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

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Message-ID: <CAPcyv4gWD7sTLdmwoX7Ce+McMvtLKhOf60e-1ax12qQs7=Qzdg@mail.gmail.com>
Date:   Thu, 24 Aug 2017 23:00:42 -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 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.