linux-kernel - Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings

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Date:   Mon, 9 Apr 2018 09:51:33 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Jeff Moyer <jmoyer@...hat.com>,
        Dave Chinner <david@...morbit.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mike Snitzer <snitzer@...hat.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh.triplett@...el.com>
Subject: Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings

On Mon, Apr 9, 2018 at 9:49 AM, Jan Kara <jack@...e.cz> wrote:
> On Sat 07-04-18 12:38:24, Dan Williams wrote:
[..]
>> I wonder if this can be trivially solved by using srcu. I.e. we don't
>> need to wait for a global quiescent state, just a
>> get_user_pages_fast() quiescent state. ...or is that an abuse of the
>> srcu api?
>
> Well, I'd rather use the percpu rwsemaphore (linux/percpu-rwsem.h) than
> SRCU. It is a more-or-less standard locking mechanism rather than relying
> on implementation properties of SRCU which is a data structure protection
> method. And the overhead of percpu rwsemaphore for your use case should be
> about the same as that of SRCU.

I was just about to ask that. Yes, it seems they would share similar
properties and it would be better to use the explicit implementation
rather than a side effect of srcu.