From: Dan Williams <email@example.com> To: Jan Kara <firstname.lastname@example.org> Cc: Christoph Hellwig <email@example.com>, Johannes Thumshirn <firstname.lastname@example.org>, Dave Jiang <email@example.com>, linux-nvdimm <firstname.lastname@example.org>, Linux MM <email@example.com>, linux-fsdevel <firstname.lastname@example.org>, linux-ext4 <email@example.com>, linux-xfs <firstname.lastname@example.org>, Linux API <email@example.com> Subject: Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps Date: Wed, 3 Oct 2018 07:38:50 -0700 Message-ID: <CAPcyv4jfV10yuTiPg6ijsPRRL2-c_48ovfpU5TK1Zu7BWnfk3g@mail.gmail.com> (raw) In-Reply-To: <20181003125056.GA21043@quack2.suse.cz> On Wed, Oct 3, 2018 at 5:51 AM Jan Kara <firstname.lastname@example.org> wrote: > > On Tue 02-10-18 13:18:54, Dan Williams wrote: > > On Tue, Oct 2, 2018 at 8:32 AM Jan Kara <email@example.com> 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. > > So luckily this particular application wasn't widely deployed yet so we > will likely get away with the vendor asking customers to update to a > version not looking into smaps and parsing /proc/mounts instead. > > But I don't find parsing /proc/mounts that beautiful either and I'd prefer > if we had a better interface for applications to query whether they can > avoid page cache for mmaps or not. Yeah, the mount flag is not a good indicator either. I think we need to follow through on the per-inode property of DAX. Darrick and I discussed just allowing the property to be inherited from the parent directory at file creation time. That avoids the dynamic set-up / teardown races that seem intractable at this point. What's wrong with MAP_SYNC as a page-cache detector in the meantime?
next prev parent 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 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 [this message] 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
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