linux-kernel - Re: [PATCH v3 2/2] iov_iter: Don't deal with iter->copy_mc in memcpy_from_iter

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Message-ID: <665724.1692218114@warthog.procyon.org.uk>
Date:   Wed, 16 Aug 2023 21:35:14 +0100
From:   David Howells <dhowells@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     dhowells@...hat.com, David Laight <David.Laight@...lab.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@...t.de>,
        Christian Brauner <christian@...uner.io>,
        Matthew Wilcox <willy@...radead.org>,
        Jeff Layton <jlayton@...nel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] iov_iter: Don't deal with iter->copy_mc in memcpy_from_iter_mc()

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > What about ITER_BVEC_MC ??
> 
> That probably would be the best option. Just make it a proper
> ITER_xyz, instead of an odd sub-case for one ITER (but set up in such
> a way that it looks like it might happen for other ITER_xyz cases).

I'm not sure that buys us anything.  It would then require every call to
iov_iter_is_bvec()[*] to check for two values instead of one - including in
iterate_and_advance() - and *still* we'd have to have the special-casing in
_copy_from_iter() and copy_page_from_iter_atomic().

The issue is that ITER_xyz changes the iteration function - but we don't
actually want to do that; rather, we need to change the step function.

David

[*] There's a bunch of them outside of iov_iter.c.