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.
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?
> /*
> * 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
>
>
>
