Re: [PATCH v2 5/5] dax: fix clearing of holes in __dax_pmd_fault()

[Ross Zwisler]

On Fri, Jan 22, 2016 at 04:37:19PM +0100, Jan Kara wrote:
> On Thu 21-01-16 10:46:04, Ross Zwisler wrote:
> > When the user reads from a DAX hole via a mmap we service page faults using
> > zero-filled page cache pages.  These zero pages are also placed into the
> > address_space radix tree.  When we get our first write for that space, we
> > can allocate a PMD page worth of DAX storage to replace that hole.
> > 
> > When this happens we need to unmap the zero pages and remove them from the
> > radix tree.  Prior to this patch we were unmapping *all* storage in our
> > PMD's range, which is incorrect because it removed DAX entries as well on
> > non-allocating page faults.
> > 
> > Instead, keep track of when get_block() actually gives us storage so that
> > we can be sure to only remove zero pages that were covering holes.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> > Reported-by: Jan Kara <jack@xxxxxxx>
> 
> So the get_block() calls below are still racy so your allocation detection
> doesn't quite work reliably anyway. For that we need a change in the fault
> locking we were talking about and at that point we'll need to somewhat
> update __dax_pmd_fault() anyway. So I'm not sure if this intermediate
> change really makes sense... I'd rather first change the locking in the
> filesystems and then fixup remaining issues with __dax_pmd_fault().

Agreed, the fault isolation code that we talked about previously is still
needed.  I've got an initial version coded up that I will send out as an RFC
today.  Those patches build directly on this patch.

My thought behind sending this was that I worried that the fault isolation
code wouldn't make it into v4.5, as we're already in the merge window.  That
code by design has to touch both DAX and all the DAX filesystems (ext4, ext2
and XFS).  Although, I guess a race that could result in data corruption
probably warrants an -RC timed fix?

In any case, one way or another we really need this fix for v4.5.  Without it
each PMD fault we receive will unmap the PMD range - even if that range is in
use by other threads. So, you can get into the following situation:

Thread 1				Thread 2
--------				--------
PMD fault
  unmap range for all threads
  map range for just this thread
					PMD fault
					  unmap range for all threads
					  map range for just this thread
PMD fault
  unmap range for all threads
  map range for just this thread
	  				...

Where two threads that are doing I/O to the same PMD will thrash and cause
every access to trigger a PMD page fault.

> > ---
> >  fs/dax.c | 32 ++++++++++++++++++++++----------
> >  1 file changed, 22 insertions(+), 10 deletions(-)
> > 
> > diff --git a/fs/dax.c b/fs/dax.c
> > index afacc30..263aed1 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -790,9 +790,9 @@ 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 error, result = 0;
> > +	bool alloc = false;
> >  
> >  	/* dax pmd mappings require pfn_t_devmap() */
> >  	if (!IS_ENABLED(CONFIG_FS_DAX_PMD))
> > @@ -830,10 +830,17 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> >  	block = (sector_t)pgoff << (PAGE_SHIFT - blkbits);
> >  
> >  	bh.b_size = PMD_SIZE;
> > -	if (get_block(inode, block, &bh, write) != 0)
> > +
> > +	if (get_block(inode, block, &bh, 0) != 0)
> >  		return VM_FAULT_SIGBUS;
> > +
> > +	if (!buffer_mapped(&bh) && write) {
> > +		if (get_block(inode, block, &bh, 1) != 0)
> > +			return VM_FAULT_SIGBUS;
> > +		alloc = true;
> > +	}
> > +
> >  	bdev = bh.b_bdev;
> > -	i_mmap_lock_read(mapping);
> >  
> >  	/*
> >  	 * If the filesystem isn't willing to tell us the length of a hole,
> > @@ -842,15 +849,20 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> >  	 */
> >  	if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE) {
> >  		dax_pmd_dbg(&bh, address, "allocated block too small");
> > -		goto fallback;
> > +		return VM_FAULT_FALLBACK;
> > +	}
> > +
> > +	/*
> > +	 * If we allocated new storage, make sure no process has any
> > +	 * zero pages covering this hole
> > +	 */
> > +	if (alloc) {
> > +		loff_t lstart = pgoff << PAGE_SHIFT;
> > +		loff_t lend = lstart + PMD_SIZE - 1; /* inclusive */
> > +
> > +		truncate_pagecache_range(inode, lstart, lend);
> >  	}
> >  
> > -	/* 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);
> >  	i_mmap_lock_read(mapping);
> >  
> >  	/*
> > -- 
> > 2.5.0
> > 
> > 
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR