Re: [PATCH v5 4/7] dax: add support for fsync/sync

[Dan Williams <dan.j.williams@intel.com>]

On Fri, Dec 18, 2015 at 9:22 PM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> To properly handle fsync/msync in an efficient way DAX needs to track dirty
> pages so it is able to flush them durably to media on demand.
>
> The tracking of dirty pages is done via the radix tree in struct
> address_space.  This radix tree is already used by the page writeback
> infrastructure for tracking dirty pages associated with an open file, and
> it already has support for exceptional (non struct page*) entries.  We
> build upon these features to add exceptional entries to the radix tree for
> DAX dirty PMD or PTE pages at fault time.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
[..]
> +static void dax_writeback_one(struct address_space *mapping, pgoff_t index,
> +               void *entry)
> +{
> +       struct radix_tree_root *page_tree = &mapping->page_tree;
> +       int type = RADIX_DAX_TYPE(entry);
> +       struct radix_tree_node *node;
> +       void **slot;
> +
> +       if (type != RADIX_DAX_PTE && type != RADIX_DAX_PMD) {
> +               WARN_ON_ONCE(1);
> +               return;
> +       }
> +
> +       spin_lock_irq(&mapping->tree_lock);
> +       /*
> +        * Regular page slots are stabilized by the page lock even
> +        * without the tree itself locked.  These unlocked entries
> +        * need verification under the tree lock.
> +        */
> +       if (!__radix_tree_lookup(page_tree, index, &node, &slot))
> +               goto unlock;
> +       if (*slot != entry)
> +               goto unlock;
> +
> +       /* another fsync thread may have already written back this entry */
> +       if (!radix_tree_tag_get(page_tree, index, PAGECACHE_TAG_TOWRITE))
> +               goto unlock;
> +
> +       radix_tree_tag_clear(page_tree, index, PAGECACHE_TAG_TOWRITE);
> +
> +       if (type == RADIX_DAX_PMD)
> +               wb_cache_pmem(RADIX_DAX_ADDR(entry), PMD_SIZE);
> +       else
> +               wb_cache_pmem(RADIX_DAX_ADDR(entry), PAGE_SIZE);

Hi Ross, I should have realized this sooner, but what guarantees that
the address returned by RADIX_DAX_ADDR(entry) is still valid at this
point?  I think we need to store the sector in the radix tree and then
perform a new dax_map_atomic() operation to either lookup a valid
address or fail the sync request.  Otherwise, if the device is gone
we'll crash, or write into some other random vmalloc address space.

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