linux-kernel - Re: [PATCH 01/32] iov_iter: Add ITER

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Date:   Sun, 19 Jul 2020 10:51:51 +0100
From:   David Howells <dhowells@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     dhowells@...hat.com, Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jeff Layton <jlayton@...hat.com>,
        Dave Wysochanski <dwysocha@...hat.com>,
        linux-cachefs@...hat.com, linux-afs@...ts.infradead.org,
        linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
        ceph-devel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/32] iov_iter: Add ITER_MAPPING

Al Viro <viro@...iv.linux.org.uk> wrote:

> My main problem here is that your iterate_mapping() assumes that STEP is
> safe under rcu_read_lock(), with no visible mentioning of that fact.

Yeah, that's probably the biggest objection to this.

> Note, BTW, that iov_iter_for_each_range() quietly calls user-supplied
> callback in such context.

And calls kmap(), but should probably use kmap_atomic().  git grep doesn't
show any users of this, so can it be removed?

> Incidentally, do you ever have different steps for bvec and mapping?

Yes:

	csum_and_copy_from_iter_full()
	iov_iter_npages()
	iov_iter_get_pages()
	iov_iter_get_pages_alloc()

But I've tried to use the internal representation struct for bvec where I can
rather than inventing a new one.

David