linux-ext4 - Re: [PATCH 16/18] dax: New fault locking

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Date:	Wed, 27 Apr 2016 14:27:59 +1000
From:	NeilBrown <nfbrown@...ell.com>
To:	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org
Cc:	linux-ext4@...r.kernel.org, linux-mm@...ck.org,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-nvdimm@...ts.01.org, Matthew Wilcox <willy@...ux.intel.com>,
	Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 16/18] dax: New fault locking

On Tue, Apr 19 2016, 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.
>
> Signed-off-by: Jan Kara <jack@...e.cz>

Reviewed-by: NeilBrown <neilb@...e.com> (for the exception locking bits)

I really like how you have structured the code - makes it fairly
obviously correct!

Thanks,
NeilBrown

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