Re: [PATCH v6 4/7] dax: add support for fsync/msync

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Tue, Jan 05, 2016 at 12:13:58PM +0100, Jan Kara wrote:
> On Wed 23-12-15 12:39:17, Ross Zwisler 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 int dax_writeback_one(struct block_device *bdev,
> > +		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;
> > +	struct blk_dax_ctl dax;
> > +	void **slot;
> > +	int ret = 0;
> > +
> > +	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 (WARN_ON_ONCE(type != RADIX_DAX_PTE && type != RADIX_DAX_PMD)) {
> > +		ret = -EIO;
> > +		goto unlock;
> > +	}
> > +
> > +	dax.sector = RADIX_DAX_SECTOR(entry);
> > +	dax.size = (type == RADIX_DAX_PMD ? PMD_SIZE : PAGE_SIZE);
> > +	spin_unlock_irq(&mapping->tree_lock);
> > +
> > +	/*
> > +	 * We cannot hold tree_lock while calling dax_map_atomic() because it
> > +	 * eventually calls cond_resched().
> > +	 */
> > +	ret = dax_map_atomic(bdev, &dax);
> > +	if (ret < 0)
> > +		return ret;
> > +
> > +	if (WARN_ON_ONCE(ret < dax.size)) {
> > +		ret = -EIO;
> > +		dax_unmap_atomic(bdev, &dax);
> > +		return ret;
> > +	}
> > +
> > +	spin_lock_irq(&mapping->tree_lock);
> > +	/*
> > +	 * We need to revalidate our radix entry while holding tree_lock
> > +	 * before we do the writeback.
> > +	 */
> 
> Do we really need to revalidate here? dax_map_atomic() makes sure the addr
> & size is still part of the device. I guess you are concerned that due to
> truncate or similar operation those sectors needn't belong to the same file
> anymore but we don't really care about flushing sectors for someone else,
> do we?
> 
> Otherwise the patch looks good to me.

Yep, the concern is that we could have somehow raced against a truncate
operation while we weren't holding the tree_lock, and that now the address we
are about to flush belongs to another file or is unallocated by the
filesystem.

I agree that this should be non-destructive - if you think the additional
check and locking isn't worth the overhead, I'm happy to take it out.  I don't
have a strong opinion either way.

Thanks for the review!

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