linux-kernel - Re: [PATCH] mm: change size_t to unsigned int for cma

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Message-ID: <YGM8/V7iv3j6JWyx@google.com>
Date:   Tue, 30 Mar 2021 08:00:13 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: change size_t to unsigned int for cma_alloc

On Tue, Mar 30, 2021 at 09:58:37AM +0200, David Hildenbrand wrote:
> On 29.03.21 22:12, Minchan Kim wrote:
> > On Mon, Mar 29, 2021 at 07:44:31PM +0100, Matthew Wilcox wrote:
> > > On Mon, Mar 29, 2021 at 11:25:53AM -0700, Minchan Kim wrote:
> > > > size_t in cma_alloc is confusing since it makes people think
> > > > it's byte count, not pages. Fix it.
> > > 
> > > i think it has to be unsigned long.
> > > 
> > > 67a2e213e7e937c41c52ab5bc46bf3f4de469f6e
> 
> Right.
> 
> Fortunately, we don't have such large allocations yet via
> CMA/alloc_contig_range
> 
> > 
> > Thanks for the pinter. I wanted to have the smallest change.
> > The commit leads me to change cma_release, trace_cma_alloc,
> > cma_clear_bitmap and front_contig_range as well.(Not sure
> > we have more. Will check).
> > 
> > Ccing david@...hat.com for upcoming changing free_contig_range.
> 
> While at it, we might want to convert free_contig_range() to eat
> "unsigned long start, unsigned long end" like alloc_contig_range(), instead
> of "unsigned long pfn, unsigned int nr_pages" like alloc_contig_pages() ...

Well, I personllay tempted to change alloc_contig_range, not
free_contig_range because base_pfn with nr_pages was more
straightforward than base_pfn and end_pfn in that we don't
need to tell whether end_pfn is inclusive or exclusive.

When I look at callers of [alloc|free]_contig_range, many of them
already have used nr_pages based approach rather than start_pfn,
end_pfn. If your suggestion come from that "it's *range* API",
I'd like to rename it with "alloc_contig_pages|free_contig_pages".

Since it's beyond the goal of this patch and might be controversial, 
I will not deal with it in this patch.