Linux-Fsdevel Archive on lore.kernel.org
 help / color / Atom feed
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
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);

  reply index

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-02 10:05 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

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CAPcyv4jt_w-89+m4w=FcN0oF3axiGqPBTHfEcWwdhnr12_=17Q@mail.gmail.com' \
    --to=dan.j.williams@intel.com \
    --cc=hch@infradead.org \
    --cc=jack@suse.cz \
    --cc=jthumshirn@suse.de \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=linux-nvdimm@lists.01.org \
    --cc=mhocko@suse.cz \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-Fsdevel Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-fsdevel/0 linux-fsdevel/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-fsdevel linux-fsdevel/ https://lore.kernel.org/linux-fsdevel \
		linux-fsdevel@vger.kernel.org
	public-inbox-index linux-fsdevel

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-fsdevel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git