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

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在 2022/2/16 10:09, Dan Williams 写道:
On Thu, Jan 27, 2022 at 4:41 AM Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx> 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@xxxxxxxxxxx>
---
  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










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