linux-kernel - Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

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Message-ID: <20240502094612.7f92a3e4@gandalf.local.home>
Date: Thu, 2 May 2024 09:46:12 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Vincent Donnefort <vdonnefort@...gle.com>
Cc: David Hildenbrand <david@...hat.com>, mhiramat@...nel.org,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 mathieu.desnoyers@...icios.com, kernel-team@...roid.com,
 rdunlap@...radead.org, rppt@...nel.org, linux-mm@...ck.org
Subject: Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping
 functions

On Thu, 2 May 2024 14:38:32 +0100
Vincent Donnefort <vdonnefort@...gle.com> wrote:

> > > +	while (s < nr_subbufs && p < nr_pages) {
> > > +		struct page *page = virt_to_page(cpu_buffer->subbuf_ids[s]);
> > > +		int off = 0;
> > > +
> > > +		for (; off < (1 << (subbuf_order)); off++, page++) {
> > > +			if (p >= nr_pages)
> > > +				break;
> > > +
> > > +			pages[p++] = page;
> > > +		}
> > > +		s++;
> > > +	}
> > > +
> > > +	err = vm_insert_pages(vma, vma->vm_start, pages, &nr_pages);  
> > 
> > Nit: I did not immediately understand if we could end here with p < nr_pages
> > (IOW, pages[] not completely filled).
> > 
> > One source of confusion is the "s < nr_subbufs" check in the while loop: why
> > is "p < nr_pages" insufficient?  
> 
> Hum, indeed, the "s < nr_subbufs" check is superfluous, nr_pages, is already
> capped by the number of subbufs, there's no way we can overflow subbuf_ids[].

We can keep it as is, or perhaps change it to:

	while (p < nr_pages) {
		struct page *page;
		int off = 0;

		if (WARN_ON_ONCE(s >= nr_subbufs))
			break;

		page = virt_to_page(cpu_buffer->subbuf_ids[s]);
		for (; off < (1 << (subbuf_order)); off++, page++) {
			if (p >= nr_pages)
				break;

			pages[p++] = page;
		}
		s++;
	}

I don't like having an unchecked dependency between s and p.

-- Steve