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Date:   Fri, 18 Jun 2021 19:49:53 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Omar Sandoval <osandov@osandov.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-btrfs <linux-btrfs@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH RESEND x3 v9 1/9] iov_iter: add copy_struct_from_iter()
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On Fri, Jun 18, 2021 at 07:42:41PM +0000, Al Viro wrote:

> Pipe ones are strictly destinations - they can't be sources.  So if you
> see it called for one of those, you've a bug.
> 
> Xarray ones are *not* - they can be sources, and that's missing here.
> 
> Much more unpleasant, though, is that this thing has hard dependency on
> nr_seg == 1 *AND* openly suggests the use of iov_iter_single_seg_count(),
> which is completely wrong.  That sucker has some weird users left (as
> of #work.iov_iter), but all of them are actually due to API deficiencies
> and I very much hope to kill that thing off.
> 
> Why not simply add iov_iter_check_zeroes(), that would be called after
> copy_from_iter() and verified that all that's left in the iterator
> consists of zeroes?  Then this copy_struct_from_...() would be
> trivial to express through those two.  And check_zeroes would also
> be trivial, especially on top of #work.iov_iter.  With no calls of
> iov_iter_advance() at all, while we are at it...
> 
> IDGI... Omar, what semantics do you really want from that primitive?

And for pity sake, let's not do that EXPORT_SYMBOL_GPL() posturing there.
If it's a sane general-purpose API, it doesn't matter who uses it;
if it's not, it shouldn't be exported in the first place.

It can be implemented via the already exported primitives, so it's
not as if we prevented anyone from doing an equivalent...