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I think a bit a bit more detail is warranted, how about? The page ->mapping pointer can have magic values like PAGE_MAPPING_DAX_SHARED and PAGE_MAPPING_ANON for page owner specific usage. In fact, PAGE_MAPPING_DAX_SHARED and PAGE_MAPPING_ANON alias the same value. > This isn't currently a problem because FS DAX pages are treated > specially. s/are treated specially/are never seen by the anonymous mapping code and vice versa/ > However a future change will make FS DAX pages more like > normal pages, so folio_test_anon() must not return true for a FS DAX > page. > > We could explicitly test for a FS DAX page in folio_test_anon(), > etc. however the PAGE_MAPPING_DAX_SHARED flag isn't actually > needed. Instead we can use the page->mapping field to implicitly track > the first mapping of a page. If page->mapping is non-NULL it implies > the page is associated with a single mapping at page->index. If the > page is associated with a second mapping clear page->mapping and set > page->share to 1. > > This is possible because a shared mapping implies the file-system > implements dax_holder_operations which makes the ->mapping and > ->index, which is a union with ->share, unused. > > The page is considered shared when page->mapping == NULL and > page->share > 0 or page->mapping != NULL, implying it is present in at > least one address space. This also makes it easier for a future change > to detect when a page is first mapped into an address space which > requires special handling. > > Signed-off-by: Alistair Popple > --- > fs/dax.c | 45 +++++++++++++++++++++++++-------------- > include/linux/page-flags.h | 6 +----- > 2 files changed, 29 insertions(+), 22 deletions(-) > > diff --git a/fs/dax.c b/fs/dax.c > index 4e49cc4..d35dbe1 100644 > --- a/fs/dax.c > +++ b/fs/dax.c > @@ -351,38 +351,41 @@ static unsigned long dax_end_pfn(void *entry) > for (pfn = dax_to_pfn(entry); \ > pfn < dax_end_pfn(entry); pfn++) > > +/* > + * A DAX page is considered shared if it has no mapping set and ->share (which > + * shares the ->index field) is non-zero. Note this may return false even if the > + * page is shared between multiple files but has not yet actually been mapped > + * into multiple address spaces. > + */ > static inline bool dax_page_is_shared(struct page *page) > { > - return page->mapping == PAGE_MAPPING_DAX_SHARED; > + return !page->mapping && page->share; > } > > /* > - * Set the page->mapping with PAGE_MAPPING_DAX_SHARED flag, increase the > - * refcount. > + * Increase the page share refcount, warning if the page is not marked as shared. > */ > static inline void dax_page_share_get(struct page *page) > { > - if (page->mapping != PAGE_MAPPING_DAX_SHARED) { > - /* > - * Reset the index if the page was already mapped > - * regularly before. > - */ > - if (page->mapping) > - page->share = 1; > - page->mapping = PAGE_MAPPING_DAX_SHARED; > - } > + WARN_ON_ONCE(!page->share); > + WARN_ON_ONCE(page->mapping); Given the only caller of this function is dax_associate_entry() it seems like overkill to check that a function only a few lines away manipulated ->mapping correctly. I don't see much reason for dax_page_share_get() to exist after your changes. Perhaps all that is needed is a dax_make_shared() helper that does the initial fiddling of '->mapping = NULL' and '->share = 1'? > page->share++; > } > > static inline unsigned long dax_page_share_put(struct page *page) > { > + WARN_ON_ONCE(!page->share); > return --page->share; > } > > /* > - * 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 > - * PAGE_MAPPING_DAX_SHARED, and use page->share as refcount. > + * When it is called in dax_insert_entry(), the shared flag will indicate > + * whether this entry is shared by multiple files. If the page has not > + * previously been associated with any mappings the ->mapping and ->index > + * fields will be set. If it has already been associated with a mapping > + * the mapping will be cleared and the share count set. It's then up to the > + * file-system to track which mappings contain which pages, ie. by implementing > + * dax_holder_operations. This feels like a good comment for a new dax_make_shared() not dax_associate_entry(). I would also: s/up to the file-system to track which mappings contain which pages, ie. by implementing dax_holder_operations/up to reverse map users like memory_failure() to call back into the filesystem to recover ->mapping and ->index information/ > */ > static void dax_associate_entry(void *entry, struct address_space *mapping, > struct vm_area_struct *vma, unsigned long address, bool shared) > @@ -397,7 +400,17 @@ static void dax_associate_entry(void *entry, struct address_space *mapping, > for_each_mapped_pfn(entry, pfn) { > struct page *page = pfn_to_page(pfn); > > - if (shared) { > + if (shared && page->mapping && page->share) { How does this case happen? I don't think any page would ever enter with both ->mapping and ->share set, right? If the file was mapped then reflinked then ->share should be zero at the first mapping attempt. It might not be zero because it is aliased with index until it is converted to a shared page.