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From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>,
	Greg KH <gregkh@linuxfoundation.org>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/6] seq_file: add seq_read_iter
Date: Wed, 11 Nov 2020 23:00:06 +0000	[thread overview]
Message-ID: <20201111230006.GB3576660@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CAHk-=wiRM_wDsz1AZ4hyWbctikjSXsYMe-ofvtnQRQ1mFcTqnA@mail.gmail.com>

On Wed, Nov 11, 2020 at 02:27:02PM -0800, Linus Torvalds wrote:
> On Wed, Nov 11, 2020 at 2:21 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > Something like below (build-tested only):
> 
> Apart from my usual "oh, Gods, the iter model really does confuse me"
> this looks more like what I expected, yes.
> 
> Considering the original bug, I'm clearly not the only one confused by
> the iov_iter helper functions and the rules..

copy_to_iter() returns the amount it has actually copied, that's all;
the cause of that bug is not the primitives used, it's the rules for
->read_iter().  The rules are actually fairly simple - "->read_iter()
should not report less data than it has actually left there".  For read(2)
it's a matter of QoI - if we hit an unmapped page, POSIX pretty much says
that all bets are off; read(fd, unmapped - 5, 8) might copy 5 bytes and
return 4.  It is allowed (and read(2) on those files used to do just that),
but it's nicer not to do so.  For generic_file_splice_read(), OTOH, it's
a bug - we end up with stray data spewed into pipe.  So converting
to ->read_iter() needs some care.  Probably something along those
lines should go into D/f/porting...

  reply	other threads:[~2020-11-12  1:34 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-04  8:27 support splice reads on seq_file based procfs files v2 Christoph Hellwig
2020-11-04  8:27 ` [PATCH 1/6] seq_file: add seq_read_iter Christoph Hellwig
2020-11-10 21:32   ` Al Viro
2020-11-10 21:35     ` Al Viro
2020-11-10 23:20       ` Al Viro
2020-11-11  7:55         ` Christoph Hellwig
2020-11-11 17:54         ` Linus Torvalds
2020-11-11 21:52           ` Al Viro
2020-11-11 22:21             ` Al Viro
2020-11-11 22:27               ` Linus Torvalds
2020-11-11 23:00                 ` Al Viro [this message]
2020-11-13 23:54               ` Nathan Chancellor
2020-11-14  1:17                 ` Al Viro
2020-11-14  3:01                   ` Nathan Chancellor
2020-11-14  3:54                     ` Al Viro
2020-11-14  4:14                       ` Nathan Chancellor
2020-11-14  5:50                         ` Al Viro
2020-11-14  6:19                           ` Nathan Chancellor
2020-11-14  7:00                             ` Al Viro
2020-11-14 20:50                               ` Al Viro
2020-11-15 15:53                                 ` Al Viro
2020-11-15 16:56                                   ` Linus Torvalds
2020-11-15 21:41                                   ` Nathan Chancellor
2020-11-15 23:38                                     ` Al Viro
2020-11-15 23:51                                       ` Nathan Chancellor
2020-11-16  0:25                                         ` Al Viro
2020-11-16  0:34                                           ` Nathan Chancellor
2020-11-16  3:29                                             ` Al Viro
2020-11-27 16:29                                               ` Christoph Hellwig
2020-12-08 16:35                                                 ` Christoph Hellwig
2020-12-08 18:34                                                   ` Linus Torvalds
2020-12-08 19:49                                                     ` Al Viro
2020-12-08 20:25                                                       ` Linus Torvalds
2020-12-08 20:53                                                         ` Al Viro
2020-12-08 21:01                                                           ` Linus Torvalds
2020-12-08 19:49                                                     ` Greg KH
2020-11-14 21:44                 ` Al Viro
2020-11-04  8:27 ` [PATCH 2/6] proc: wire up generic_file_splice_read for iter ops Christoph Hellwig
2020-11-04  8:27 ` [PATCH 3/6] proc/cpuinfo: switch to ->read_iter Christoph Hellwig
2020-11-04  8:27 ` [PATCH 4/6] proc/stat: " Christoph Hellwig
2020-11-04  8:27 ` [PATCH 5/6] proc "single files": " Christoph Hellwig
2020-11-04  8:27 ` [PATCH 6/6] proc "seq " Christoph Hellwig
2020-11-04 17:53 ` support splice reads on seq_file based procfs files v2 Linus Torvalds

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