Re: [PATCH v7 2/9] dax: fix conversion of holes to PMDs

[Dan Williams]

On Wed, Jan 6, 2016 at 10:00 AM, Ross Zwisler
<ross.zwisler@xxxxxxxxxxxxxxx> wrote:
> When we get a DAX PMD fault for a write it is possible that there could be
> some number of 4k zero pages already present for the same range that were
> inserted to service reads from a hole.  These 4k zero pages need to be
> unmapped from the VMAs and removed from the struct address_space radix tree
> before the real DAX PMD entry can be inserted.
>
> For PTE faults this same use case also exists and is handled by a
> combination of unmap_mapping_range() to unmap the VMAs and
> delete_from_page_cache() to remove the page from the address_space radix
> tree.
>
> For PMD faults we do have a call to unmap_mapping_range() (protected by a
> buffer_new() check), but nothing clears out the radix tree entry.  The
> buffer_new() check is also incorrect as the current ext4 and XFS filesystem
> code will never return a buffer_head with BH_New set, even when allocating
> new blocks over a hole.  Instead the filesystem will zero the blocks
> manually and return a buffer_head with only BH_Mapped set.
>
> Fix this situation by removing the buffer_new() check and adding a call to
> truncate_inode_pages_range() to clear out the radix tree entries before we
> insert the DAX PMD.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>

Replaced the current contents of v6 in -mm from next-20160106 with
this v7 set and it looks good.

Reported-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Tested-by: Dan Williams <dan.j.williams@xxxxxxxxx>

One question below...

> ---
>  fs/dax.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 03cc4a3..9dc0c97 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -594,6 +594,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
>         bool write = flags & FAULT_FLAG_WRITE;
>         struct block_device *bdev;
>         pgoff_t size, pgoff;
> +       loff_t lstart, lend;
>         sector_t block;
>         int result = 0;
>
> @@ -647,15 +648,13 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
>                 goto fallback;
>         }
>
> -       /*
> -        * If we allocated new storage, make sure no process has any
> -        * zero pages covering this hole
> -        */
> -       if (buffer_new(&bh)) {
> -               i_mmap_unlock_read(mapping);
> -               unmap_mapping_range(mapping, pgoff << PAGE_SHIFT, PMD_SIZE, 0);
> -               i_mmap_lock_read(mapping);
> -       }
> +       /* make sure no process has any zero pages covering this hole */
> +       lstart = pgoff << PAGE_SHIFT;
> +       lend = lstart + PMD_SIZE - 1; /* inclusive */
> +       i_mmap_unlock_read(mapping);
> +       unmap_mapping_range(mapping, lstart, PMD_SIZE, 0);
> +       truncate_inode_pages_range(mapping, lstart, lend);

Do we need to do both unmap and truncate given that
truncate_inode_page() optionally does an unmap_mapping_range()
internally?
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