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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andreas Gruenbacher <agruenba@redhat.com>,
	Christoph Hellwig <hch@infradead.org>,
	"Darrick J. Wong" <djwong@kernel.org>, Jan Kara <jack@suse.cz>,
	Matthew Wilcox <willy@infradead.org>,
	cluster-devel <cluster-devel@redhat.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	ocfs2-devel@oss.oracle.com
Subject: Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable
Date: Fri, 27 Aug 2021 12:05:32 -0700	[thread overview]
Message-ID: <CAHk-=wh5p6zpgUUoY+O7e74X9BZyODhnsqvv=xqnTaLRNj3d_Q@mail.gmail.com> (raw)
In-Reply-To: <YSkz025ncjhyRmlB@zeniv-ca.linux.org.uk>

On Fri, Aug 27, 2021 at 11:52 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> Again, the calling conventions are wrong.  Make it success/failure or
> 0/-EFAULT.  And it's inconsistent for iovec and non-iovec cases as it is.

Al, the 0/-EFAULT thing DOES NOT WORK.

The whole "success vs failure" model is broken.

Because "success" for some people is "everything worked".

And for other people it is "at least _part_ of it worked".

So no, 0/-EFAULT fundamentally cannot work, because the return needs
to be able to handle that ternary situation (ie "nothing" vs
"something" vs "everything").

This is *literally* the exact same thing that we have for
copy_to/from_user(). And Andreas' solution (based on my suggestion) is
the exact same one that we have had for that code since basically day
#1.

The whole "0/-EFAULT" is simpler, yes. And it's what
"{get|put}_user()" uses, yes. And it's more common to a lot of other
functions that return zero or an error.

But see above. People *need* that ternary result, and "bytes/pages
uncopied" is not only the traditional one we use elsewhere in similar
situations, it's the one that has the easiest error tests for existing
users (because zero remains "everything worked").

Andreas originally had that "how many bytes/pages succeeded" return
value instead, and yes, that's also ternary. But it means that now the
common "complete success" test ends up being a lot uglier, and the
semantics of the function changes completely where "0" no longer means
success, and that messes up much more.

So I really think you are barking entirely up the wrong tree.

If there is any inconsistency, maybe we should make _more_ cases use
that "how many bytes/pages not copied" logic, but in a lot of cases
you don't actually need the ternary decision value.

So the inconsistency is EXACTLY the same as the one we have always had
for get|put_user() vs copy_to|from_user(), and it exists for the EXACT
same reason.

IOW, please explain how you'd solve the ternary problem without making
the code a lot uglier.

              Linus

  reply	other threads:[~2021-08-27 19:05 UTC|newest]

Thread overview: 99+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-27 16:49 [PATCH v7 00/19] gfs2: Fix mmap + page fault deadlocks Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 01/19] iov_iter: Fix iov_iter_get_pages{,_alloc} page fault return value Andreas Gruenbacher
2021-09-09 11:09   ` Christoph Hellwig
2021-08-27 16:49 ` [PATCH v7 02/19] powerpc/kvm: Fix kvm_use_magic_page Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 03/19] gup: Turn fault_in_pages_{readable,writeable} into fault_in_{readable,writeable} Andreas Gruenbacher
2021-08-27 19:08   ` Al Viro
2021-09-03 14:56   ` Filipe Manana
2021-09-28 15:02     ` Andreas Gruenbacher
2021-09-28 16:37       ` Matthew Wilcox
2021-09-28 20:41         ` Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 04/19] iov_iter: Turn iov_iter_fault_in_readable into fault_in_iov_iter_readable Andreas Gruenbacher
2021-08-27 18:53   ` Al Viro
2021-08-27 18:57     ` Linus Torvalds
2021-08-27 19:16       ` Al Viro
2021-08-27 20:56   ` Kari Argillander
2021-08-28 17:13     ` Linus Torvalds
2021-08-27 16:49 ` [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable Andreas Gruenbacher
2021-08-27 18:49   ` Al Viro
2021-08-27 19:05     ` Linus Torvalds [this message]
2021-08-27 19:23       ` Al Viro
2021-08-27 19:33         ` Linus Torvalds
2021-08-27 19:37           ` Al Viro
2021-08-27 21:48             ` Al Viro
2021-08-27 21:57               ` Al Viro
2021-08-27 23:22                 ` Luck, Tony
2021-08-28  2:20                   ` Luck, Tony
2021-08-28 21:47                   ` Thomas Gleixner
2021-08-28 22:04                     ` Al Viro
2021-08-28 22:11                       ` Al Viro
2021-08-28 22:19                         ` Al Viro
2021-08-28 22:51                           ` Al Viro
2021-08-29 18:44                             ` Thomas Gleixner
2021-08-29 19:46                               ` Al Viro
2021-08-29 19:51                                 ` Thomas Gleixner
2021-08-28 22:20                         ` Tony Luck
2021-08-29  1:40                           ` Matthew Wilcox
2021-08-30 15:41                             ` Luck, Tony
2021-08-28 22:23                       ` Thomas Gleixner
2021-08-28 19:28               ` [RFC][arm64] possible infinite loop in btrfs search_ioctl() Al Viro
2021-08-31 13:54                 ` Catalin Marinas
2021-08-31 15:28                   ` Al Viro
2021-08-31 16:01                     ` Catalin Marinas
2021-10-11 17:37                     ` Catalin Marinas
2021-10-11 19:15                       ` Linus Torvalds
2021-10-11 21:08                         ` Catalin Marinas
2021-10-11 23:59                           ` Linus Torvalds
2021-10-12 17:27                             ` Catalin Marinas
2021-10-12 17:58                               ` Linus Torvalds
2021-10-18 17:13                                 ` Catalin Marinas
2021-10-21  0:46                             ` Andreas Gruenbacher
2021-10-21 10:05                               ` Catalin Marinas
2021-10-21 14:42                                 ` Andreas Gruenbacher
2021-10-21 17:09                                   ` Catalin Marinas
2021-10-21 18:00                                     ` Andreas Gruenbacher
2021-10-22 18:41                                       ` Catalin Marinas
2021-10-25 19:37                                         ` Andreas Gruenbacher
2021-10-22  2:30                                   ` Linus Torvalds
2021-10-22  9:34                                     ` Catalin Marinas
2021-08-29  0:58               ` [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable Al Viro
2021-08-27 16:49 ` [PATCH v7 06/19] gfs2: Add wrapper for iomap_file_buffered_write Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 07/19] gfs2: Clean up function may_grant Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 08/19] gfs2: Eliminate vestigial HIF_FIRST Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 09/19] gfs2: Remove redundant check from gfs2_glock_dq Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 10/19] gfs2: Introduce flag for glock holder auto-demotion Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 11/19] gfs2: Move the inode glock locking to gfs2_file_buffered_write Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 12/19] gfs2: Eliminate ip->i_gh Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 13/19] gfs2: Fix mmap + page fault deadlocks for buffered I/O Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 14/19] iomap: Fix iomap_dio_rw return value for user copies Andreas Gruenbacher
2021-09-03 18:54   ` Darrick J. Wong
2021-09-09 11:17   ` Christoph Hellwig
2021-08-27 16:49 ` [PATCH v7 15/19] iomap: Support partial direct I/O on user copy failures Andreas Gruenbacher
2021-09-03 18:54   ` Darrick J. Wong
2021-09-09 11:20   ` Christoph Hellwig
2021-09-28 15:05     ` Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 16/19] iomap: Add done_before argument to iomap_dio_rw Andreas Gruenbacher
2021-08-27 18:30   ` Darrick J. Wong
2021-08-27 20:15     ` Andreas Gruenbacher
2021-08-27 21:32       ` Darrick J. Wong
2021-08-27 21:49         ` Andreas Grünbacher
2021-08-27 22:35         ` Linus Torvalds
2021-09-03 18:47           ` Darrick J. Wong
2021-09-03 18:53   ` Darrick J. Wong
2021-09-09 11:30   ` Christoph Hellwig
2021-09-09 17:22     ` Linus Torvalds
2021-09-10  7:36       ` Christoph Hellwig
2021-08-27 16:49 ` [PATCH v7 17/19] gup: Introduce FOLL_NOFAULT flag to disable page faults Andreas Gruenbacher
2021-09-09 11:36   ` Christoph Hellwig
2021-09-09 17:17     ` Linus Torvalds
2021-09-10  7:24       ` Christoph Hellwig
2021-08-27 16:49 ` [PATCH v7 18/19] iov_iter: Introduce nofault " Andreas Gruenbacher
2021-08-27 18:47   ` Al Viro
2021-08-27 19:56     ` Andreas Gruenbacher
2021-08-27 16:49 ` [PATCH v7 19/19] gfs2: Fix mmap + page fault deadlocks for direct I/O Andreas Gruenbacher
2021-08-27 17:16 ` [PATCH v7 00/19] gfs2: Fix mmap + page fault deadlocks Linus Torvalds
2021-09-01 19:52   ` Andreas Gruenbacher
2021-09-03 15:52     ` Linus Torvalds
2021-09-03 18:25       ` Al Viro
2021-09-03 18:47         ` Linus Torvalds
2021-09-03 15:07 ` Filipe Manana

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