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Date:   Sat, 24 Jul 2021 21:57:28 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Andreas Gruenbacher <agruenba@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Christoph Hellwig <hch@infradead.org>,
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        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
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Subject: Re: [PATCH v4 1/8] iov_iter: Introduce iov_iter_fault_in_writeable
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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".