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

From: Dan Williams
Date: Thu Sep 24 2015 - 20:03:24 EST

On Thu, Sep 24, 2015 at 8:15 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> 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)?

That makes sense to me, especially because drivers/nvdimm/blk.c is
broken in the same way as drivers/nvdimm/pmem.c and it would be
awkward to have it use blk_dax_get() / blk_dax_put(). The
percpu_refcount should be valid for all queues and it will only ever
be > 1 in the blk_mq and libnvdimm cases (for now). Will fix.
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