On Tue, Apr 19, 2022 at 12:50:45PM +0800, Shiyang Ruan wrote: > 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 | 50 +++++++++++++++++++++++++++++++------- > include/linux/page-flags.h | 6 +++++ > 2 files changed, 47 insertions(+), 9 deletions(-) > > diff --git a/fs/dax.c b/fs/dax.c > index 57efd3f73655..4d3dfc8bee33 100644 > --- a/fs/dax.c > +++ b/fs/dax.c > @@ -334,13 +334,35 @@ static unsigned long dax_end_pfn(void *entry) > for (pfn = dax_to_pfn(entry); \ > pfn < dax_end_pfn(entry); pfn++) > > +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 the page->mapping with FS_DAX_MAPPING_COW flag, increase the refcount. > + */ > +static inline void dax_mapping_set_cow(struct page *page) > +{ > + if ((uintptr_t)page->mapping != PAGE_MAPPING_DAX_COW) { > + /* > + * Reset the index if the page was already mapped > + * regularly before. > + */ > + if (page->mapping) > + page->index = 1; > + page->mapping = (void *)PAGE_MAPPING_DAX_COW; > + } > + page->index++; > +} > + > +/* > + * 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 > + * 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 +374,13 @@ 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) { > + dax_mapping_set_cow(page); > + } else { > + WARN_ON_ONCE(page->mapping); > + page->mapping = mapping; > + page->index = index + i++; > + } > } > } > > @@ -370,7 +396,12 @@ 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 +860,8 @@ 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 d725a2d17806..5b601e375773 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -650,6 +650,12 @@ __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 The logic looks sound enough, I guess. Though I do wonder -- if this were defined like this: #define PAGE_MAPPING_DAX_COW ((struct address_space *)0x1) Could you then avoid all uintptr_t/unsigned long casts above? It's probably not worth holding up the whole patchset though, so Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx> --D > + > static __always_inline int PageMappingFlags(struct page *page) > { > return ((unsigned long)page->mapping & PAGE_MAPPING_FLAGS) != 0; > -- > 2.35.1 > > >
