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Date:   Tue, 28 Mar 2023 19:42:20 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     linux-fsdevel@vger.kernel.org, torvalds@linux-foundation.org,
        brauner@kernel.org
Subject: Re: [PATCH 2/8] iov_iter: add iovec_nr_user_vecs() helper
Message-ID: <20230328184220.GL3390869@ZenIV>
References: <20230328173613.555192-1-axboe@kernel.dk>
 <20230328173613.555192-3-axboe@kernel.dk>
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On Tue, Mar 28, 2023 at 11:36:07AM -0600, Jens Axboe wrote:
> This returns the number of user segments in an iov_iter. The input can
> either be an ITER_IOVEC, where it'll return the number of iovecs. Or it
> can be an ITER_UBUF, in which case the number of segments is always 1.
> 
> Outside of those two, no user backed iterators exist. Just return 0 for
> those.

Umm...  Why not set ->nr_segs to 1 in iov_iter_ubuf() instead?  Note that
it won't be more costly; that part of struct iov_iter (8 bytes at offset 40
on amd64) is *not* left uninitialized - zero gets stored there.  That way
you'll get constant 1 stored there, which is just as cheap...