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

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Message-ID: <20181003164407.GK24030@quack2.suse.cz>
Date:   Wed, 3 Oct 2018 18:44:07 +0200
From:   Jan Kara <jack@...e.cz>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jan Kara <jack@...e.cz>, 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 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...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR