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Date: Thu, 24 Sep 2015 08:15:03 -0700
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 08/15] block, dax, pmem: reference counting infrastructure
Message-ID: <20150924151503.GF24375@infradead.org>
References: <20150923043737.36490.70547.stgit@dwillia2-desk3.jf.intel.com>
 <20150923044155.36490.2017.stgit@dwillia2-desk3.jf.intel.com>
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To: Dan Williams <dan.j.williams@intel.com>
Cc: akpm@linux-foundation.org, Jens Axboe <axboe@kernel.dk>, linux-nvdimm@ml01.01.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Ross Zwisler <ross.zwisler@linux.intel.com>

On Wed, Sep 23, 2015 at 12:41:55AM -0400, Dan Williams wrote:
> Enable DAX to use a reference count for keeping the virtual address
> returned by ->direct_access() valid for the duration of its usage in
> fs/dax.c, or otherwise hold off blk_cleanup_queue() while
> pmem_make_request is active.  The blk-mq code is already in a position
> to need low overhead referece counting for races against request_queue
> destruction (blk_cleanup_queue()).  Given DAX-enabled block drivers do
> not enable blk-mq, share the storage in 'struct request_queue' between
> the two implementations.

Can we just move the refcounting to common code with the same field
name, and even initialize it for non-mq, non-dax queues but just never
tage a reference there (for now)?

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