linux-kernel - Re: [PATCH v6 2/5] fs, xfs: introduce S_IOMAP

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Date:   Fri, 25 Aug 2017 12:44:29 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Dave Chinner <david@...morbit.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Jeff Moyer <jmoyer@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH v6 2/5] fs, xfs: introduce S_IOMAP_SEALED

On Thu, Aug 24, 2017 at 11:00 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Thu, Aug 24, 2017 at 9:13 AM, Christoph Hellwig <hch@....de> wrote:
>> I'm still very unhappy about the get/set flag state.  What is the
>> reason you can't use/extend leases? (take a look at the fcntl
>> man page and look for Leases).  A variant of the concept is what
>> the pNFS block server uses.
>
> So I think leases could potentially be extended to replace the inode
> flag. A MAP_DIRECT operation would take out a lease that is broken by
> break_layouts(). However, like the pNFS case the lease break would
> need to held off while any DMA might be in-flight. We can use an
> elevated page count as that indication as ZONE_DEVICE pages only ever
> have an elevated page count in response to get_user_pages().
>
> However, I think the only practical difference is turning an immediate
> ETXTBSY response that S_IOMAP_SEALED provides into an indefinite
> blocking wait for break_layouts() to complete. Can pNFS run
> break_layouts() in bounded time?
>
> As far I can see a lease and S_IOMAP_SEALED have the same DMA
> cancelling problem, so a lease is not better in that regard. Absent an
> overlaying protocol like pNFS, I think S_IOMAP_SEALED is cleaner
> because it fails incompatible operations outright rather than stalls
> them in break_layouts(). Were their other benefits to a lease over an
> inode flag that you had in mind for this case where the protocol is
> userspace defined? Maybe I'm thinking too small on the ways a lease
> might be extended.

At a minimum I can at least use a new lease type as an indication of
when to bail out an block-map operation with ETXTBSY, and reuse the
lease security model. That way we at least start to converge the
in-kernel lease machinery for pinning blocks with this userspace
mechanism.