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