Re: [PATCH v9 10/10] fsdax: set a CoW flag when associate reflink mappings
在 2022/1/21 15:16, Christoph Hellwig 写道: On Fri, Jan 21, 2022 at 10:33:58AM +0800, Shiyang Ruan wrote: But different question, how does this not conflict with: #define PAGE_MAPPING_ANON 0x1 in page-flags.h? Now we are treating dax pages, so I think its flags should be different from normal page. In another word, PAGE_MAPPING_ANON is a flag of rmap mechanism for normal page, it doesn't work for dax page. And now, we have dax rmap for dax page. So, I think this two kinds of flags are supposed to be used in different mechanisms and won't conflect. It just needs someone to use folio_test_anon in a place where a DAX folio can be passed. This probably should not happen, but we need to clearly document that. Either way I think this flag should move to page-flags.h and be integrated with the PAGE_MAPPING_FLAGS infrastucture. And that's why I keep them in this dax.c file. But that does not integrate it with the infrastructure. For people to debug things it needs to be next to PAGE_MAPPING_ANON and have documentation explaining why they are exclusive. Ok, understood. -- Thanks, Ruan.
Re: [PATCH v9 10/10] fsdax: set a CoW flag when associate reflink mappings
On Fri, Jan 21, 2022 at 10:33:58AM +0800, Shiyang Ruan wrote: > > > > But different question, how does this not conflict with: > > > > #define PAGE_MAPPING_ANON 0x1 > > > > in page-flags.h? > > Now we are treating dax pages, so I think its flags should be different from > normal page. In another word, PAGE_MAPPING_ANON is a flag of rmap mechanism > for normal page, it doesn't work for dax page. And now, we have dax rmap > for dax page. So, I think this two kinds of flags are supposed to be used > in different mechanisms and won't conflect. It just needs someone to use folio_test_anon in a place where a DAX folio can be passed. This probably should not happen, but we need to clearly document that. > > Either way I think this flag should move to page-flags.h and be > > integrated with the PAGE_MAPPING_FLAGS infrastucture. > > And that's why I keep them in this dax.c file. But that does not integrate it with the infrastructure. For people to debug things it needs to be next to PAGE_MAPPING_ANON and have documentation explaining why they are exclusive.
Re: [PATCH v9 10/10] fsdax: set a CoW flag when associate reflink mappings
在 2022/1/20 16:59, Christoph Hellwig 写道: On Sun, Dec 26, 2021 at 10:34:39PM +0800, Shiyang Ruan wrote: +#define FS_DAX_MAPPING_COW 1UL + +#define MAPPING_SET_COW(m) (m = (struct address_space *)FS_DAX_MAPPING_COW) +#define MAPPING_TEST_COW(m)(((unsigned long)m & FS_DAX_MAPPING_COW) == \ + FS_DAX_MAPPING_COW) These really should be inline functions and probably use lower case names. OK. But different question, how does this not conflict with: #define PAGE_MAPPING_ANON 0x1 in page-flags.h? Now we are treating dax pages, so I think its flags should be different from normal page. In another word, PAGE_MAPPING_ANON is a flag of rmap mechanism for normal page, it doesn't work for dax page. And now, we have dax rmap for dax page. So, I think this two kinds of flags are supposed to be used in different mechanisms and won't conflect. Either way I think this flag should move to page-flags.h and be integrated with the PAGE_MAPPING_FLAGS infrastucture. And that's why I keep them in this dax.c file. -- Thanks, Ruan.
Re: [PATCH v9 10/10] fsdax: set a CoW flag when associate reflink mappings
On Sun, Dec 26, 2021 at 10:34:39PM +0800, Shiyang Ruan wrote: > +#define FS_DAX_MAPPING_COW 1UL > + > +#define MAPPING_SET_COW(m) (m = (struct address_space *)FS_DAX_MAPPING_COW) > +#define MAPPING_TEST_COW(m) (((unsigned long)m & FS_DAX_MAPPING_COW) == \ > + FS_DAX_MAPPING_COW) These really should be inline functions and probably use lower case names. But different question, how does this not conflict with: #define PAGE_MAPPING_ANON 0x1 in page-flags.h? Either way I think this flag should move to page-flags.h and be integrated with the PAGE_MAPPING_FLAGS infrastucture.
[PATCH v9 10/10] fsdax: set a CoW flag when associate reflink mappings
Introduce a FS_DAX_MAPPING_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
---
fs/dax.c | 66
1 file changed, 57 insertions(+), 9 deletions(-)
diff --git a/fs/dax.c b/fs/dax.c
index ad8ceea1f54c..e72ec712c002 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -335,12 +335,46 @@ static unsigned long dax_end_pfn(void *entry)
pfn < dax_end_pfn(entry); pfn++)
/*
- * 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 FS_DAX_MAPPING_COW flag on the last bit of page->mapping to indicate
that
+ * this is a reflink case. In this case, we associate this page->mapping with
+ * file mapping at the first time and only once.
+ */
+#define FS_DAX_MAPPING_COW 1UL
+
+#define MAPPING_SET_COW(m) (m = (struct address_space *)FS_DAX_MAPPING_COW)
+#define MAPPING_TEST_COW(m)(((unsigned long)m & FS_DAX_MAPPING_COW) == \
+ FS_DAX_MAPPING_COW)
+
+/*
+ * 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 (MAPPING_TEST_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.
+*/
+ MAPPING_SET_COW(page->mapping);
+ return true;
+ }
+ /* Newly associate CoW mapping */
+ MAPPING_SET_COW(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 +386,17 @@ 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 +412,12 @@ 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 (!MAPPING_TEST_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;
}
@@ -829,7 +876,8 @@ 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
--
2.34.1
