linux-kernel - Re: [PATCH] mm: Cleanup __put_devmap_managed_page() vs ->page

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Date:   Thu, 14 Nov 2019 08:19:03 +0100
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     jhubbard@...dia.com, Jan Kara <jack@...e.cz>,
        Christoph Hellwig <hch@....de>,
        Ira Weiny <ira.weiny@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        linux-nvdimm@...ts.01.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: Cleanup __put_devmap_managed_page() vs
 ->page_free()

On Wed, Nov 13, 2019 at 04:07:22PM -0800, Dan Williams wrote:
>  static int devmap_managed_enable_get(struct dev_pagemap *pgmap)
>  {
> -	if (!pgmap->ops || !pgmap->ops->page_free) {
> +	if (!pgmap->ops || (pgmap->type == MEMORY_DEVICE_PRIVATE
> +				&& !pgmap->ops->page_free)) {

I don't think this check is correct.  You only want the the ops null check
or MEMORY_DEVICE_PRIVATE as well now, i.e.:

	if (pgmap->type == MEMORY_DEVICE_PRIVATE &&
	    (!pgmap->ops || !pgmap->ops->page_free)) {

> @@ -476,10 +471,17 @@ void __put_devmap_managed_page(struct page *page)
>  		 * handled differently or not done at all, so there is no need
>  		 * to clear page->mapping.
>  		 */
> -		if (is_device_private_page(page))
> -			page->mapping = NULL;
> +		if (is_device_private_page(page)) {
> +			/* Clear Active bit in case of parallel mark_page_accessed */

This adds a > 80 char line.  But that whole flow of the function seems
rather odd now.

Why can't we do:

	if (count == 0) {
		__put_page(page);
	} else if (is_device_private_page(page)) {
		__ClearPageActive(page);
		__ClearPageWaiters(page);

		mem_cgroup_uncharge(page);
		page->mapping = NULL;
		page->pgmap->ops->page_free(page);
	} else {
		wake_up_var(&page->_refcount);
	}

(except for the fact that I don't get the point of calling __put_page
on a refcount of zero, but that is separate from this patch).