linux-kernel - Re: [PATCH v2 0/2] Revert locking changes in DAX for v4.3

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Date:	Sat, 3 Oct 2015 23:24:33 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-nvdimm@...ts.01.org
Subject: Re: [PATCH v2 0/2] Revert locking changes in DAX for v4.3

On Fri, Oct 02, 2015 at 06:01:32PM -0600, Ross Zwisler wrote:
> This series reverts some recent changes to the locking scheme in DAX introduced
> by these two commits:
> 
> commit 843172978bb9 ("dax: fix race between simultaneous faults")
> commit 46c043ede471 ("mm: take i_mmap_lock in unmap_mapping_range() for DAX")
> 
> Changes from v1:
>  -  Squashed patches 1 and 2 from the first series into a single patch to avoid
>     adding another spot in the git history where we could end up referencing an
>     uninitialized pointer.
> 
> Ross Zwisler (2):
>   Revert "mm: take i_mmap_lock in unmap_mapping_range() for DAX"
>   Revert "dax: fix race between simultaneous faults"
> 
>  fs/dax.c    | 83 +++++++++++++++++++++++++------------------------------------
>  mm/memory.c |  2 ++
>  2 files changed, 36 insertions(+), 49 deletions(-)
> 
> -- 
> 2.1.0

*sigh* - even after these reverts we can deadlock on in the DAX PMD code with
its original locking scheme.  I can hit them 100% of the time with either
generic/074 or generic/198 using either XFS or ext4.  I'll debug exactly
what's going on on Monday.

The quick and easy workaround for this is to do a "return VM_FAULT_FALLBACK;"
at the beginning of __dax_pmd_fault() to just turn off PMD faults while we
rework the locking for v4.4.  This saves us reverting and re-adding all the
PMD code, and will let us ship v4.3 without known deadlocks.

Other better ideas?

- Ross
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