and it
> feels incredibly pointless and stupid to me.
>
> I *think* we should be able to just do something like
>
> len = size;
> while (len > 0) {
> struct pipe_buffer *buf;
> unsigned int tail = pipe->tail;
> unsigned int head = pipe->head;
> unsigned int mask = pipe->ring_size - 1;
>
> if (pipe_full(head, tail, pipe->max_usage))
> break;
> buf = &pipe->bufs[iter_head & p_mask];
> buf->ops = &zero_pipe_buf_ops;
> buf->page = ZERO_PAGE(0);
> buf->offset = 0;
> buf->len = min_t(ssize_t, len, PAGE_SIZE);
> len -= buf->len;
> pipe->head = head+1;
> }
> return size - len;
>
> but honestly, I haven't thought a lot about it.
>
> Al? This is another of those "right up your alley" things.
Umm... That would do wonders to anything that used to do
copy_to_user()/clear_user()/copy_to_user() and got converted
to copy_to_iter()/iov_iter_zero()/copy_to_iter()...
Are you sure we can shove zero page into pipe, anyway?
IIRC, get_page()/put_page() on that is not allowed, and
I'm not at all sure that nothing in e.g. fuse splice-related
logics would go ahead an do just that. Or am I confused
about the page refcounting for those?
next prev parent reply other threads:[~2021-05-07 19:18 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-07 18:05 splice() from /dev/zero to a pipe does not work (5.9+) Colin Ian King
2021-05-07 18:21 ` Kees Cook
2021-05-07 19:06 ` Linus Torvalds
2021-05-07 19:17 ` Al Viro [this message]
2021-05-07 19:29 ` Linus Torvalds
2021-05-07 20:31 ` Al Viro
2021-05-12 15:13 ` Colin Ian King
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=YJWSYDk4gAT1hkf6@zeniv-ca.linux.org.uk \
--to=viro@zeniv.linux.org.uk \
--cc=colin.king@canonical.com \
--cc=hch@lst.de \
--cc=johannes@sipsolutions.net \
--cc=keescook@chromium.org \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=torvalds@linux-foundation.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).