linux-kernel - Re: [PATCH v2 0/5] fs, xfs: block map immutable files for dax, dma-to-storage, and swap

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Message-ID: <20170811104429.GA13736@lst.de>
Date:   Fri, 11 Aug 2017 12:44:29 +0200
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@....de>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jan Kara <jack@...e.cz>,
        "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, 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>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 0/5] fs, xfs: block map immutable files for dax,
        dma-to-storage, and swap

On Sun, Aug 06, 2017 at 11:51:50AM -0700, Dan Williams wrote:
> Of course it's a useful API. An application already needs to worry
> about the block map, that's why we have fallocate, msync, fiemap
> and...

Fallocate and msync do not expose the block map in any way.  Proof:
they work just fine over say nfs.

fiemap does indeed expose the block map, which is the whole point.
But it's a debug tool that we don't event have a man page for.  And
it's not usable for anything else, if only for the fact that it doesn't
tell you what device your returned extents are relative to.

> > We've been through this a few times but let me repeat it:  The only
> > sensible API gurantee is one that is observable and usable.
> 
> I'm missing how block-map immutable files violate this observable and
> usable constraint?

What is the observable behavior of an extent map change?  How can you
describe your immutable extent map behavior so that when I violate
them by e.g. moving one extent to a different place on disk you can
observe that in userspace?

> This immutable approach should also go in, it solves the same problem
> without the the latency drawback,

How is your latency going to be any different from MAP_SYNC on
a fully allocated and pre-zeroed file?

> Beyond flush from userspace it also
> can be used to solve the swapfile problems you highlighted

Which swapfile problem?

> and it
> allows safe ongoing dma to a filesystem-dax mapping beyond what we can
> already do with direct-I/O.

Please explain how this interface allows for any sort of safe userspace
DMA.