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Date:   Fri, 2 Dec 2022 18:07:46 -0800
From:   Dan Williams <dan.j.williams@intel.com>
To:     Shiyang Ruan <ruansy.fnst@fujitsu.com>,
        <linux-kernel@vger.kernel.org>, <linux-xfs@vger.kernel.org>,
        <nvdimm@lists.linux.dev>, <linux-fsdevel@vger.kernel.org>
CC:     <djwong@kernel.org>, <david@fromorbit.com>,
        <dan.j.williams@intel.com>, <akpm@linux-foundation.org>
Subject: RE: [PATCH v2.1 1/8] fsdax: introduce page->share for fsdax in
 reflink mode
Message-ID: <638aaf72cba2a_3cbe029479@dwillia2-xfh.jf.intel.com.notmuch>
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Shiyang Ruan wrote:
> fsdax page is used not only when CoW, but also mapread. To make the it
> easily understood, use 'share' to indicate that the dax page is shared
> by more than one extent.  And add helper functions to use it.
> 
> Also, the flag needs to be renamed to PAGE_MAPPING_DAX_SHARED.
> 
> Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
> ---
>  fs/dax.c                   | 38 ++++++++++++++++++++++----------------
>  include/linux/mm_types.h   |  5 ++++-
>  include/linux/page-flags.h |  2 +-
>  3 files changed, 27 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 1c6867810cbd..edbacb273ab5 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -334,35 +334,41 @@ static unsigned long dax_end_pfn(void *entry)
>  	for (pfn = dax_to_pfn(entry); \
>  			pfn < dax_end_pfn(entry); pfn++)
>  
> -static inline bool dax_mapping_is_cow(struct address_space *mapping)
> +static inline bool dax_page_is_shared(struct page *page)
>  {
> -	return (unsigned long)mapping == PAGE_MAPPING_DAX_COW;
> +	return (unsigned long)page->mapping == PAGE_MAPPING_DAX_SHARED;
>  }
>  
>  /*
> - * Set the page->mapping with FS_DAX_MAPPING_COW flag, increase the refcount.
> + * Set the page->mapping with PAGE_MAPPING_DAX_SHARED flag, increase the
> + * refcount.
>   */
> -static inline void dax_mapping_set_cow(struct page *page)
> +static inline void dax_page_bump_sharing(struct page *page)

Similar to page_ref naming I would call this page_share_get() and the
corresponding function page_share_put().

>  {
> -	if ((uintptr_t)page->mapping != PAGE_MAPPING_DAX_COW) {
> +	if ((uintptr_t)page->mapping != PAGE_MAPPING_DAX_SHARED) {
>  		/*
>  		 * 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->share = 1;
> +		page->mapping = (void *)PAGE_MAPPING_DAX_SHARED;

Small nit, You could save a cast here by defining
PAGE_MAPPING_DAX_SHARED as "((void *) 1)".

>  	}
> -	page->index++;
> +	page->share++;
> +}
> +
> +static inline unsigned long dax_page_drop_sharing(struct page *page)
> +{
> +	return --page->share;
>  }
>  
>  /*
> - * When it is called in dax_insert_entry(), the cow flag will indicate that
> + * When it is called in dax_insert_entry(), the shared flag will indicate that
>   * whether this entry is shared by multiple files.  If so, set the page->mapping
> - * FS_DAX_MAPPING_COW, and use page->index as refcount.
> + * PAGE_MAPPING_DAX_SHARED, and use page->share as refcount.
>   */
>  static void dax_associate_entry(void *entry, struct address_space *mapping,
> -		struct vm_area_struct *vma, unsigned long address, bool cow)
> +		struct vm_area_struct *vma, unsigned long address, bool shared)
>  {
>  	unsigned long size = dax_entry_size(entry), pfn, index;
>  	int i = 0;
> @@ -374,8 +380,8 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
>  	for_each_mapped_pfn(entry, pfn) {
>  		struct page *page = pfn_to_page(pfn);
>  
> -		if (cow) {
> -			dax_mapping_set_cow(page);
> +		if (shared) {
> +			dax_page_bump_sharing(page);
>  		} else {
>  			WARN_ON_ONCE(page->mapping);
>  			page->mapping = mapping;
> @@ -396,9 +402,9 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
>  		struct page *page = pfn_to_page(pfn);
>  
>  		WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
> -		if (dax_mapping_is_cow(page->mapping)) {
> -			/* keep the CoW flag if this page is still shared */
> -			if (page->index-- > 0)
> +		if (dax_page_is_shared(page)) {
> +			/* keep the shared flag if this page is still shared */
> +			if (dax_page_drop_sharing(page) > 0)
>  				continue;

I think part of what makes this hard to read is trying to preserve the
same code paths for shared pages and typical pages.

page_share_put() should, in addition to decrementing the share, clear
out page->mapping value.

>  		} else
>  			WARN_ON_ONCE(page->mapping && page->mapping != mapping);
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 500e536796ca..f46cac3657ad 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -103,7 +103,10 @@ struct page {
>  			};
>  			/* See page-flags.h for PAGE_MAPPING_FLAGS */
>  			struct address_space *mapping;
> -			pgoff_t index;		/* Our offset within mapping. */
> +			union {
> +				pgoff_t index;		/* Our offset within mapping. */
> +				unsigned long share;	/* share count for fsdax */
> +			};
>  			/**
>  			 * @private: Mapping-private opaque data.
>  			 * Usually used for buffer_heads if PagePrivate.
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 0b0ae5084e60..c8a3aa02278d 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -641,7 +641,7 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
>   * Different with flags above, this flag is used only for fsdax mode.  It
>   * indicates that this page->mapping is now under reflink case.
>   */
> -#define PAGE_MAPPING_DAX_COW	0x1
> +#define PAGE_MAPPING_DAX_SHARED	0x1
>  
>  static __always_inline bool folio_mapping_flags(struct folio *folio)
>  {
> -- 
> 2.38.1
>