linux-kernel - Re: [PATCH v7 1/8] dax: Use rwsem for dax_{read,write}

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Date:   Thu, 14 Oct 2021 23:30:03 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        nvdimm@...ts.linux.dev, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, djwong@...nel.org,
        dan.j.williams@...el.com, david@...morbit.com, hch@...radead.org,
        jane.chu@...cle.com
Subject: Re: [PATCH v7 1/8] dax: Use rwsem for dax_{read,write}_lock()

On Fri, Sep 24, 2021 at 09:09:52PM +0800, Shiyang Ruan wrote:
> In order to introduce dax holder registration, we need a write lock for
> dax.  Because of the rarity of notification failures and the infrequency
> of registration events, it would be better to be a global lock rather
> than per-device.  So, change the current lock to rwsem and introduce a
> write lock for registration.

I don't think taking the rw_semaphore everywhere will scale, as
basically any DAX based I/O will take it (in read mode).

So at a minimum we'd need a per-device percpu_rw_semaphore if we want
to go there.