dax: use common 4k zero page for dax mmap reads

When servicing mmap() reads from file holes the current DAX code
allocates a page cache page of all zeroes and places the struct page
pointer in the mapping->page_tree radix tree.

This has three major drawbacks:

1) It consumes memory unnecessarily. For every 4k page that is read via
   a DAX mmap() over a hole, we allocate a new page cache page. This
   means that if you read 1GiB worth of pages, you end up using 1GiB of
   zeroed memory. This is easily visible by looking at the overall
   memory consumption of the system or by looking at /proc/[pid]/smaps:

	7f62e72b3000-7f63272b3000 rw-s 00000000 103:00 12   /root/dax/data
	Size:            1048576 kB
	Rss:             1048576 kB
	Pss:             1048576 kB
	Shared_Clean:          0 kB
	Shared_Dirty:          0 kB
	Private_Clean:   1048576 kB
	Private_Dirty:         0 kB
	Referenced:      1048576 kB
	Anonymous:             0 kB
	LazyFree:              0 kB
	AnonHugePages:         0 kB
	ShmemPmdMapped:        0 kB
	Shared_Hugetlb:        0 kB
	Private_Hugetlb:       0 kB
	Swap:                  0 kB
	SwapPss:               0 kB
	KernelPageSize:        4 kB
	MMUPageSize:           4 kB
	Locked:                0 kB

2) It is slower than using a common zero page because each page fault
   has more work to do. Instead of just inserting a common zero page we
   have to allocate a page cache page, zero it, and then insert it. Here
   are the average latencies of dax_load_hole() as measured by ftrace on
   a random test box:

    Old method, using zeroed page cache pages:	3.4 us
    New method, using the common 4k zero page:	0.8 us

   This was the average latency over 1 GiB of sequential reads done by
   this simple fio script:

     [global]
     size=1G
     filename=/root/dax/data
     fallocate=none
     [io]
     rw=read
     ioengine=mmap

3) The fact that we had to check for both DAX exceptional entries and
   for page cache pages in the radix tree made the DAX code more
   complex.

Solve these issues by following the lead of the DAX PMD code and using a
common 4k zero page instead.  As with the PMD code we will now insert a
DAX exceptional entry into the radix tree instead of a struct page
pointer which allows us to remove all the special casing in the DAX
code.

Note that we do still pretty aggressively check for regular pages in the
DAX radix tree, especially where we take action based on the bits set in
the page.  If we ever find a regular page in our radix tree now that
most likely means that someone besides DAX is inserting pages (which has
happened lots of times in the past), and we want to find that out early
and fail loudly.

This solution also removes the extra memory consumption.  Here is that
same /proc/[pid]/smaps after 1GiB of reading from a hole with the new
code:

	7f2054a74000-7f2094a74000 rw-s 00000000 103:00 12   /root/dax/data
	Size:            1048576 kB
	Rss:                   0 kB
	Pss:                   0 kB
	Shared_Clean:          0 kB
	Shared_Dirty:          0 kB
	Private_Clean:         0 kB
	Private_Dirty:         0 kB
	Referenced:            0 kB
	Anonymous:             0 kB
	LazyFree:              0 kB
	AnonHugePages:         0 kB
	ShmemPmdMapped:        0 kB
	Shared_Hugetlb:        0 kB
	Private_Hugetlb:       0 kB
	Swap:                  0 kB
	SwapPss:               0 kB
	KernelPageSize:        4 kB
	MMUPageSize:           4 kB
	Locked:                0 kB

Overall system memory consumption is similarly improved.

Another major change is that we remove dax_pfn_mkwrite() from our fault
flow, and instead rely on the page fault itself to make the PTE dirty
and writeable.  The following description from the patch adding the
vm_insert_mixed_mkwrite() call explains this a little more:

   "To be able to use the common 4k zero page in DAX we need to have our
    PTE fault path look more like our PMD fault path where a PTE entry
    can be marked as dirty and writeable as it is first inserted rather
    than waiting for a follow-up dax_pfn_mkwrite() =>
    finish_mkwrite_fault() call.

    Right now we can rely on having a dax_pfn_mkwrite() call because we
    can distinguish between these two cases in do_wp_page():

            case 1: 4k zero page => writable DAX storage
            case 2: read-only DAX storage => writeable DAX storage

    This distinction is made by via vm_normal_page(). vm_normal_page()
    returns false for the common 4k zero page, though, just as it does
    for DAX ptes. Instead of special casing the DAX + 4k zero page case
    we will simplify our DAX PTE page fault sequence so that it matches
    our DAX PMD sequence, and get rid of the dax_pfn_mkwrite() helper.
    We will instead use dax_iomap_fault() to handle write-protection
    faults.

    This means that insert_pfn() needs to follow the lead of
    insert_pfn_pmd() and allow us to pass in a 'mkwrite' flag. If
    'mkwrite' is set insert_pfn() will do the work that was previously
    done by wp_page_reuse() as part of the dax_pfn_mkwrite() call path"

Link: http://lkml.kernel.org/r/20170724170616.25810-4-ross.zwisler@linux.intel.com
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 76 insertions, 167 deletions

diff --git a/fs/dax.c b/fs/dax.c
index b8882b5ce6ed..ab67ae30ccbf 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -66,7 +66,7 @@ static int dax_is_pte_entry(void *entry)
 
 static int dax_is_zero_entry(void *entry)
 {
-        return (unsigned long)entry & RADIX_DAX_HZP;
+        return (unsigned long)entry & RADIX_DAX_ZERO_PAGE;
 }
 
 static int dax_is_empty_entry(void *entry)
@@ -206,7 +206,8 @@ static void *get_unlocked_mapping_entry(struct address_space *mapping,
         for (;;) {
                 entry = __radix_tree_lookup(&mapping->page_tree, index, NULL,
                                           &slot);
-                if (!entry || !radix_tree_exceptional_entry(entry) ||
+                if (!entry ||
+                    WARN_ON_ONCE(!radix_tree_exceptional_entry(entry)) ||
                     !slot_locked(mapping, slot)) {
                         if (slotp)
                                 *slotp = slot;
@@ -241,14 +242,9 @@ static void dax_unlock_mapping_entry(struct address_space *mapping,
 }
 
 static void put_locked_mapping_entry(struct address_space *mapping,
-                                     pgoff_t index, void *entry)
+                pgoff_t index)
 {
-        if (!radix_tree_exceptional_entry(entry)) {
-                unlock_page(entry);
-                put_page(entry);
-        } else {
-                dax_unlock_mapping_entry(mapping, index);
-        }
+        dax_unlock_mapping_entry(mapping, index);
 }
 
 /*
@@ -258,7 +254,7 @@ static void put_locked_mapping_entry(struct address_space *mapping,
 static void put_unlocked_mapping_entry(struct address_space *mapping,
                                        pgoff_t index, void *entry)
 {
-        if (!radix_tree_exceptional_entry(entry))
+        if (!entry)
                 return;
 
         /* We have to wake up next waiter for the radix tree entry lock */
@@ -266,15 +262,15 @@ static void put_unlocked_mapping_entry(struct address_space *mapping,
 }
 
 /*
- * Find radix tree entry at given index. If it points to a page, return with
- * the page locked. If it points to the exceptional entry, return with the
- * radix tree entry locked. If the radix tree doesn't contain given index,
- * create empty exceptional entry for the index and return with it locked.
+ * Find radix tree entry at given index. If it points to an exceptional entry,
+ * return it with the radix tree entry locked. If the radix tree doesn't
+ * contain given index, create an empty exceptional entry for the index and
+ * return with it locked.
  *
  * When requesting an entry with size RADIX_DAX_PMD, grab_mapping_entry() will
  * either return that locked entry or will return an error.  This error will
- * happen if there are any 4k entries (either zero pages or DAX entries)
- * within the 2MiB range that we are requesting.
+ * happen if there are any 4k entries within the 2MiB range that we are
+ * requesting.
  *
  * We always favor 4k entries over 2MiB entries. There isn't a flow where we
  * evict 4k entries in order to 'upgrade' them to a 2MiB entry.  A 2MiB
@@ -301,18 +297,21 @@ restart:
         spin_lock_irq(&mapping->tree_lock);
         entry = get_unlocked_mapping_entry(mapping, index, &slot);
 
+        if (WARN_ON_ONCE(entry && !radix_tree_exceptional_entry(entry))) {
+                entry = ERR_PTR(-EIO);
+                goto out_unlock;
+        }
+
         if (entry) {
                 if (size_flag & RADIX_DAX_PMD) {
-                        if (!radix_tree_exceptional_entry(entry) ||
-                            dax_is_pte_entry(entry)) {
+                        if (dax_is_pte_entry(entry)) {
                                 put_unlocked_mapping_entry(mapping, index,
                                                 entry);
                                 entry = ERR_PTR(-EEXIST);
                                 goto out_unlock;
                         }
                 } else { /* trying to grab a PTE entry */
-                        if (radix_tree_exceptional_entry(entry) &&
-                            dax_is_pmd_entry(entry) &&
+                        if (dax_is_pmd_entry(entry) &&
                             (dax_is_zero_entry(entry) ||
                              dax_is_empty_entry(entry))) {
                                 pmd_downgrade = true;
@@ -346,7 +345,7 @@ restart:
                                 mapping_gfp_mask(mapping) & ~__GFP_HIGHMEM);
                 if (err) {
                         if (pmd_downgrade)
-                                put_locked_mapping_entry(mapping, index, entry);
+                                put_locked_mapping_entry(mapping, index);
                         return ERR_PTR(err);
                 }
                 spin_lock_irq(&mapping->tree_lock);
@@ -396,21 +395,6 @@ restart:
                 spin_unlock_irq(&mapping->tree_lock);
                 return entry;
         }
-        /* Normal page in radix tree? */
-        if (!radix_tree_exceptional_entry(entry)) {
-                struct page *page = entry;
-
-                get_page(page);
-                spin_unlock_irq(&mapping->tree_lock);
-                lock_page(page);
-                /* Page got truncated? Retry... */
-                if (unlikely(page->mapping != mapping)) {
-                        unlock_page(page);
-                        put_page(page);
-                        goto restart;
-                }
-                return page;
-        }
         entry = lock_slot(mapping, slot);
  out_unlock:
         spin_unlock_irq(&mapping->tree_lock);
@@ -426,7 +410,7 @@ static int __dax_invalidate_mapping_entry(struct address_space *mapping,
 
         spin_lock_irq(&mapping->tree_lock);
         entry = get_unlocked_mapping_entry(mapping, index, NULL);
-        if (!entry || !radix_tree_exceptional_entry(entry))
+        if (!entry || WARN_ON_ONCE(!radix_tree_exceptional_entry(entry)))
                 goto out;
         if (!trunc &&
             (radix_tree_tag_get(page_tree, index, PAGECACHE_TAG_DIRTY) ||
@@ -508,47 +492,27 @@ static void *dax_insert_mapping_entry(struct address_space *mapping,
                                       unsigned long flags)
 {
         struct radix_tree_root *page_tree = &mapping->page_tree;
-        int error = 0;
-        bool hole_fill = false;
         void *new_entry;
         pgoff_t index = vmf->pgoff;
 
         if (vmf->flags & FAULT_FLAG_WRITE)
                 __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
 
-        /* Replacing hole page with block mapping? */
-        if (!radix_tree_exceptional_entry(entry)) {
-                hole_fill = true;
-                /*
-                 * Unmap the page now before we remove it from page cache below.
-                 * The page is locked so it cannot be faulted in again.
-                 */
-                unmap_mapping_range(mapping, vmf->pgoff << PAGE_SHIFT,
-                                    PAGE_SIZE, 0);
-                error = radix_tree_preload(vmf->gfp_mask & ~__GFP_HIGHMEM);
-                if (error)
-                        return ERR_PTR(error);
-        } else if (dax_is_zero_entry(entry) && !(flags & RADIX_DAX_HZP)) {
-                /* replacing huge zero page with PMD block mapping */
-                unmap_mapping_range(mapping,
-                        (vmf->pgoff << PAGE_SHIFT) & PMD_MASK, PMD_SIZE, 0);
+        if (dax_is_zero_entry(entry) && !(flags & RADIX_DAX_ZERO_PAGE)) {
+                /* we are replacing a zero page with block mapping */
+                if (dax_is_pmd_entry(entry))
+                        unmap_mapping_range(mapping,
+                                        (vmf->pgoff << PAGE_SHIFT) & PMD_MASK,
+                                        PMD_SIZE, 0);
+                else /* pte entry */
+                        unmap_mapping_range(mapping, vmf->pgoff << PAGE_SHIFT,
+                                        PAGE_SIZE, 0);
         }
 
         spin_lock_irq(&mapping->tree_lock);
         new_entry = dax_radix_locked_entry(sector, flags);
 
-        if (hole_fill) {
-                __delete_from_page_cache(entry, NULL);
-                /* Drop pagecache reference */
-                put_page(entry);
-                error = __radix_tree_insert(page_tree, index,
-                                dax_radix_order(new_entry), new_entry);
-                if (error) {
-                        new_entry = ERR_PTR(error);
-                        goto unlock;
-                }
-                mapping->nrexceptional++;
-        } else if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
+        if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
                 /*
                  * Only swap our new entry into the radix tree if the current
                  * entry is a zero page or an empty entry.  If a normal PTE or
@@ -565,23 +529,14 @@ static void *dax_insert_mapping_entry(struct address_space *mapping,
                 WARN_ON_ONCE(ret != entry);
                 __radix_tree_replace(page_tree, node, slot,
                                      new_entry, NULL, NULL);
+                entry = new_entry;
         }
+
         if (vmf->flags & FAULT_FLAG_WRITE)
                 radix_tree_tag_set(page_tree, index, PAGECACHE_TAG_DIRTY);
- unlock:
+
         spin_unlock_irq(&mapping->tree_lock);
-        if (hole_fill) {
-                radix_tree_preload_end();
-                /*
-                 * We don't need hole page anymore, it has been replaced with
-                 * locked radix tree entry now.
-                 */
-                if (mapping->a_ops->freepage)
-                        mapping->a_ops->freepage(entry);
-                unlock_page(entry);
-                put_page(entry);
-        }
-        return new_entry;
+        return entry;
 }
 
 static inline unsigned long
@@ -683,7 +638,7 @@ static int dax_writeback_one(struct block_device *bdev,
         spin_lock_irq(&mapping->tree_lock);
         entry2 = get_unlocked_mapping_entry(mapping, index, &slot);
         /* Entry got punched out / reallocated? */
-        if (!entry2 || !radix_tree_exceptional_entry(entry2))
+        if (!entry2 || WARN_ON_ONCE(!radix_tree_exceptional_entry(entry2)))
                 goto put_unlocked;
         /*
          * Entry got reallocated elsewhere? No need to writeback. We have to
@@ -755,7 +710,7 @@ static int dax_writeback_one(struct block_device *bdev,
         trace_dax_writeback_one(mapping->host, index, size >> PAGE_SHIFT);
  dax_unlock:
         dax_read_unlock(id);
-        put_locked_mapping_entry(mapping, index, entry);
+        put_locked_mapping_entry(mapping, index);
         return ret;
 
  put_unlocked:
@@ -830,11 +785,10 @@ EXPORT_SYMBOL_GPL(dax_writeback_mapping_range);
 
 static int dax_insert_mapping(struct address_space *mapping,
                 struct block_device *bdev, struct dax_device *dax_dev,
-                sector_t sector, size_t size, void **entryp,
+                sector_t sector, size_t size, void *entry,
                 struct vm_area_struct *vma, struct vm_fault *vmf)
 {
         unsigned long vaddr = vmf->address;
-        void *entry = *entryp;
         void *ret, *kaddr;
         pgoff_t pgoff;
         int id, rc;
@@ -855,87 +809,44 @@ static int dax_insert_mapping(struct address_space *mapping,
         ret = dax_insert_mapping_entry(mapping, vmf, entry, sector, 0);
         if (IS_ERR(ret))
                 return PTR_ERR(ret);
-        *entryp = ret;
 
         trace_dax_insert_mapping(mapping->host, vmf, ret);
-        return vm_insert_mixed(vma, vaddr, pfn);
-}
-
-/**
- * dax_pfn_mkwrite - handle first write to DAX page
- * @vmf: The description of the fault
- */
-int dax_pfn_mkwrite(struct vm_fault *vmf)
-{
-        struct file *file = vmf->vma->vm_file;
-        struct address_space *mapping = file->f_mapping;
-        struct inode *inode = mapping->host;
-        void *entry, **slot;
-        pgoff_t index = vmf->pgoff;
-
-        spin_lock_irq(&mapping->tree_lock);
-        entry = get_unlocked_mapping_entry(mapping, index, &slot);
-        if (!entry || !radix_tree_exceptional_entry(entry)) {
-                if (entry)
-                        put_unlocked_mapping_entry(mapping, index, entry);
-                spin_unlock_irq(&mapping->tree_lock);
-                trace_dax_pfn_mkwrite_no_entry(inode, vmf, VM_FAULT_NOPAGE);
-                return VM_FAULT_NOPAGE;
-        }
-        radix_tree_tag_set(&mapping->page_tree, index, PAGECACHE_TAG_DIRTY);
-        entry = lock_slot(mapping, slot);
-        spin_unlock_irq(&mapping->tree_lock);
-        /*
-         * If we race with somebody updating the PTE and finish_mkwrite_fault()
-         * fails, we don't care. We need to return VM_FAULT_NOPAGE and retry
-         * the fault in either case.
-         */
-        finish_mkwrite_fault(vmf);
-        put_locked_mapping_entry(mapping, index, entry);
-        trace_dax_pfn_mkwrite(inode, vmf, VM_FAULT_NOPAGE);
-        return VM_FAULT_NOPAGE;
+        if (vmf->flags & FAULT_FLAG_WRITE)
+                return vm_insert_mixed_mkwrite(vma, vaddr, pfn);
+        else
+                return vm_insert_mixed(vma, vaddr, pfn);
 }
-EXPORT_SYMBOL_GPL(dax_pfn_mkwrite);
 
 /*
- * The user has performed a load from a hole in the file.  Allocating
- * a new page in the file would cause excessive storage usage for
- * workloads with sparse files.  We allocate a page cache page instead.
- * We'll kick it out of the page cache if it's ever written to,
- * otherwise it will simply fall out of the page cache under memory
- * pressure without ever having been dirtied.
+ * The user has performed a load from a hole in the file.  Allocating a new
+ * page in the file would cause excessive storage usage for workloads with
+ * sparse files.  Instead we insert a read-only mapping of the 4k zero page.
+ * If this page is ever written to we will re-fault and change the mapping to
+ * point to real DAX storage instead.
  */
-static int dax_load_hole(struct address_space *mapping, void **entry,
+static int dax_load_hole(struct address_space *mapping, void *entry,
                          struct vm_fault *vmf)
 {
         struct inode *inode = mapping->host;
-        struct page *page;
-        int ret;
-
-        /* Hole page already exists? Return it...  */
-        if (!radix_tree_exceptional_entry(*entry)) {
-                page = *entry;
-                goto finish_fault;
-        }
+        unsigned long vaddr = vmf->address;
+        int ret = VM_FAULT_NOPAGE;
+        struct page *zero_page;
+        void *entry2;
 
-        /* This will replace locked radix tree entry with a hole page */
-        page = find_or_create_page(mapping, vmf->pgoff,
-                                   vmf->gfp_mask | __GFP_ZERO);
-        if (!page) {
+        zero_page = ZERO_PAGE(0);
+        if (unlikely(!zero_page)) {
                 ret = VM_FAULT_OOM;
                 goto out;
         }
 
-finish_fault:
-        vmf->page = page;
-        ret = finish_fault(vmf);
-        vmf->page = NULL;
-        *entry = page;
-        if (!ret) {
-                /* Grab reference for PTE that is now referencing the page */
-                get_page(page);
-                ret = VM_FAULT_NOPAGE;
+        entry2 = dax_insert_mapping_entry(mapping, vmf, entry, 0,
+                        RADIX_DAX_ZERO_PAGE);
+        if (IS_ERR(entry2)) {
+                ret = VM_FAULT_SIGBUS;
+                goto out;
         }
+
+        vm_insert_mixed(vmf->vma, vaddr, page_to_pfn_t(zero_page));
 out:
         trace_dax_load_hole(inode, vmf, ret);
         return ret;
@@ -1223,7 +1134,7 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf,
                         major = VM_FAULT_MAJOR;
                 }
                 error = dax_insert_mapping(mapping, iomap.bdev, iomap.dax_dev,
-                                sector, PAGE_SIZE, &entry, vmf->vma, vmf);
+                                sector, PAGE_SIZE, entry, vmf->vma, vmf);
                 /* -EBUSY is fine, somebody else faulted on the same PTE */
                 if (error == -EBUSY)
                         error = 0;
@@ -1231,7 +1142,7 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf,
         case IOMAP_UNWRITTEN:
         case IOMAP_HOLE:
                 if (!(vmf->flags & FAULT_FLAG_WRITE)) {
-                        vmf_ret = dax_load_hole(mapping, &entry, vmf);
+                        vmf_ret = dax_load_hole(mapping, entry, vmf);
                         goto finish_iomap;
                 }
                 /*FALLTHRU*/
@@ -1258,7 +1169,7 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf,
                 ops->iomap_end(inode, pos, PAGE_SIZE, copied, flags, &iomap);
         }
  unlock_entry:
-        put_locked_mapping_entry(mapping, vmf->pgoff, entry);
+        put_locked_mapping_entry(mapping, vmf->pgoff);
  out:
         trace_dax_pte_fault_done(inode, vmf, vmf_ret);
         return vmf_ret;
@@ -1272,7 +1183,7 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf,
 #define PG_PMD_COLOUR   ((PMD_SIZE >> PAGE_SHIFT) - 1)
 
 static int dax_pmd_insert_mapping(struct vm_fault *vmf, struct iomap *iomap,
-                loff_t pos, void **entryp)
+                loff_t pos, void *entry)
 {
         struct address_space *mapping = vmf->vma->vm_file->f_mapping;
         const sector_t sector = dax_iomap_sector(iomap, pos);
@@ -1303,11 +1214,10 @@ static int dax_pmd_insert_mapping(struct vm_fault *vmf, struct iomap *iomap,
                 goto unlock_fallback;
         dax_read_unlock(id);
 
-        ret = dax_insert_mapping_entry(mapping, vmf, *entryp, sector,
+        ret = dax_insert_mapping_entry(mapping, vmf, entry, sector,
                         RADIX_DAX_PMD);
         if (IS_ERR(ret))
                 goto fallback;
-        *entryp = ret;
 
         trace_dax_pmd_insert_mapping(inode, vmf, length, pfn, ret);
         return vmf_insert_pfn_pmd(vmf->vma, vmf->address, vmf->pmd,
@@ -1321,7 +1231,7 @@ fallback:
 }
 
 static int dax_pmd_load_hole(struct vm_fault *vmf, struct iomap *iomap,
-                void **entryp)
+                void *entry)
 {
         struct address_space *mapping = vmf->vma->vm_file->f_mapping;
         unsigned long pmd_addr = vmf->address & PMD_MASK;
@@ -1336,11 +1246,10 @@ static int dax_pmd_load_hole(struct vm_fault *vmf, struct iomap *iomap,
         if (unlikely(!zero_page))
                 goto fallback;
 
-        ret = dax_insert_mapping_entry(mapping, vmf, *entryp, 0,
-                        RADIX_DAX_PMD | RADIX_DAX_HZP);
+        ret = dax_insert_mapping_entry(mapping, vmf, entry, 0,
+                        RADIX_DAX_PMD | RADIX_DAX_ZERO_PAGE);
         if (IS_ERR(ret))
                 goto fallback;
-        *entryp = ret;
 
         ptl = pmd_lock(vmf->vma->vm_mm, vmf->pmd);
         if (!pmd_none(*(vmf->pmd))) {
@@ -1416,10 +1325,10 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
                 goto fallback;
 
         /*
-         * grab_mapping_entry() will make sure we get a 2M empty entry, a DAX
-         * PMD or a HZP entry.  If it can't (because a 4k page is already in
-         * the tree, for instance), it will return -EEXIST and we just fall
-         * back to 4k entries.
+         * grab_mapping_entry() will make sure we get a 2MiB empty entry, a
+         * 2MiB zero page entry or a DAX PMD.  If it can't (because a 4k page
+         * is already in the tree, for instance), it will return -EEXIST and
+         * we just fall back to 4k entries.
          */
         entry = grab_mapping_entry(mapping, pgoff, RADIX_DAX_PMD);
         if (IS_ERR(entry))
@@ -1452,13 +1361,13 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
 
         switch (iomap.type) {
         case IOMAP_MAPPED:
-                result = dax_pmd_insert_mapping(vmf, &iomap, pos, &entry);
+                result = dax_pmd_insert_mapping(vmf, &iomap, pos, entry);
                 break;
         case IOMAP_UNWRITTEN:
         case IOMAP_HOLE:
                 if (WARN_ON_ONCE(write))
                         break;
-                result = dax_pmd_load_hole(vmf, &iomap, &entry);
+                result = dax_pmd_load_hole(vmf, &iomap, entry);
                 break;
         default:
                 WARN_ON_ONCE(1);
@@ -1481,7 +1390,7 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
                                 &iomap);
         }
  unlock_entry:
-        put_locked_mapping_entry(mapping, pgoff, entry);
+        put_locked_mapping_entry(mapping, pgoff);
  fallback:
         if (result == VM_FAULT_FALLBACK) {
                 split_huge_pmd(vma, vmf->pmd, vmf->address);