linux-ext4 - Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps

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Date:   Wed, 3 Oct 2018 14:13:59 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Dave Jiang <dave.jiang@...el.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps

On Wed, Oct 3, 2018 at 9:46 AM Jan Kara <jack@...e.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@...e.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.