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

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Message-ID: <62cbe523-e8a4-cdfd-90c2-80260cefa5de@arm.com>
Date:   Wed, 7 Aug 2019 15:30:38 +0100
From:   Steven Price <steven.price@....com>
To:     Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.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>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: drm pull for v5.3-rc1

On 07/08/2019 15:15, Matthew Wilcox wrote:
> On Tue, Aug 06, 2019 at 11:40:00PM -0700, Christoph Hellwig wrote:
>> On Tue, Aug 06, 2019 at 12:09:38PM -0700, Matthew Wilcox wrote:
>>> 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.
>>
>> I'm not an all that huge fan of super magic macro loops.  But in this
>> case I don't see how it could even work, as we get special callbacks
>> for huge pages and holes, and people are trying to add a few more ops
>> as well.
> 
> We could have bits in the mm_walk_state which indicate what things to return
> and what things to skip.  We could (and probably should) also use different
> iterator names if people actually want to iterate different things.  eg
> for_each_pte_range(&state, pte) as well as for_each_page_range().
> 

The iterator approach could be awkward for the likes of my generic
ptdump implementation[1]. It would require an iterator which returns all
levels and allows skipping levels when required (to prevent KASAN
slowing things down too much). So something like:

start_walk_range(&state);
for_each_page_range(&state, page) {
	switch(page->level) {
	case PTE:
		...
	case PMD:
		if (...)
			skip_pmd(&state);
		...
	case HOLE:
		....
	...
	}
}
end_walk_range(&state);

It seems a little fragile - e.g. we wouldn't (easily) get type checking
that you are actually treating a PTE as a pte_t. The state mutators like
skip_pmd() also seem a bit clumsy.

Steve

[1]
https://lore.kernel.org/lkml/20190731154603.41797-20-steven.price@arm.com/