Re: [PATCH v2 4/5] dax: fix PMD handling for fsync/msync

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

On Fri, Jan 22, 2016 at 04:11:41PM +0100, Jan Kara wrote:
> On Thu 21-01-16 10:46:03, Ross Zwisler wrote:
> > Fix the way that DAX PMD radix tree entries are handled.  With this patch
> > we now check to see if a PMD entry exists in the radix tree on write, even
> > if we are just trying to insert a PTE.  If it exists, we dirty that instead
> > of inserting our own PTE entry.
> > 
> > Fix a bug in the PMD path in dax_writeback_mapping_range() where we were
> > previously passing a loff_t into radix_tree_lookup instead of a pgoff_t.
> 
> Ah, good catch!
> 
> > Account for the fact that multiple fsync/msync operations may be happening
> > at the same time and don't flush entries that are beyond end_index.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> 
> Just one nit below. You can add:
> 
> Reviewed-by: Jan Kara <jack@suse.cz>
> 
> > ---
> >  fs/dax.c | 39 +++++++++++++++++++++++++++------------
> >  1 file changed, 27 insertions(+), 12 deletions(-)
> > 
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 55ae394..afacc30 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> ...
> > @@ -460,31 +468,33 @@ int dax_writeback_mapping_range(struct address_space *mapping, loff_t start,
> >  {
> >  	struct inode *inode = mapping->host;
> >  	struct block_device *bdev = inode->i_sb->s_bdev;
> > +	pgoff_t start_index, end_index, pmd_index;
> >  	pgoff_t indices[PAGEVEC_SIZE];
> > -	pgoff_t start_page, end_page;
> >  	struct pagevec pvec;
> > -	void *entry;
> > +	bool done = false;
> >  	int i, ret = 0;
> > +	void *entry;
> >  
> >  	if (WARN_ON_ONCE(inode->i_blkbits != PAGE_SHIFT))
> >  		return -EIO;
> >  
> > +	start_index = start >> PAGE_CACHE_SHIFT;
> > +	end_index = end >> PAGE_CACHE_SHIFT;
> > +	pmd_index = DAX_PMD_INDEX(start_index);
> > +
> >  	rcu_read_lock();
> > -	entry = radix_tree_lookup(&mapping->page_tree, start & PMD_MASK);
> > +	entry = radix_tree_lookup(&mapping->page_tree, pmd_index);
> >  	rcu_read_unlock();
> >  
> >  	/* see if the start of our range is covered by a PMD entry */
> > -	if (entry && RADIX_DAX_TYPE(entry) == RADIX_DAX_PMD)
> > -		start &= PMD_MASK;
> > -
> > -	start_page = start >> PAGE_CACHE_SHIFT;
> > -	end_page = end >> PAGE_CACHE_SHIFT;
> > +	if (RADIX_DAX_TYPE(entry) == RADIX_DAX_PMD)
> 
> Don't you miss a check that entry != NULL? I agree that RADIX_DAX_TYPE(NULL)
> is != from RADIX_DAX_PMD so it works as desired but it looks a bit
> dangerous.

Yea, the initial version had a NULL check first, but I realized I could
optimize it out and make things simpler since RADIX_DAX_TYPE() just looked at
the value of 'entry' and never treated it like a pointer (because to us, it
isn't a pointer).  I will add it back if you think that not having it may
confuse the reader.