linux-ext4 - Re: [PATCH 5/7] dax: New fault locking

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Date:	Thu, 12 May 2016 13:36:24 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-nvdimm@...ts.01.org, Ted Tso <tytso@....edu>,
	linux-ext4@...r.kernel.org,
	Vishal Verma <vishal.l.verma@...el.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 5/7] dax: New fault locking

On Thu, May 12, 2016 at 06:29:18PM +0200, Jan Kara wrote:
> Currently DAX page fault locking is racy.
> 
> CPU0 (write fault)		CPU1 (read fault)
> 
> __dax_fault()			__dax_fault()
>   get_block(inode, block, &bh, 0) -> not mapped
> 				  get_block(inode, block, &bh, 0)
> 				    -> not mapped
>   if (!buffer_mapped(&bh))
>     if (vmf->flags & FAULT_FLAG_WRITE)
>       get_block(inode, block, &bh, 1) -> allocates blocks
>   if (page) -> no
> 				  if (!buffer_mapped(&bh))
> 				    if (vmf->flags & FAULT_FLAG_WRITE) {
> 				    } else {
> 				      dax_load_hole();
> 				    }
>   dax_insert_mapping()
> 
> And we are in a situation where we fail in dax_radix_entry() with -EIO.
> 
> Another problem with the current DAX page fault locking is that there is
> no race-free way to clear dirty tag in the radix tree. We can always
> end up with clean radix tree and dirty data in CPU cache.
> 
> We fix the first problem by introducing locking of exceptional radix
> tree entries in DAX mappings acting very similarly to page lock and thus
> synchronizing properly faults against the same mapping index. The same
> lock can later be used to avoid races when clearing radix tree dirty
> tag.
> 
> Reviewed-by: NeilBrown <neilb@...e.com>
> Reviewed-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
<>
> @@ -897,13 +1166,10 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
>  		 * the write to insert a dirty entry.
>  		 */
>  		if (write) {
> -			error = dax_radix_entry(mapping, pgoff, dax.sector,
> -					true, true);
> -			if (error) {
> -				dax_pmd_dbg(&bh, address,
> -						"PMD radix insertion failed");
> -				goto fallback;
> -			}
> +			/*
> +			 * We should insert radix-tree entry and dirty it here.
> +			 * For now this is broken...
> +			 */

With this change the 'error' variable in __dax_pmd_fault() is now unused,
resulting in a compiler warning.

fs/dax.c: In function ‘__dax_pmd_fault’:
fs/dax.c:1019:6: warning: unused variable ‘error’ [-Wunused-variable]
  int error, result = 0;
        ^

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