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Date:   Sun, 10 Apr 2022 23:55:35 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Shiyang Ruan <ruansy.fnst@fujitsu.com>
Cc:     linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org,
        nvdimm@lists.linux.dev, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, djwong@kernel.org,
        dan.j.williams@intel.com, david@fromorbit.com, hch@infradead.org,
        jane.chu@oracle.com
Subject: Re: [PATCH v12 7/7] fsdax: set a CoW flag when associate reflink
 mappings
Message-ID: <YlPQ59w4L4pnDYWq@infradead.org>
References: <20220410160904.3758789-1-ruansy.fnst@fujitsu.com>
 <20220410160904.3758789-8-ruansy.fnst@fujitsu.com>
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> + * Set or Update the page->mapping with FS_DAX_MAPPING_COW flag.
> + * Return true if it is an Update.
> + */
> +static inline bool dax_mapping_set_cow(struct page *page)
> +{
> +	if (page->mapping) {
> +		/* flag already set */
> +		if (dax_mapping_is_cow(page->mapping))
> +			return false;
> +
> +		/*
> +		 * This page has been mapped even before it is shared, just
> +		 * need to set this FS_DAX_MAPPING_COW flag.
> +		 */
> +		dax_mapping_set_cow_flag(&page->mapping);
> +		return true;
> +	}
> +	/* Newly associate CoW mapping */
> +	dax_mapping_set_cow_flag(&page->mapping);
> +	return false;

Given that this is the only place calling dax_mapping_set_cow I wonder
if we should just open code it here, and also lift the page->index logic
from the caller into this helper.

static inline void dax_mapping_set_cow(struct page *page)
{
	if ((uintptr_t)page->mapping != PAGE_MAPPING_DAX_COW) {
		/*
		 * Reset the index if the page was already mapped
		 * regularly before.
		 */
		if (page->mapping)
			page->index = 1;
		page->mapping = (void *)PAGE_MAPPING_DAX_COW;
	}
	page->index++;
}

> +		if (!dax_mapping_is_cow(page->mapping)) {
> +			/* keep the CoW flag if this page is still shared */
> +			if (page->index-- > 0)
> +				continue;
> +		} else
> +			WARN_ON_ONCE(page->mapping && page->mapping != mapping);

Isnt the dax_mapping_is_cow check above inverted?