linux-kernel - Re: what's in nvdimm.git for v4.4?

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Date:	Wed, 21 Oct 2015 15:50:02 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	"Elliott, Robert (Persistent Memory)" <elliott@....com>
Cc:	Jan Kara <jack@...e.cz>,
	"martin.petersen@...cle.com" <martin.petersen@...cle.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
	Dave Chinner <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"axboe@...com" <axboe@...com>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"hch@....de" <hch@....de>
Subject: Re: what's in nvdimm.git for v4.4?

On Wed, Oct 21, 2015 at 3:36 PM, Elliott, Robert (Persistent Memory)
<elliott@....com> wrote:
>
>
>> -----Original Message-----
>> From: Linux-nvdimm [mailto:linux-nvdimm-bounces@...ts.01.org] On Behalf Of
>> Jan Kara
>> Sent: Wednesday, October 21, 2015 4:08 AM
> ...
>> On Tue 20-10-15 17:31:18, Dan Williams wrote:
>> > On Tue, Oct 20, 2015 at 5:01 PM, Dave Chinner <david@...morbit.com>
> ...
>> > I'm wondering if we can take a conservative step forward with those
>> > patches for 4.4.  if XFS and EXT4 interactions need more time to get
>> > worked out, which I believe they do, I can conceive just turning on
>> > get_user_pages() support for DAX-mappings of the raw block device.
>> > This would be via the new facility I posted yesterday:
>> > https://lists.01.org/pipermail/linux-nvdimm/2015-October/002512.html.
>> > While not very functional for applications it makes testing base DAX
>> > mechanisms straightforward.
>>
>> I had a look at the patch and I miss one thing: Why do we need bd_mutex
>> to protect faults? I see a comment there:
>>
>> /* check that the faulting page hasn't raced with bdev resize */
>>
>> Is it really possible that bdev gets shrunk under us? Hum, looking into
>> fs/block_dev.c, probably it is. But there are other places - like DIO path
>> - assuming that block device mapping cannot just disappear from under us.
>> I wonder how that would cope with bdev size change...
>
> DIO_IGNORE_TRUNCATE was added to eliminate an awful lot of CPU
> time incrementing and decrementing i_dio_count for direct IO
> to block devices, especially at pmem type speeds.  I hope
> that kind of accounting doesn't need to be brought back.
>
> https://lkml.org/lkml/2015/4/3/557
> https://lkml.org/lkml/2015/4/15/590

Thanks for that reminder Robert.  I think this backs up my suspicion
that the base mmap_sem protections should be enough for raw block mmap
+ DAX.
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