Introduce a PAGE_MAPPING_DAX_COW flag to support association with CoW file mappings. In this case, since the dax-rmap has already took 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. == PS: The @cow added for dax_associate_entry(), is used to let it know whether the entry is to be shared during iomap operation. It is decided by iomap,srcmap's flag, and will be used in another patchset( fsdax,xfs: Add reflink&dedupe support for fsdax[1]). In this patch, we set @cow always false for now. [1] https://lore.kernel.org/linux-xfs/20210928062311.4012070-1-ruansy.fnst@xxxxxxxxxxx/ == 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 f164cf64c611..b97106a0a0b1 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; } @@ -829,7 +875,8 @@ 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; -- 2.35.1
