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

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Date:   Thu, 22 Jun 2017 20:07:59 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Dan Williams <dan.j.williams@...el.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>,
        Christoph Hellwig <hch@....de>
Subject: Re: [RFC PATCH 2/2] mm, fs: daxfile, an interface for
 byte-addressable updates to pmem

On Thu, Jun 22, 2017 at 5:52 PM, Dave Chinner <david@...morbit.com> wrote:
> On Wed, Jun 21, 2017 at 09:07:57PM -0700, Andy Lutomirski wrote:
>> On Wed, Jun 21, 2017 at 5:02 PM, Dave Chinner <david@...morbit.com> wrote:
>> >
>> > You seem to be calling the "fdatasync on every page fault" the
>>
>> It's the opposite of fdatasync().  It needs to sync whatever metadata
>> is needed to find the data.  The data doesn't need to be synced.
>
> So much wrong with that statement.
>
> Andy, what does fdatasync() do when you have a data-clean,
> metadata-dirty file (e.g. you just punched a hole  or preallocated
> more space via fallocate())?  Hint: it doesn't sync any data
> because the mapping tree is clean, but it still syncs the dirty
> metadata needed to access the data.
>
> Now, what does a file where we do direct IO writes look like? Yup,
> the mapping tree always remains clean and so it's only ever going to
> appear to the kernel as a *data-clean, metadata-dirty* file. So,
> after a direct IO write is done, what operation do we need to run to
> ensure that we can always access the data?
>
> Yup, it's fdatasync().

Fair enough.  Except that fdatasync() goes through dax_writeback_one()
(I think), which deals with cache flushes (via wb_cache_pmem()).  This
special type of sync shouldn't need to do that, so it's not really
quite fdatasync().

>> > "lightweight" option. That's the brute-force-with-big-hammer
>> > solution - it's most definitely not lightweight as every page fault
>> > has extra overhead to call ->fsync(). Sure, the API is simple, but
>> > the runtime overhead is significant.
>>
>> It's lightweight in terms of its impact on the filesystem.  It doesn't
>> need any persistent setup -- you can just use it.
>
> Well, no, that's wrong, because we have to co-ordinate multiple
> concurrent accesses to the data in the kernel. What happens when
> some other process writes to the file *at the same time* but does
> not use userspace sync? We aren't tracking dirty regions on the
> inode mapping because we've been told not to do that, so fsync()
> from that other process *won't sync the data it wrote*. IOws, the
> kernel has failed to provide the guarantee that userspace wants it
> to provide.

...

> What I'd like to avoid is creating another kernel bypass mechanism
> where we allow coherency and/or integrity to be fucked up in a way that
> we can't fix without giving up all the performance that the kernel
> bypass provides userspace apps. Constrain the cases where kernel
> bypass is allowed, and we avoid all the crappy corner cases where
> our only answer to users with corrupt data is "the man page advises
> application developers not to do that".

Ah, I see, a DAX file makes regular write() flush out the cache
automatically.  But I think the situation may be fucked up
integrity-wise anyway.  If you make an immutable-extent DAX file and a
DAX-unaware process mmaps() it and writes to the mapping, what flushes
the CPU cache?  Isn't part of the point of the magic immutable-extent
mode that it wouldn't have to track dirty extents?  Can it keep track
of which mappings are DAX-aware (via an mmap() flag, I assume)?  Would
all mappings of a DAX immutable-extent file be forced to be uncached
(or writethrough or WC or some type that allows fsync to be fast)?

Can you send a link to your fallocate email?  I'm having trouble finding it.