From: Dan Williams <dan.j.williams@intel.com>
To: Jan Kara <jack@suse.cz>
Cc: Christoph Hellwig <hch@infradead.org>,
Johannes Thumshirn <jthumshirn@suse.de>,
Dave Jiang <dave.jiang@intel.com>,
linux-nvdimm <linux-nvdimm@lists.01.org>,
Linux MM <linux-mm@kvack.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linux-ext4 <linux-ext4@vger.kernel.org>,
linux-xfs <linux-xfs@vger.kernel.org>,
Linux API <linux-api@vger.kernel.org>
Subject: Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps
Date: Wed, 3 Oct 2018 14:13:59 -0700 [thread overview]
Message-ID: <CAPcyv4iCXNYTSzwU5OoZc_b+xxbqKwjyQV74CRxEfvDjaMPDfg@mail.gmail.com> (raw)
In-Reply-To: <20181003164407.GK24030@quack2.suse.cz>
On Wed, Oct 3, 2018 at 9:46 AM Jan Kara <jack@suse.cz> wrote:
>
> On Wed 03-10-18 08:13:37, Dan Williams wrote:
> > On Wed, Oct 3, 2018 at 8:07 AM Jan Kara <jack@suse.cz> wrote:
> > > WRT per-inode DAX property, AFAIU that inode flag is just going to be
> > > advisory thing - i.e., use DAX if possible. If you mount a filesystem with
> > > these inode flags set in a configuration which does not allow DAX to be
> > > used, you will still be able to access such inodes but the access will use
> > > page cache instead. And querying these flags should better show real
> > > on-disk status and not just whether DAX is used as that would result in an
> > > even bigger mess. So this feature seems to be somewhat orthogonal to the
> > > API I'm looking for.
> >
> > True, I imagine once we have that flag we will be able to distinguish
> > the "saved" property and the "effective / live" property of DAX...
> > Also it's really not DAX that applications care about as much as "is
> > there page-cache indirection / overhead for this mapping?". That seems
> > to be a narrower guarantee that we can make than what "DAX" might
> > imply.
>
> Right. So what do people think about my suggestion earlier in the thread to
> use madvise(MADV_DIRECT_ACCESS) for this? Currently it would return success
> when DAX is in use, failure otherwise. Later we could extend it to be also
> used as a hint for caching policy for the inode...
The only problem is that you can't use it purely as a query. If we
ever did plumb it to be a hint you could not read the state without
writing the state.
mincore(2) seems to be close the intent of discovering whether RAM is
being consumed for a given address range, but it currently is
implemented to only indicate if *any* mapping is established, not
whether RAM is consumed. I can see an argument that a dax mapped file
should always report an empty mincore vector.
next prev parent reply other threads:[~2018-10-03 21:13 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
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 [this message]
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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