linux-kernel - Re: [PATCH v5 5/9] mm: Introduce mf_dax_kill

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Message-ID: <YNm3VeeWuI0m4Vcx@casper.infradead.org>
Date:   Mon, 28 Jun 2021 12:49:41 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        nvdimm@...ts.linux.dev, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, dm-devel@...hat.com,
        darrick.wong@...cle.com, dan.j.williams@...el.com,
        david@...morbit.com, hch@....de, agk@...hat.com,
        snitzer@...hat.com, rgoldwyn@...e.de
Subject: Re: [PATCH v5 5/9] mm: Introduce mf_dax_kill_procs() for fsdax case

On Mon, Jun 28, 2021 at 08:02:14AM +0800, Shiyang Ruan wrote:
> +/*
> + * dax_load_pfn - Load pfn of the DAX entry corresponding to a page
> + * @mapping:	The file whose entry we want to load
> + * @index:	offset where the DAX entry located in
> + *
> + * Return:	pfn number of the DAX entry
> + */

This is an externally visible function; why not add the second '*' and
make this kernel-doc?

> +unsigned long dax_load_pfn(struct address_space *mapping, unsigned long index)
> +{
> +	XA_STATE(xas, &mapping->i_pages, index);
> +	void *entry;
> +	unsigned long pfn;
> +
> +	xas_lock_irq(&xas);
> +	entry = xas_load(&xas);
> +	pfn = dax_to_pfn(entry);
> +	xas_unlock_irq(&xas);

Why do you need the i_pages lock to do this?  is the rcu_read_lock()
insufficient?  For that matter, why use the xas functions?  Why not
simply:

	void *entry = xa_load(&mapping->i_pages, index);
	return dax_to_pfn(entry);

Looking at it more though, how do you know this is a PFN entry?
It could be locked, for example.  Or the zero page, or empty.

But I think this is unnecessary; why not just pass the PFN into
mf_dax_kill_procs?