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

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Message-ID: <CAPcyv4j0tTD+rENqFExA68aw=-MmtCBaOe1qJovyrmJC=yBg-Q@mail.gmail.com>
Date:   Tue, 2 Oct 2018 13:18:54 -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 Tue, Oct 2, 2018 at 8:32 AM Jan Kara <jack@...e.cz> wrote:
>
> On Tue 02-10-18 07:52:06, Christoph Hellwig wrote:
> > On Tue, Oct 02, 2018 at 04:44:13PM +0200, Johannes Thumshirn wrote:
> > > On Tue, Oct 02, 2018 at 07:37:13AM -0700, Christoph Hellwig wrote:
> > > > No, it should not.  DAX is an implementation detail thay may change
> > > > or go away at any time.
> > >
> > > Well we had an issue with an application checking for dax, this is how
> > > we landed here in the first place.
> >
> > So what exacty is that "DAX" they are querying about (and no, I'm not
> > joking, nor being philosophical).
>
> I believe the application we are speaking about is mostly concerned about
> the memory overhead of the page cache. Think of a machine that has ~ 1TB of
> DRAM, the database running on it is about that size as well and they want
> database state stored somewhere persistently - which they may want to do by
> modifying mmaped database files if they do small updates... So they really
> want to be able to use close to all DRAM for the DB and not leave slack
> space for the kernel page cache to cache 1TB of database files.

VM_MIXEDMAP was never a reliable indication of DAX because it could be
set for random other device-drivers that use vm_insert_mixed(). The
MAP_SYNC flag positively indicates that page cache is disabled for a
given mapping, although whether that property is due to "dax" or some
other kernel mechanics is purely an internal detail.

I'm not opposed to faking out VM_MIXEDMAP if this broken check has
made it into production, but again, it's unreliable.