linux-kernel - Re: [PATCH 08/15] block, dax, pmem: reference counting infrastructure

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Message-ID: <20150924151503.GF24375@infradead.org>
Date:	Thu, 24 Sep 2015 08:15:03 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	akpm@...ux-foundation.org, Jens Axboe <axboe@...nel.dk>,
	linux-nvdimm@...1.01.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH 08/15] block, dax, pmem: reference counting infrastructure

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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