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: <CAHk-=wg8G7teERgR7ExNUjHj0yx3dNRopjefnN3zOWWvYADXCw@mail.gmail.com>
Date:   Thu, 17 Aug 2023 06:18:53 +0200
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Howells <dhowells@...hat.com>
Cc:     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()

On Wed, 16 Aug 2023 at 22:35, David Howells <dhowells@...hat.com> wrote:
>
> 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

Well, that part is trivially fixable, and we should do that anyway for
other reasons.

See the attached patch.

> 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.

Yeah, that may be the fundamental issue. But making the ITER_xyz flags
be bit masks would help - partly exactly because it makes it so
trivial yo say "for this set of ITER_xyz, do this".

This patch only does that for the 'user_backed' thing, which was a similar case.

Hmm?

               Linus

View attachment "patch.diff" of type "text/x-patch" (5325 bytes)