linux-kernel - Re: [PATCH 12/15] mm, dax: enable filesystems to trigger page-idle callbacks

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Date:   Fri, 10 Nov 2017 10:04:53 +0100
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-nvdimm@...ts.01.org, Michal Hocko <mhocko@...e.com>,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-mm@...ck.org,
        Jérôme Glisse <jglisse@...hat.com>,
        linux-fsdevel@...r.kernel.org, akpm@...ux-foundation.org,
        hch@....de
Subject: Re: [PATCH 12/15] mm, dax: enable filesystems to trigger page-idle
        callbacks

> +DEFINE_MUTEX(devmap_lock);

static?

> +#if IS_ENABLED(CONFIG_FS_DAX)
> +static void generic_dax_pagefree(struct page *page, void *data)
> +{
> +}
> +
> +struct dax_device *fs_dax_claim_bdev(struct block_device *bdev, void *owner)
> +{
> +	struct dax_device *dax_dev;
> +	struct dev_pagemap *pgmap;
> +
> +	if (!blk_queue_dax(bdev->bd_queue))
> +		return NULL;
> +	dax_dev = fs_dax_get_by_host(bdev->bd_disk->disk_name);
> +	if (!dax_dev->pgmap)
> +		return dax_dev;
> +	pgmap = dax_dev->pgmap;

> +	mutex_lock(&devmap_lock);
> +	if ((pgmap->data && pgmap->data != owner) || pgmap->page_free
> +			|| pgmap->page_fault
> +			|| pgmap->type != MEMORY_DEVICE_HOST) {
> +		put_dax(dax_dev);
> +		mutex_unlock(&devmap_lock);
> +		return NULL;
> +	}
> +
> +	pgmap->type = MEMORY_DEVICE_FS_DAX;
> +	pgmap->page_free = generic_dax_pagefree;
> +	pgmap->data = owner;
> +	mutex_unlock(&devmap_lock);

All this deep magic will need some explanation.  So far I don't understand
it at all, but maybe the later patches will help..