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From: Andreas Gruenbacher <agruenba@redhat.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: cluster-devel <cluster-devel@redhat.com>, Jan Kara <jack@suse.cz>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	ocfs2-devel@oss.oracle.com
Subject: Re: [Ocfs2-devel] [PATCH v4 1/8] iov_iter: Introduce iov_iter_fault_in_writeable helper
Date: Sat, 24 Jul 2021 23:38:20 +0200	[thread overview]
Message-ID: <CAHc6FU5nGRn1_oc-8rSOCPfkasWknH1Wb3FeeQYP29zb_5fFGQ@mail.gmail.com> (raw)
In-Reply-To: <YPx28cEvrVl6YrDk@zeniv-ca.linux.org.uk>

On Sat, Jul 24, 2021 at 10:24 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Sat, Jul 24, 2021 at 12:52:34PM -0700, Linus Torvalds wrote:
> > ...
> > > +                       if (fault_in_user_pages(start, len, true) != len)
> > > +                               return -EFAULT;
> >
> > Looking at this once more, I think this is likely wrong.
> >
> > Why?
> >
> > Because any user can/should only care about at least *part* of the
> > area being writable.
> >
> > Imagine that you're doing a large read. If the *first* page is
> > writable, you should still return the partial read, not -EFAULT.
>
> Agreed.
>
> > So I think the code needs to return 0 if _any_ fault was successful.
>
> s/any/the first/...
>
> The same goes for fault-in for read, of course; I've a half-baked conversion
> to such semantics (-EFAULT vs. 0; precise length is unreliable anyway,
> especially if you have sub-page failure areas), need to finish and post
> it...

Hmm, how could we have sub-page failure areas when this is about if
and how pages are mapped? If we return the number of bytes that are
accessible, then users will know if they got nothing, something, or
everything, and they can act accordingly.

Thanks,
Andreas


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  parent reply	other threads:[~2021-07-24 21:41 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-24 19:34 [Ocfs2-devel] [PATCH v4 0/8] gfs2: Fix mmap + page fault deadlocks Andreas Gruenbacher
2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 1/8] iov_iter: Introduce iov_iter_fault_in_writeable helper Andreas Gruenbacher
2021-07-24 19:52   ` Linus Torvalds
2021-07-24 20:24     ` Al Viro
2021-07-24 20:37       ` Linus Torvalds
2021-07-24 21:38       ` Andreas Gruenbacher [this message]
2021-07-24 21:57         ` Al Viro
2021-07-24 22:06           ` Andreas Gruenbacher
2021-07-24 23:39             ` Al Viro
2021-07-27  9:30     ` David Laight
2021-07-27 11:13       ` Andreas Gruenbacher
2021-07-27 17:51         ` Linus Torvalds
2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 2/8] gfs2: Add wrapper for iomap_file_buffered_write Andreas Gruenbacher
2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 3/8] gfs2: Fix mmap + page fault deadlocks for buffered I/O Andreas Gruenbacher
2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 4/8] iomap: Fix iomap_dio_rw return value for user copies Andreas Gruenbacher
2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 5/8] iomap: Add done_before argument to iomap_dio_rw Andreas Gruenbacher
2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 6/8] iomap: Support restarting direct I/O requests after user copy failures Andreas Gruenbacher
2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 7/8] iov_iter: Introduce noio flag to disable page faults Andreas Gruenbacher
2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 8/8] gfs2: Fix mmap + page fault deadlocks for direct I/O Andreas Gruenbacher

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