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

[Andy Lutomirski]

On Tue, Jun 20, 2017 at 9:17 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Tue, Jun 20, 2017 at 1:49 AM, Christoph Hellwig <hch@xxxxxx> 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.