Re: [PATCH v8 2/9] dax: fix conversion of holes to PMDs

[Ross Zwisler]

On Tue, Jan 12, 2016 at 10:44:51AM +0100, Jan Kara wrote:
> On Thu 07-01-16 22:27:52, Ross Zwisler 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>
> > Reported-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Tested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> 
> Just two nits below. Nothing serious so you can add:
> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>

Cool, thank you for the review!

> > ---
> >  fs/dax.c | 20 ++++++++++----------
> >  1 file changed, 10 insertions(+), 10 deletions(-)
> > 
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 513bba5..5b84a46 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -589,6 +589,7 @@ 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;
> >  
> > @@ -643,15 +644,13 @@ 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);
> 
> Just a nit but is there reason why we grab i_mmap_lock_read(mapping) only
> to release it a few lines below? The bh checks inside the locked region
> don't seem to rely on i_mmap_lock...

I think we can probably just take it when we're done with the truncate() -
I'll fix for v9.

> > +	unmap_mapping_range(mapping, lstart, PMD_SIZE, 0);
> > +	truncate_inode_pages_range(mapping, lstart, lend);
> 
> These two calls can be shortened as:
> 
> truncate_pagecache_range(inode, lstart, lend);

Nice.  I'll change it for v9.