linux-kernel - Re: [PATCH v5 2/5] allow mapping page-less memremaped areas into KVA

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Date:	Thu, 13 Aug 2015 17:48:55 +0300
From:	Boaz Harrosh <boaz@...xistor.com>
To:	Christoph Hellwig <hch@....de>
CC:	Dan Williams <dan.j.williams@...el.com>,
	linux-kernel@...r.kernel.org, axboe@...nel.dk, riel@...hat.com,
	linux-nvdimm@...ts.01.org, linux-mm@...ck.org, mgorman@...e.de,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH v5 2/5] allow mapping page-less memremaped areas into
 KVA

On 08/13/2015 05:37 PM, Christoph Hellwig wrote:
> Hi Boaz,
> 
> can you please fix your quoting?  I read down about 10 pages but still
> couldn't find a comment from you.  For now I gave up on this mail.
> 

Sorry here:

> +void *kmap_atomic_pfn_t(__pfn_t pfn)
> +{
> +	struct page *page = __pfn_t_to_page(pfn);
> +	resource_size_t addr;
> +	struct kmap *kmap;
> +
> +	rcu_read_lock();
> +	if (page)
> +		return kmap_atomic(page);

Right even with pages I pay rcu_read_lock(); for every access?

> +	addr = __pfn_t_to_phys(pfn);
> +	list_for_each_entry_rcu(kmap, &ranges, list)
> +		if (addr >= kmap->res->start && addr <= kmap->res->end)
> +			return kmap->base + addr - kmap->res->start;
> +

Good god! This loop is a real *joke*. You have just dropped memory access
performance by 10 fold.

The all point of pages and memory_model.h was to have a one to one
relation-ships between Kernel-virtual vs physical vs page *

There is already an object that holds a relationship of physical
to Kernel-virtual. It is called a memory-section. Why not just
widen its definition?

If you are willing to accept this loop. In current Linux 2015 Kernel
Then I have nothing farther to say.

Boaz - go mourning for the death of the Linux Kernel alone in the corner ;-(

> +	/* only unlock in the error case */
> +	rcu_read_unlock();
> +	return NULL;
> +}
> +EXPORT_SYMBOL(kmap_atomic_pfn_t);
> +

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