linux-kernel - Re: [RFC PATCH 2/2] mm, fs: daxfile, an interface for byte-addressable updates to pmem

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Date:   Tue, 20 Jun 2017 09:26:59 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@....de>, Andy Lutomirski <luto@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "Rudoff, Andy" <andy.rudoff@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Jeff Moyer <jmoyer@...hat.com>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC PATCH 2/2] mm, fs: daxfile, an interface for
 byte-addressable updates to pmem

On Tue, Jun 20, 2017 at 9:17 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Tue, Jun 20, 2017 at 1:49 AM, Christoph Hellwig <hch@....de> wrote:
>> [stripped giant fullquotes]
>>
>> On Mon, Jun 19, 2017 at 10:53:12PM -0700, Andy Lutomirski wrote:
>>> But that's my whole point.  The kernel doesn't really need to prevent
>>> all these background maintenance operations -- it just needs to block
>>> .page_mkwrite until they are synced.  I think that whatever new
>>> mechanism we add for this should be sticky, but I see no reason why
>>> the filesystem should have to block reflink on a DAX file entirely.
>>
>> Agreed - IFF we want to support write through semantics this is the
>> only somewhat feasible way.  It still has massive downsides of forcing
>> the full sync machinery to run from the page fauly handler, which
>> I'm rather scared off, but that's still better than creating a magic
>> special case that isn't managable at all.
>
> An immutable-extent DAX-file and a reflink-capable DAX-file are not
> mutually exclusive, and I have yet to hear a need for reflink support
> without fsync/msync. Instead I have heard the need for an immutable
> file for RDMA purposes, especially for hardware that can't trigger an
> mmu fault. The special management of an immutable file is acceptable
> to get these capabilities.

I guess this applies to any user of get_user_pages() on a DAX-mapped file.  Hmm.