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Subject: RE: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps
Date: Wed, 31 Oct 2018 05:59:17 +0000
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References: <20181002100531.GC4135@quack2.suse.cz>
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Hello,

> On Mon, Oct 29, 2018 at 11:30:41PM -0700, Dan Williams wrote:
> > On Thu, Oct 18, 2018 at 5:58 PM Dave Chinner <david@fromorbit.com> wrot=
e:
> > > On Thu, Oct 18, 2018 at 04:55:55PM +0200, Jan Kara wrote:
> > > > On Thu 18-10-18 11:25:10, Dave Chinner wrote:
> > > > > On Wed, Oct 17, 2018 at 04:23:50PM -0400, Jeff Moyer wrote:
> > > > > > MAP_SYNC
> > > > > > - file system guarantees that metadata required to reach faulte=
d-in file
> > > > > >   data is consistent on media before a write fault is completed=
.  A
> > > > > >   side-effect is that the page cache will not be used for
> > > > > >   writably-mapped pages.
> > > > >
> > > > > I think you are conflating current implementation with API
> > > > > requirements - MAP_SYNC doesn't guarantee anything about page cac=
he
> > > > > use. The man page definition simply says "supported only for file=
s
> > > > > supporting DAX" and that it provides certain data integrity
> > > > > guarantees. It does not define the implementation.
> ....
> > > > With O_DIRECT the fallback to buffered IO is quite rare (at least f=
or major
> > > > filesystems) so usually people just won't notice. If fallback for
> > > > MAP_DIRECT will be easy to hit, I'm not sure it would be very usefu=
l.
> > >
> > > Which is just like the situation where O_DIRECT on ext3 was not very
> > > useful, but on other filesystems like XFS it was fully functional.
> > >
> > > IMO, the fact that a specific filesytem has a suboptimal fallback
> > > path for an uncommon behaviour isn't an argument against MAP_DIRECT
> > > as a hint - it's actually a feature. If MAP_DIRECT can't be used
> > > until it's always direct access, then most filesystems wouldn't be
> > > able to provide any faster paths at all. It's much better to have
> > > partial functionality now than it is to never have the functionality
> > > at all, and so we need to design in the flexibility we need to
> > > iteratively improve implementations without needing API changes that
> > > will break applications.
> >
> > The hard guarantee requirement still remains though because an
> > application that expects combined MAP_SYNC|MAP_DIRECT semantics will
> > be surprised if the MAP_DIRECT property silently disappears.
>=20
> Why would they be surprised? They won't even notice it if the
> filesystem can provide MAP_SYNC without MAP_DIRECT.
>=20
> And that's the whole point.
>=20
> MAP_DIRECT is a private mapping state. So is MAP_SYNC. They are not
> visible to the filesystem and the filesystem does nothing to enforce
> them. If someone does something that requires the page cache (e.g.
> calls do_splice_direct()) then that MAP_DIRECT mapping has a whole
> heap of new work to do. And, in some cases, the filesystem may not
> be able to provide MAP_DIRECT as a result..
>=20
> IOWs, the filesystem cannot guarantee MAP_DIRECT and the
> circumstances under which MAP_DIRECT will and will not work are
> dynamic. If MAP_DIRECT is supposed to be a guarantee then we'll have
> applications randomly segfaulting in production as things like
> backups, indexers, etc run over the filesystem and do their work.
>=20
> This is why MAP_DIRECT needs to be an optimisation, not a
> requirement - things will still work if MAP_DIRECT is not used. What
> matters to these applications is MAP_SYNC - if we break MAP_SYNC,
> then the application data integrity model is violated. That's not an
> acceptible outcome.
>=20
> The problem, it seems to me, is that people are unable to separate
> MAP_DIRECT and MAP_SYNC. I suspect that is because, at present,
> MAP_SYNC on XFS and ext4 requires MAP_DIRECT. i.e. we can only
> provide MAP_SYNC functionality on DAX mappings. However, that's a
> /filesystem implementation issue/, not an API guarantee we need to
> provide to userspace.
>=20
> If we implement a persistent page cache (e.g. allocate page cache
> pages out of ZONE_DEVICE pmem), then filesystems like XFS and ext4
> could provide applications with the MAP_SYNC data integrity model
> without MAP_DIRECT. Indeed, those filesystems would not even be able
> to provide MAP_DIRECT semantics because they aren't backed by pmem.
>=20
> Hence if applications that want MAP_SYNC are hard coded
> MAP_SYNC|MAP_DIRECT and we make MAP_DIRECT a hard guarantee, then
> those applications are going to fail on a filesystem that provides
> only MAP_SYNC. This is despite the fact the applications would
> function correctly and the data integrity model would be maintained.
> i.e. the failure is because applications have assumed MAP_SYNC can
> only be provided by a DAX implementation, not because MAP_SYNC is
> not supported.
>=20
> MAP_SYNC really isn't about DAX at all. It's about enabling a data
> integrity model that requires the filesystem to provide userspace
> access to CPU addressable persistent memory.  DAX+MAP_DIRECT is just
> one method of providing this functionality, but it's not the only
> method. Our API needs to be future proof rather than an encoding of
> the existing implementation limitations, otherwise apps will have to
> be re-written as every new MAP_SYNC capable technology comes along.
>=20
> In summary:
>=20
> 	MAP_DIRECT is an access hint.
>=20
> 	MAP_SYNC provides a data integrity model guarantee.
>=20
> 	MAP_SYNC may imply MAP_DIRECT for specific implementations,
> 	but it does not require or guarantee MAP_DIRECT.
>=20
> Let's compare that with O_DIRECT:
>=20
> 	O_DIRECT in an access hint.
>=20
> 	O_DSYNC provides a data integrity model guarantee.
>=20
> 	O_DSYNC may imply O_DIRECT for specific implementations, but
> 	it does not require or guarantee O_DIRECT.
>=20
> Consistency in access and data integrity models is a good thing. DAX
> and pmem is not an exception. We need to use a model we know works
> and has proven itself over a long period of time.

Hmmm, then, I would like to know all of the reasons of breakage of MAP_DIRE=
CT.
(I'm not opposed to your opinion, but I need to know it.)

In O_DIRECT case, in my understanding, the reason of breakage of O_DIRECT i=
s=20
"wrong alignment is specified by application", right?

When filesystem can not use O_DIRECT and it uses page cache instead,
then system uses more memory resource than user's expectation.
So, there is a side effect, and it may cause other trouble.
(memory pressure, expected performance can not be gained, and so on ..)
=20
In such case its administrator (or technical support engineer) needs to str=
uggle to
investigate what is the reason.
If the reason of the breakage is clear, then it is helpful to find the root=
 cause,
and they can require the developer of wrong application to fix the problem.
"Please fix the alignment!".

So, I would like to know in MAP_DIRECT case, what is the reasons?=20
I think it will be helpful for users.
Only splice?

(Maybe such document will be necessary....)

Thanks,

>=20
> > I think
> > it still makes some sense as a hint for apps that want to minimize
> > page cache, but for the applications with a flush from userspace model
> > I think that wants to be an F_SETLEASE / F_DIRECTLCK operation. This
> > still gives the filesystem the option to inject page-cache at will,
> > but with an application coordination point.
>=20
> Why make it more complex for applications than it needs to be?
>=20
> Cheers,
>=20
> Dave.
> --
> Dave Chinner
> david@fromorbit.com