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

From: Christoph Hellwig
Date: Thu Sep 24 2015 - 11:15:10 EST

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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