diff mbox series

[v10,9/9] fsdax: set a CoW flag when associate reflink mappings

Message ID 20220127124058.1172422-10-ruansy.fnst@fujitsu.com (mailing list archive)
State New
Headers show
Series fsdax: introduce fs query to support reflink | expand

Commit Message

Shiyang Ruan Jan. 27, 2022, 12:40 p.m. UTC
Introduce a PAGE_MAPPING_DAX_COW flag to support association with CoW file
mappings.  In this case, the dax-RMAP already takes the responsibility
to look up for shared files by given dax page.  The page->mapping is no
longer to used for rmap but for marking that this dax page is shared.
And to make sure disassociation works fine, we use page->index as
refcount, and clear page->mapping to the initial state when page->index
is decreased to 0.

With the help of this new flag, it is able to distinguish normal case
and CoW case, and keep the warning in normal case.

Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
---
 fs/dax.c                   | 65 ++++++++++++++++++++++++++++++++------
 include/linux/page-flags.h |  6 ++++
 2 files changed, 62 insertions(+), 9 deletions(-)

Comments

Dan Williams Feb. 16, 2022, 2:09 a.m. UTC | #1
On Thu, Jan 27, 2022 at 4:41 AM Shiyang Ruan <ruansy.fnst@fujitsu.com> wrote:
>
> Introduce a PAGE_MAPPING_DAX_COW flag to support association with CoW file
> mappings.  In this case, the dax-RMAP already takes the responsibility
> to look up for shared files by given dax page.  The page->mapping is no
> longer to used for rmap but for marking that this dax page is shared.
> And to make sure disassociation works fine, we use page->index as
> refcount, and clear page->mapping to the initial state when page->index
> is decreased to 0.
>
> With the help of this new flag, it is able to distinguish normal case
> and CoW case, and keep the warning in normal case.
>
> Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
> ---
>  fs/dax.c                   | 65 ++++++++++++++++++++++++++++++++------
>  include/linux/page-flags.h |  6 ++++
>  2 files changed, 62 insertions(+), 9 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 250794a5b789..88879c579c1f 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -334,13 +334,46 @@ static unsigned long dax_end_pfn(void *entry)
>         for (pfn = dax_to_pfn(entry); \
>                         pfn < dax_end_pfn(entry); pfn++)
>
> +static inline void dax_mapping_set_cow_flag(struct address_space *mapping)
> +{
> +       mapping = (struct address_space *)PAGE_MAPPING_DAX_COW;
> +}
> +
> +static inline bool dax_mapping_is_cow(struct address_space *mapping)
> +{
> +       return (unsigned long)mapping == PAGE_MAPPING_DAX_COW;
> +}
> +
>  /*
> - * TODO: for reflink+dax we need a way to associate a single page with
> - * multiple address_space instances at different linear_page_index()
> - * offsets.
> + * 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;
> +}
> +
> +/*
> + * When it is called in dax_insert_entry(), the cow flag will indicate that
> + * whether this entry is shared by multiple files.  If so, set the page->mapping
> + * to be FS_DAX_MAPPING_COW, and use page->index as refcount.
>   */
>  static void dax_associate_entry(void *entry, struct address_space *mapping,
> -               struct vm_area_struct *vma, unsigned long address)
> +               struct vm_area_struct *vma, unsigned long address, bool cow)
>  {
>         unsigned long size = dax_entry_size(entry), pfn, index;
>         int i = 0;
> @@ -352,9 +385,17 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
>         for_each_mapped_pfn(entry, pfn) {
>                 struct page *page = pfn_to_page(pfn);
>
> -               WARN_ON_ONCE(page->mapping);
> -               page->mapping = mapping;
> -               page->index = index + i++;
> +               if (cow) {
> +                       if (dax_mapping_set_cow(page)) {
> +                               /* Was normal, now updated to CoW */
> +                               page->index = 2;
> +                       } else
> +                               page->index++;
> +               } else {
> +                       WARN_ON_ONCE(page->mapping);
> +                       page->mapping = mapping;
> +                       page->index = index + i++;
> +               }
>         }
>  }
>
> @@ -370,7 +411,12 @@ 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);
> -               WARN_ON_ONCE(page->mapping && page->mapping != mapping);
> +               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);
>                 page->mapping = NULL;
>                 page->index = 0;
>         }
> @@ -810,7 +856,8 @@ static void *dax_insert_entry(struct xa_state *xas,
>                 void *old;
>
>                 dax_disassociate_entry(entry, mapping, false);
> -               dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address);
> +               dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address,
> +                               false);

Where is the caller that passes 'true'? Also when that caller arrives
introduce a separate dax_associate_cow_entry() as that's easier to
read than dax_associate_entry(..., true) in case someone does not
remember what that boolean flag means.

However, it's not clear to me that this approach is a good idea given
that the filesystem is the source of truth for how many address_spaces
this page mapping might be duplicated. What about a iomap_page_ops for
fsdax to ask the filesystem when it is ok to clear the mapping
association for a page?

>                 /*
>                  * Only swap our new entry into the page cache if the current
>                  * entry is a zero page or an empty entry.  If a normal PTE or
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 1c3b6e5c8bfd..6370d279795a 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -572,6 +572,12 @@ __PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
>  #define PAGE_MAPPING_KSM       (PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
>  #define PAGE_MAPPING_FLAGS     (PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
>
> +/*
> + * 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
> +
>  static __always_inline int PageMappingFlags(struct page *page)
>  {
>         return ((unsigned long)page->mapping & PAGE_MAPPING_FLAGS) != 0;
> --
> 2.34.1
>
>
>
Shiyang Ruan Feb. 16, 2022, 2:55 a.m. UTC | #2
在 2022/2/16 10:09, Dan Williams 写道:
> On Thu, Jan 27, 2022 at 4:41 AM Shiyang Ruan <ruansy.fnst@fujitsu.com> wrote:
>>
>> Introduce a PAGE_MAPPING_DAX_COW flag to support association with CoW file
>> mappings.  In this case, the dax-RMAP already takes the responsibility
>> to look up for shared files by given dax page.  The page->mapping is no
>> longer to used for rmap but for marking that this dax page is shared.
>> And to make sure disassociation works fine, we use page->index as
>> refcount, and clear page->mapping to the initial state when page->index
>> is decreased to 0.
>>
>> With the help of this new flag, it is able to distinguish normal case
>> and CoW case, and keep the warning in normal case.
>>
>> Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
>> ---
>>   fs/dax.c                   | 65 ++++++++++++++++++++++++++++++++------
>>   include/linux/page-flags.h |  6 ++++
>>   2 files changed, 62 insertions(+), 9 deletions(-)
>>
>> diff --git a/fs/dax.c b/fs/dax.c
>> index 250794a5b789..88879c579c1f 100644
>> --- a/fs/dax.c
>> +++ b/fs/dax.c
>> @@ -334,13 +334,46 @@ static unsigned long dax_end_pfn(void *entry)
>>          for (pfn = dax_to_pfn(entry); \
>>                          pfn < dax_end_pfn(entry); pfn++)
>>
>> +static inline void dax_mapping_set_cow_flag(struct address_space *mapping)
>> +{
>> +       mapping = (struct address_space *)PAGE_MAPPING_DAX_COW;
>> +}
>> +
>> +static inline bool dax_mapping_is_cow(struct address_space *mapping)
>> +{
>> +       return (unsigned long)mapping == PAGE_MAPPING_DAX_COW;
>> +}
>> +
>>   /*
>> - * TODO: for reflink+dax we need a way to associate a single page with
>> - * multiple address_space instances at different linear_page_index()
>> - * offsets.
>> + * 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;
>> +}
>> +
>> +/*
>> + * When it is called in dax_insert_entry(), the cow flag will indicate that
>> + * whether this entry is shared by multiple files.  If so, set the page->mapping
>> + * to be FS_DAX_MAPPING_COW, and use page->index as refcount.
>>    */
>>   static void dax_associate_entry(void *entry, struct address_space *mapping,
>> -               struct vm_area_struct *vma, unsigned long address)
>> +               struct vm_area_struct *vma, unsigned long address, bool cow)
>>   {
>>          unsigned long size = dax_entry_size(entry), pfn, index;
>>          int i = 0;
>> @@ -352,9 +385,17 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
>>          for_each_mapped_pfn(entry, pfn) {
>>                  struct page *page = pfn_to_page(pfn);
>>
>> -               WARN_ON_ONCE(page->mapping);
>> -               page->mapping = mapping;
>> -               page->index = index + i++;
>> +               if (cow) {
>> +                       if (dax_mapping_set_cow(page)) {
>> +                               /* Was normal, now updated to CoW */
>> +                               page->index = 2;
>> +                       } else
>> +                               page->index++;
>> +               } else {
>> +                       WARN_ON_ONCE(page->mapping);
>> +                       page->mapping = mapping;
>> +                       page->index = index + i++;
>> +               }
>>          }
>>   }
>>
>> @@ -370,7 +411,12 @@ 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);
>> -               WARN_ON_ONCE(page->mapping && page->mapping != mapping);
>> +               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);
>>                  page->mapping = NULL;
>>                  page->index = 0;
>>          }
>> @@ -810,7 +856,8 @@ static void *dax_insert_entry(struct xa_state *xas,
>>                  void *old;
>>
>>                  dax_disassociate_entry(entry, mapping, false);
>> -               dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address);
>> +               dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address,
>> +                               false);
> 
> Where is the caller that passes 'true'? Also when that caller arrives
> introduce a separate dax_associate_cow_entry() as that's easier to
> read than dax_associate_entry(..., true) in case someone does not
> remember what that boolean flag means.

This flag is supposed to be used when CoW support is introduced.  When 
it is a CoW operation, which is decided by iomap & srcmap's flag, this 
flag will be set true.

I think I should describe it in detail in the commit message.

> 
> However, it's not clear to me that this approach is a good idea given
> that the filesystem is the source of truth for how many address_spaces
> this page mapping might be duplicated. What about a iomap_page_ops for
> fsdax to ask the filesystem when it is ok to clear the mapping
> association for a page?

I'll think how to implement it in this way.


--
Thanks,
Ruan.

> 
>>                  /*
>>                   * Only swap our new entry into the page cache if the current
>>                   * entry is a zero page or an empty entry.  If a normal PTE or
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index 1c3b6e5c8bfd..6370d279795a 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -572,6 +572,12 @@ __PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
>>   #define PAGE_MAPPING_KSM       (PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
>>   #define PAGE_MAPPING_FLAGS     (PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
>>
>> +/*
>> + * 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
>> +
>>   static __always_inline int PageMappingFlags(struct page *page)
>>   {
>>          return ((unsigned long)page->mapping & PAGE_MAPPING_FLAGS) != 0;
>> --
>> 2.34.1
>>
>>
>>
Dan Williams Feb. 16, 2022, 3:09 a.m. UTC | #3
On Tue, Feb 15, 2022 at 6:55 PM Shiyang Ruan <ruansy.fnst@fujitsu.com> wrote:
>
>
>
> 在 2022/2/16 10:09, Dan Williams 写道:
> > On Thu, Jan 27, 2022 at 4:41 AM Shiyang Ruan <ruansy.fnst@fujitsu.com> wrote:
> >>
> >> Introduce a PAGE_MAPPING_DAX_COW flag to support association with CoW file
> >> mappings.  In this case, the dax-RMAP already takes the responsibility
> >> to look up for shared files by given dax page.  The page->mapping is no
> >> longer to used for rmap but for marking that this dax page is shared.
> >> And to make sure disassociation works fine, we use page->index as
> >> refcount, and clear page->mapping to the initial state when page->index
> >> is decreased to 0.
> >>
> >> With the help of this new flag, it is able to distinguish normal case
> >> and CoW case, and keep the warning in normal case.
> >>
> >> Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
> >> ---
> >>   fs/dax.c                   | 65 ++++++++++++++++++++++++++++++++------
> >>   include/linux/page-flags.h |  6 ++++
> >>   2 files changed, 62 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/fs/dax.c b/fs/dax.c
> >> index 250794a5b789..88879c579c1f 100644
> >> --- a/fs/dax.c
> >> +++ b/fs/dax.c
> >> @@ -334,13 +334,46 @@ static unsigned long dax_end_pfn(void *entry)
> >>          for (pfn = dax_to_pfn(entry); \
> >>                          pfn < dax_end_pfn(entry); pfn++)
> >>
> >> +static inline void dax_mapping_set_cow_flag(struct address_space *mapping)
> >> +{
> >> +       mapping = (struct address_space *)PAGE_MAPPING_DAX_COW;
> >> +}
> >> +
> >> +static inline bool dax_mapping_is_cow(struct address_space *mapping)
> >> +{
> >> +       return (unsigned long)mapping == PAGE_MAPPING_DAX_COW;
> >> +}
> >> +
> >>   /*
> >> - * TODO: for reflink+dax we need a way to associate a single page with
> >> - * multiple address_space instances at different linear_page_index()
> >> - * offsets.
> >> + * 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;
> >> +}
> >> +
> >> +/*
> >> + * When it is called in dax_insert_entry(), the cow flag will indicate that
> >> + * whether this entry is shared by multiple files.  If so, set the page->mapping
> >> + * to be FS_DAX_MAPPING_COW, and use page->index as refcount.
> >>    */
> >>   static void dax_associate_entry(void *entry, struct address_space *mapping,
> >> -               struct vm_area_struct *vma, unsigned long address)
> >> +               struct vm_area_struct *vma, unsigned long address, bool cow)
> >>   {
> >>          unsigned long size = dax_entry_size(entry), pfn, index;
> >>          int i = 0;
> >> @@ -352,9 +385,17 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
> >>          for_each_mapped_pfn(entry, pfn) {
> >>                  struct page *page = pfn_to_page(pfn);
> >>
> >> -               WARN_ON_ONCE(page->mapping);
> >> -               page->mapping = mapping;
> >> -               page->index = index + i++;
> >> +               if (cow) {
> >> +                       if (dax_mapping_set_cow(page)) {
> >> +                               /* Was normal, now updated to CoW */
> >> +                               page->index = 2;
> >> +                       } else
> >> +                               page->index++;
> >> +               } else {
> >> +                       WARN_ON_ONCE(page->mapping);
> >> +                       page->mapping = mapping;
> >> +                       page->index = index + i++;
> >> +               }
> >>          }
> >>   }
> >>
> >> @@ -370,7 +411,12 @@ 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);
> >> -               WARN_ON_ONCE(page->mapping && page->mapping != mapping);
> >> +               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);
> >>                  page->mapping = NULL;
> >>                  page->index = 0;
> >>          }
> >> @@ -810,7 +856,8 @@ static void *dax_insert_entry(struct xa_state *xas,
> >>                  void *old;
> >>
> >>                  dax_disassociate_entry(entry, mapping, false);
> >> -               dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address);
> >> +               dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address,
> >> +                               false);
> >
> > Where is the caller that passes 'true'? Also when that caller arrives
> > introduce a separate dax_associate_cow_entry() as that's easier to
> > read than dax_associate_entry(..., true) in case someone does not
> > remember what that boolean flag means.
>
> This flag is supposed to be used when CoW support is introduced.

Ok, so should this patch wait and be a part of that series? It's
otherwise confusing to introduce a new capability in a patch set and
not take advantage of it until a separate / later patch set.

> When
> it is a CoW operation, which is decided by iomap & srcmap's flag, this
> flag will be set true.
>
> I think I should describe it in detail in the commit message.

That could help, or move it to the COW support series.

> > However, it's not clear to me that this approach is a good idea given
> > that the filesystem is the source of truth for how many address_spaces
> > this page mapping might be duplicated. What about a iomap_page_ops for
> > fsdax to ask the filesystem when it is ok to clear the mapping
> > association for a page?
>
> I'll think how to implement it in this way.
>
>
> --
> Thanks,
> Ruan.
>
> >
> >>                  /*
> >>                   * Only swap our new entry into the page cache if the current
> >>                   * entry is a zero page or an empty entry.  If a normal PTE or
> >> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> >> index 1c3b6e5c8bfd..6370d279795a 100644
> >> --- a/include/linux/page-flags.h
> >> +++ b/include/linux/page-flags.h
> >> @@ -572,6 +572,12 @@ __PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
> >>   #define PAGE_MAPPING_KSM       (PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
> >>   #define PAGE_MAPPING_FLAGS     (PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
> >>
> >> +/*
> >> + * 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
> >> +
> >>   static __always_inline int PageMappingFlags(struct page *page)
> >>   {
> >>          return ((unsigned long)page->mapping & PAGE_MAPPING_FLAGS) != 0;
> >> --
> >> 2.34.1
> >>
> >>
> >>
>
>
diff mbox series

Patch

diff --git a/fs/dax.c b/fs/dax.c
index 250794a5b789..88879c579c1f 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -334,13 +334,46 @@  static unsigned long dax_end_pfn(void *entry)
 	for (pfn = dax_to_pfn(entry); \
 			pfn < dax_end_pfn(entry); pfn++)
 
+static inline void dax_mapping_set_cow_flag(struct address_space *mapping)
+{
+	mapping = (struct address_space *)PAGE_MAPPING_DAX_COW;
+}
+
+static inline bool dax_mapping_is_cow(struct address_space *mapping)
+{
+	return (unsigned long)mapping == PAGE_MAPPING_DAX_COW;
+}
+
 /*
- * TODO: for reflink+dax we need a way to associate a single page with
- * multiple address_space instances at different linear_page_index()
- * offsets.
+ * 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;
+}
+
+/*
+ * When it is called in dax_insert_entry(), the cow flag will indicate that
+ * whether this entry is shared by multiple files.  If so, set the page->mapping
+ * to be FS_DAX_MAPPING_COW, and use page->index as refcount.
  */
 static void dax_associate_entry(void *entry, struct address_space *mapping,
-		struct vm_area_struct *vma, unsigned long address)
+		struct vm_area_struct *vma, unsigned long address, bool cow)
 {
 	unsigned long size = dax_entry_size(entry), pfn, index;
 	int i = 0;
@@ -352,9 +385,17 @@  static void dax_associate_entry(void *entry, struct address_space *mapping,
 	for_each_mapped_pfn(entry, pfn) {
 		struct page *page = pfn_to_page(pfn);
 
-		WARN_ON_ONCE(page->mapping);
-		page->mapping = mapping;
-		page->index = index + i++;
+		if (cow) {
+			if (dax_mapping_set_cow(page)) {
+				/* Was normal, now updated to CoW */
+				page->index = 2;
+			} else
+				page->index++;
+		} else {
+			WARN_ON_ONCE(page->mapping);
+			page->mapping = mapping;
+			page->index = index + i++;
+		}
 	}
 }
 
@@ -370,7 +411,12 @@  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);
-		WARN_ON_ONCE(page->mapping && page->mapping != mapping);
+		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);
 		page->mapping = NULL;
 		page->index = 0;
 	}
@@ -810,7 +856,8 @@  static void *dax_insert_entry(struct xa_state *xas,
 		void *old;
 
 		dax_disassociate_entry(entry, mapping, false);
-		dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address);
+		dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address,
+				false);
 		/*
 		 * Only swap our new entry into the page cache if the current
 		 * entry is a zero page or an empty entry.  If a normal PTE or
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 1c3b6e5c8bfd..6370d279795a 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -572,6 +572,12 @@  __PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
 #define PAGE_MAPPING_KSM	(PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
 #define PAGE_MAPPING_FLAGS	(PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
 
+/*
+ * 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
+
 static __always_inline int PageMappingFlags(struct page *page)
 {
 	return ((unsigned long)page->mapping & PAGE_MAPPING_FLAGS) != 0;