linux-ext4 - Re: [RFC PATCH 0/7] dax, ext4: Synchronous page faults

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Message-ID: <2ac0880f-d661-2493-0f80-1b930a8b858b@netapp.com>
Date:   Tue, 15 Aug 2017 12:06:02 +0300
From:   Boaz Harrosh <boazh@...app.com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Dave Chinner <david@...morbit.com>,
        <linux-xfs@...r.kernel.org>, Andy Lutomirski <luto@...nel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        Amit Golander <amitg@...app.com>
Subject: Re: [RFC PATCH 0/7] dax, ext4: Synchronous page faults

On 14/08/17 19:03, Dan Williams wrote:
> On Mon, Aug 14, 2017 at 7:04 AM, Boaz Harrosh <boazh@...app.com> wrote:
>>
>> Thank you Jan, I'm patiently waiting for this MAP_SYNC flag since I asked for
>> it in 2014. I'm so glad its time is finally do.
>>

<>

>> 4] Once we have this flag. And properly implemented at least in one FS
>>    and optionally in /dev/pmemX we no longer have any justification for
>>    /dev/daxX and it can die a slow and happy death.
> 
> I'm all for replacing /dev/dax with filesystem equivalent
> functionality, but I don't think MAP_SYNC gets us fully there. That's
> what the MAP_DIRECT proposal [1] is meant to address.
> 

OK This is true. Could you please summarise for us the exact semantics of both
proposed flags?

That said I think the big difference is the movability of physical blocks
underneath the mmap mapping. Now for swap files that is a problem. because
of the deadlocks that can happen with memory needed if blocks start moving.
But for an application like nvml? why does it care. Why can't an nvml image file
not be cloned and COWed underneath the NVM application transparently.

Sorry for being slow but I don't see why you need MAP_DIRECT from user-mode
If you have MAP_SYNC. Please advise

(not that the immutable patchset is not a very needed fixing)

> [1]: https://lkml.org/lkml/2017/8/13/160
> 

Thanks
Boaz