linux-kernel - Re: [PATCH v5] mm: cma: support sysfs

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Date:   Wed, 24 Mar 2021 04:34:34 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, gregkh@...uxfoundation.org,
        surenb@...gle.com, joaodias@...gle.com, jhubbard@...dia.com,
        digetx@...il.com
Subject: Re: [PATCH v5] mm: cma: support sysfs

On Tue, Mar 23, 2021 at 08:31:31PM -0700, Minchan Kim wrote:
> On Wed, Mar 24, 2021 at 03:02:24AM +0000, Matthew Wilcox wrote:
> > On Tue, Mar 23, 2021 at 12:50:50PM -0700, Minchan Kim wrote:
> > > +	/* the number of CMA page successful allocations */
> > > +	atomic64_t nr_pages_succeeded;
> > 
> > > +void cma_sysfs_alloc_pages_count(struct cma *cma, size_t count)
> > > +{
> > > +	atomic64_add(count, &cma->nr_pages_succeeded);
> > > +}
> > 
> > I don't understand.  A size_t is a byte count.  But the variable is called
> > 'nr_pages'.  So which is it, a byte count or a page count?
> 
> It's page count. I followed the cma_alloc interface since it has
> size_t count variable for nr_pages.

That's very confusing.  cma_alloc is wrong; if it needs to be an
unsigned long, that's fine.  But it shouldn't be size_t.

7.17 of n1256 defines:

	size_t
which is the unsigned integer type of the result of the sizeof operator

Do you want to submit a patch to fix cma_alloc as well?

> Let's go with unsigned long nr_pages:
> void cma_sysfs_alloc_pages_count(struct cma *cma, unsigned long
> nr_pages)

Works for me!