Re: [PATCH v2.1 1/8] fsdax: introduce page->share for fsdax in reflink mode

[Shiyang Ruan <ruansy.fnst@fujitsu.com>]

在 2022/12/3 10:07, Dan Williams 写道:
> 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)".

Ok.

> 
>>   	}
>> -	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.

In order to be consistent, how about naming the 3 helper functions like 
this:

bool          dax_page_is_shared(struct page *page);
void          dax_page_share_get(struct page *page);
unsigned long dax_page_share_put(struct page *page);


--
Thanks,
Ruan.

> 
>>   		} 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
>>
> 
>