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From: Al Viro <viro@zeniv.linux.org.uk>
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>,
	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 21:57:28 +0000	[thread overview]
Message-ID: <YPyMyPCpZKGlfAGk@zeniv-ca.linux.org.uk> (raw)
In-Reply-To: <CAHc6FU5nGRn1_oc-8rSOCPfkasWknH1Wb3FeeQYP29zb_5fFGQ@mail.gmail.com>

On Sat, Jul 24, 2021 at 11:38:20PM +0200, Andreas Gruenbacher wrote:

> 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.

What I'm saying is that in situation when you have cacheline-sized
poisoned areas, there's no way to get an accurate count of readable
area other than try and copy it out.

What's more, "something" is essentially useless information - the
pages might get unmapped right as your function returns; the caller
still needs to deal with partial copies.  And that's a slow path
by definition, so informing them of a partial fault-in is not
going to be useful.

As far as callers are concerned, it's "nothing suitable in the
beginning of the area" vs. "something might be accessible".

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  reply	other threads:[~2021-07-24 21:57 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
2021-07-24 21:57         ` Al Viro [this message]
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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