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Date: Fri, 29 Nov 2024 09:52:52 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Namhyung Kim <namhyung@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@kernel.org>, Ian Rogers <irogers@google.com>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Kan Liang <kan.liang@linux.intel.com>,
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] perf: map pages in advance
Message-ID: <20241129085252.GA15382@noisy.programming.kicks-ass.net>
References: <20241128113714.492474-1-lorenzo.stoakes@oracle.com>
 <2550a3c6-b0d9-4ad2-ae04-5735d7b4cf5f@lucifer.local>
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In-Reply-To: <2550a3c6-b0d9-4ad2-ae04-5735d7b4cf5f@lucifer.local>

On Thu, Nov 28, 2024 at 08:47:28PM +0000, Lorenzo Stoakes wrote:
> Peter - not sure whether it's easy for you to make a simple adjustment to this
> patch or if you want me to just send a v2, but I have to pop an #ifdef CONFIG_MMU
> into the code.
> 

> > +static int map_range(struct perf_buffer *rb, struct vm_area_struct *vma)
> > +{
> > +	unsigned long nr_pages = vma_pages(vma);
> > +	int err = 0;
> > +	unsigned long pgoff;
> > +
> > +	for (pgoff = 0; pgoff < nr_pages; pgoff++) {
> > +		unsigned long va = vma->vm_start + PAGE_SIZE * pgoff;
> > +		struct page *page = perf_mmap_to_page(rb, pgoff);
> > +
> > +		if (page == NULL) {
> > +			err = -EINVAL;
> > +			break;
> > +		}
> > +
> > +		/* Map readonly, perf_mmap_pfn_mkwrite() called on write fault. */
> > +		err = remap_pfn_range(vma, va, page_to_pfn(page), PAGE_SIZE,
> > +				      vm_get_page_prot(vma->vm_flags & ~VM_SHARED));
> > +		if (err)
> > +			break;
> > +	}
> > +
> 
> Need a:
> 
> #ifdef CONFIG_MMU
> > +	/* Clear any partial mappings on error. */
> > +	if (err)
> > +		zap_page_range_single(vma, vma->vm_start, nr_pages * PAGE_SIZE, NULL);
> #endif
> 
> Here to work around the wonders of nommu :)

All good, I'll edit the thing.