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

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Date:   Fri, 11 Aug 2017 20:57:18 -0700
From:   Andy Lutomirski <luto@...nel.org>
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 Fri, Aug 11, 2017 at 3:26 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Fri, Aug 11, 2017 at 3:44 AM, Christoph Hellwig <hch@....de> wrote:
>> Please explain how this interface allows for any sort of safe userspace
>> DMA.
>
> So this is where I continue to see S_IOMAP_IMMUTABLE being able to
> support applications that MAP_SYNC does not. Dave mentioned userspace
> pNFS4 servers, but there's also Samba and other protocols that want to
> negotiate a direct path to pmem outside the kernel. Xen support has
> thus far not been able to follow in the footsteps of KVM enabling due
> to a dependence on static M2P tables that assume a static
> guest-physical to host-physical relationship [1]. Immutable files
> would allow Xen to follow the same "mmap a file" semantic as KVM.

One thing that makes me quite nervous about S_IOMAP_IMMUTABLE is the
degree to which things go badly if one program relies on it while
another program clears the flag: you risk corrupting unrelated
filesystem metadata.  I think a userspace interface to pin the extent
mapping of a file really wants a way to reliably keep it pinned (or to
reliably zap the userspace application if it gets unpinned).