linux-kernel - Re: drm pull for v5.3-rc1

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Date:   Tue, 6 Aug 2019 12:09:38 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Thomas Hellström (VMware) 
        <thomas@...pmail.org>, Dave Airlie <airlied@...il.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Price <steven.price@....com>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: drm pull for v5.3-rc1

On Tue, Aug 06, 2019 at 11:50:42AM -0700, Linus Torvalds wrote:
> In fact, I do note that a lot of the users don't actually use the
> "void *private" argument at all - they just want the walker - and just
> pass in a NULL private pointer. So we have things like this:
> 
> > +       if (walk_page_range(&init_mm, va, va + size, &set_nocache_walk_ops,
> > +                       NULL)) {
> 
> and in a perfect world we'd have arguments with default values so that
> we could skip those entirely for when people just don't need it.
> 
> I'm not a huge fan of C++ because of a lot of the complexity (and some
> really bad decisions), but many of the _syntactic_ things in C++ would
> be nice to use. This one doesn't seem to be one that the gcc people
> have picked up as an extension ;(
> 
> Yes, yes, we could do it with a macro, I guess.
> 
>    #define walk_page_range(mm, start,end, ops, ...) \
>        __walk_page_range(mm, start, end, (NULL , ## __VA_ARGS__))
> 
> but I'm not sure it's worthwhile.

Has anyone looked at turning the interface inside-out?  ie something like:

	struct mm_walk_state state = { .mm = mm, .start = start, .end = end, };

	for_each_page_range(&state, page) {
		... do something with page ...
	}

with appropriate macrology along the lines of:

#define for_each_page_range(state, page)				\
	while ((page = page_range_walk_next(state)))

Then you don't need to package anything up into structs that are shared
between the caller and the iterated function.