linux-kernel - Re: [RFC PATCH] iov_iter: Convert iterate*() to inline funcs

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Date:   Mon, 14 Aug 2023 14:13:09 +0100
From:   David Howells <dhowells@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     dhowells@...hat.com, Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        Christian Brauner <christian@...uner.io>,
        Matthew Wilcox <willy@...radead.org>, jlayton@...nel.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] iov_iter: Convert iterate*() to inline funcs

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

> 
> So I think you need to remove the changes you did to
> memcpy_from_iter(). The old code was an explicit conditional of direct
> calls:
> 
>         if (iov_iter_is_copy_mc(i))
>                 return (void *)copy_mc_to_kernel(to, from, size);
>         return memcpy(to, from, size);
> 
> and now you do that
> 
>                                    iov_iter_is_copy_mc(i) ?
>                                    memcpy_from_iter_mc : memcpy_from_iter);
> 
> to pass in a function pointer.
> 
> Not ok. Not ok at all. It may look clever, but function pointers are
> bad. Avoid them like the plague.

Yeah.  I was hoping that the compiler would manage to inline that, but it just
does an indirect call.  I'm trying to avoid passing the iterator as that makes
things bigger.  I think I can probably share the extra argument used for
passing checksums.

David