From: Dan Williams <dan.j.williams@intel.com>
To: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Christoph Hellwig <hch@infradead.org>, Jan Kara <jack@suse.cz>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Linux MM <linux-mm@kvack.org>,
linux-nvdimm <linux-nvdimm@lists.01.org>,
Michal Hocko <mhocko@suse.cz>
Subject: Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps
Date: Wed, 17 Oct 2018 13:01:15 -0700 [thread overview]
Message-ID: <CAPcyv4jt_w-89+m4w=FcN0oF3axiGqPBTHfEcWwdhnr12_=17Q@mail.gmail.com> (raw)
In-Reply-To: <CAPcyv4i7WJsq3BMASozjjbpMmEiS4AqmRS0kt3=rHdGfb5YvLA@mail.gmail.com>
On Sun, Oct 14, 2018 at 8:47 AM Dan Williams <dan.j.williams@intel.com> wrote:
>
> On Fri, Oct 5, 2018 at 6:17 PM Dan Williams <dan.j.williams@intel.com> wrote:
> >
> > On Thu, Oct 4, 2018 at 11:35 PM Johannes Thumshirn <jthumshirn@suse.de> wrote:
> > >
> > > On Thu, Oct 04, 2018 at 11:25:24PM -0700, Christoph Hellwig wrote:
> > > > Since when is an article on some website a promise (of what exactly)
> > > > by linux kernel developers?
> > >
> > > Let's stop it here, this doesn't make any sort of forward progress.
> > >
> >
> > I do think there is some progress we can make if we separate DAX as an
> > access mechanism vs DAX as a resource utilization contract. My attempt
> > at representing Christoph's position is that the kernel should not be
> > advertising / making access mechanism guarantees. That makes sense.
> > Even with MAP_SYNC+DAX the kernel reserves the right to write-protect
> > mappings at will and trap access into a kernel handler. Additionally,
> > whether read(2) / write(2) does anything different behind the scenes
> > in DAX mode, or not should be irrelevant to the application.
> >
> > That said what is certainly not irrelevant is a kernel giving
> > userspace visibility and control into resource utilization. Jan's
> > MADV_DIRECT_ACCESS let's the application make assumptions about page
> > cache utilization, we just need to another mechanism to read if a
> > mapping is effectively already in that state.
>
> I thought more about this today while reviewing the virtio-pmem driver
> that will behave mostly like a DAX-capable pmem device except it will
> be implemented by passing host page cache through to the guest as a
> pmem device with a paravirtualized / asynchronous flush interface.
> MAP_SYNC obviously needs to be disabled for this case, but still need
> allow to some semblance of DAX operation to save allocating page cache
> in the guest. The need to explicitly clarify the state of DAX is
> growing with the different nuances of DAX operation.
>
> Lets use a new MAP_DIRECT flag to positively assert that a given
> mmap() call is setting up a memory mapping without page-cache or
> buffered indirection. To be clear not my original MAP_DIRECT proposal
> from a while back, instead just a flag to mmap() that causes the
> mapping attempt to fail if there is any software buffering fronting
> the memory mapping, or any requirement for software to manage flushing
> outside of pushing writes through the cpu cache. This way, if we ever
> extend MAP_SYNC for a buffered use case we can still definitely assert
> that the mapping is "direct". So, MAP_DIRECT would fail for
> traditional non-DAX block devices, and for this new virtio-pmem case.
> It would also fail for any pmem device where we cannot assert that the
> platform will take care of flushing write-pending-queues on power-loss
> events.
After letting this set for a few days I think I'm back to liking
MADV_DIRECT_ACCESS more since madvise() is more closely related to the
page-cache management than mmap. It does not solve the query vs enable
problem, but it's still a step towards giving applications what they
want with respect to resource expectations.
Perhaps a new syscall to retrieve the effective advice for a range?
int madvice(void *addr, size_t length, int *advice);
next prev parent reply other threads:[~2018-10-17 20:01 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-02 10:05 Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps Jan Kara
2018-10-02 10:50 ` Michal Hocko
2018-10-02 13:32 ` Jan Kara
2018-10-02 12:10 ` Johannes Thumshirn
2018-10-02 14:20 ` Johannes Thumshirn
2018-10-02 14:45 ` Christoph Hellwig
2018-10-02 15:01 ` Johannes Thumshirn
2018-10-02 15:06 ` Christoph Hellwig
2018-10-04 10:09 ` Johannes Thumshirn
2018-10-05 6:25 ` Christoph Hellwig
2018-10-05 6:35 ` Johannes Thumshirn
2018-10-06 1:17 ` Dan Williams
2018-10-14 15:47 ` Dan Williams
2018-10-17 20:01 ` Dan Williams [this message]
2018-10-18 17:43 ` Jan Kara
2018-10-18 19:10 ` Dan Williams
2018-10-19 3:01 ` Dave Chinner
2018-10-02 14:29 ` Jan Kara
2018-10-02 14:37 ` Christoph Hellwig
2018-10-02 14:44 ` Johannes Thumshirn
2018-10-02 14:52 ` Christoph Hellwig
2018-10-02 15:31 ` Jan Kara
2018-10-02 20:18 ` Dan Williams
2018-10-03 12:50 ` Jan Kara
2018-10-03 14:38 ` Dan Williams
2018-10-03 15:06 ` Jan Kara
2018-10-03 15:13 ` Dan Williams
2018-10-03 16:44 ` Jan Kara
2018-10-03 21:13 ` Dan Williams
2018-10-04 10:04 ` Johannes Thumshirn
2018-10-02 15:07 ` Jan Kara
2018-10-17 20:23 ` Jeff Moyer
2018-10-18 0:25 ` Dave Chinner
2018-10-18 14:55 ` Jan Kara
2018-10-19 0:43 ` Dave Chinner
2018-10-30 6:30 ` Dan Williams
2018-10-30 22:49 ` Dave Chinner
2018-10-30 22:59 ` Dan Williams
2018-10-31 5:59 ` y-goto
2018-11-01 23:00 ` Dave Chinner
2018-11-02 1:43 ` y-goto
2018-10-18 21:05 ` Jeff Moyer
2018-10-09 19:43 ` Jeff Moyer
2018-10-16 8:25 ` Jan Kara
2018-10-16 12:35 ` Jeff Moyer
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