On Mon, Feb 08, 2021 at 01:09:21AM +0800, Shiyang Ruan wrote:
> We replace the existing entry to the newly allocated one
> in case of CoW. Also, we mark the entry as PAGECACHE_TAG_TOWRITE
> so writeback marks this entry as writeprotected. This
> helps us snapshots so new write pagefaults after snapshots
> trigger a CoW.
> 
> Signed-off-by: Goldwyn Rodrigues <rgold...@suse.com>
> Signed-off-by: Shiyang Ruan <ruansy.f...@cn.fujitsu.com>
> ---
>  fs/dax.c | 31 +++++++++++++++++++++++--------
>  1 file changed, 23 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index b2195cbdf2dc..29698a3d2e37 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -722,6 +722,9 @@ static int copy_cow_page_dax(struct block_device *bdev, 
> struct dax_device *dax_d
>       return 0;
>  }
>  
> +#define DAX_IF_DIRTY         (1ULL << 0)
> +#define DAX_IF_COW           (1ULL << 1)

The constants are ULL, but I see other flags only 'unsigned long'

> +
>  /*
>   * By this point grab_mapping_entry() has ensured that we have a locked entry
>   * of the appropriate size so we don't have to worry about downgrading PMDs 
> to
> @@ -731,14 +734,16 @@ static int copy_cow_page_dax(struct block_device *bdev, 
> struct dax_device *dax_d
>   */
>  static void *dax_insert_entry(struct xa_state *xas,
>               struct address_space *mapping, struct vm_fault *vmf,
> -             void *entry, pfn_t pfn, unsigned long flags, bool dirty)
> +             void *entry, pfn_t pfn, unsigned long flags, bool insert_flags)

insert_flags is bool

>  {
>       void *new_entry = dax_make_entry(pfn, flags);
> +     bool dirty = insert_flags & DAX_IF_DIRTY;

"insert_flags & DAX_IF_DIRTY" is "bool & ULL", this can't be right

> +     bool cow = insert_flags & DAX_IF_COW;

Same

>  
>       if (dirty)
>               __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
>  
> -     if (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE)) {
> +     if (cow || (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE))) {
>               unsigned long index = xas->xa_index;
>               /* we are replacing a zero page with block mapping */
>               if (dax_is_pmd_entry(entry))
> @@ -750,7 +755,7 @@ static void *dax_insert_entry(struct xa_state *xas,
>  
>       xas_reset(xas);
>       xas_lock_irq(xas);
> -     if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
> +     if (cow || dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
>               void *old;
>  
>               dax_disassociate_entry(entry, mapping, false);
> @@ -774,6 +779,9 @@ static void *dax_insert_entry(struct xa_state *xas,
>       if (dirty)
>               xas_set_mark(xas, PAGECACHE_TAG_DIRTY);
>  
> +     if (cow)
> +             xas_set_mark(xas, PAGECACHE_TAG_TOWRITE);
> +
>       xas_unlock_irq(xas);
>       return entry;
>  }
> @@ -1319,6 +1327,7 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault 
> *vmf, pfn_t *pfnp,
>       void *entry;
>       pfn_t pfn;
>       void *kaddr;
> +     unsigned long insert_flags = 0;
>  
>       trace_dax_pte_fault(inode, vmf, ret);
>       /*
> @@ -1444,8 +1453,10 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault 
> *vmf, pfn_t *pfnp,
>  
>               goto finish_iomap;
>       case IOMAP_UNWRITTEN:
> -             if (write && iomap.flags & IOMAP_F_SHARED)
> +             if (write && (iomap.flags & IOMAP_F_SHARED)) {
> +                     insert_flags |= DAX_IF_COW;

Here's an example of 'unsigned long = unsigned long long', though it'll
work, it would be better to unify all the types.

>                       goto cow;
> +             }
>               fallthrough;
>       case IOMAP_HOLE:
>               if (!write) {
> @@ -1555,6 +1566,7 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault 
> *vmf, pfn_t *pfnp,
>       int error;
>       pfn_t pfn;
>       void *kaddr;
> +     unsigned long insert_flags = 0;
>  
>       /*
>        * Check whether offset isn't beyond end of file now. Caller is
> @@ -1670,14 +1682,17 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault 
> *vmf, pfn_t *pfnp,
>               result = vmf_insert_pfn_pmd(vmf, pfn, write);
>               break;
>       case IOMAP_UNWRITTEN:
> -             if (write && iomap.flags & IOMAP_F_SHARED)
> +             if (write && (iomap.flags & IOMAP_F_SHARED)) {
> +                     insert_flags |= DAX_IF_COW;
>                       goto cow;
> +             }
>               fallthrough;
>       case IOMAP_HOLE:
> -             if (WARN_ON_ONCE(write))
> +             if (!write) {
> +                     result = dax_pmd_load_hole(&xas, vmf, &iomap, &entry);
>                       break;
> -             result = dax_pmd_load_hole(&xas, vmf, &iomap, &entry);
> -             break;
> +             }
> +             fallthrough;
>       default:
>               WARN_ON_ONCE(1);
>               break;
> -- 
> 2.30.0
> 
> 

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