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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andreas Gruenbacher <agruenba@redhat.com>
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>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@vger.kernel.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 12:52:34 -0700	[thread overview]
Message-ID: <CAHk-=whodi=ZPhoJy_a47VD+-aFtz385B4_GHvQp8Bp9NdTKUg@mail.gmail.com> (raw)
In-Reply-To: <20210724193449.361667-2-agruenba@redhat.com>

On Sat, Jul 24, 2021 at 12:35 PM Andreas Gruenbacher
<agruenba@redhat.com> wrote:
>
> +int iov_iter_fault_in_writeable(const struct iov_iter *i, size_t bytes)
> +{
...
> +                       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.

So I think the code needs to return 0 if _any_ fault was successful.
Or perhaps return how much it was able to fault in. Because returning
-EFAULT if any of it failed seems wrong, and doesn't allow for partial
success being reported.

The other reaction I have is that you now only do the
iov_iter_fault_in_writeable, but then you make fault_in_user_pages()
still have that "bool write" argument.

We already have 'fault_in_pages_readable()', and that one is more
efficient (well, at least if the fault isn't needed it is). So it
would make more sense to just implement fault_in_pages_writable()
instead of that "fault_in_user_pages(, bool write)".

                 Linus

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  reply	other threads:[~2021-07-24 19:53 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 [this message]
2021-07-24 20:24     ` Al Viro
2021-07-24 20:37       ` Linus Torvalds
2021-07-24 21:38       ` Andreas Gruenbacher
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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