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From: "y-goto@fujitsu.com" <y-goto@fujitsu.com>
Subject: RE: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps
Date: Fri, 2 Nov 2018 01:43:27 +0000
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> > > 	MAP_DIRECT is an access hint.
> > >
> > > 	MAP_SYNC provides a data integrity model guarantee.
> > >
> > > 	MAP_SYNC may imply MAP_DIRECT for specific implementations,
> > > 	but it does not require or guarantee MAP_DIRECT.
> > >
> > > Let's compare that with O_DIRECT:
> > >
> > > 	O_DIRECT in an access hint.
> > >
> > > 	O_DSYNC provides a data integrity model guarantee.
> > >
> > > 	O_DSYNC may imply O_DIRECT for specific implementations, but
> > > 	it does not require or guarantee O_DIRECT.
> > >
> > > 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_DIRECT.
> > (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 is
> > "wrong alignment is specified by application", right?
> 
> O_DIRECT has defined memory and offset alignment restrictions, and
> will return an error to userspace when they are violated. It does
> not fall back to buffered IO in this case. MAP_DIRECT has no
> equivalent restriction, so IO alignment of O_DIRECT is largely
> irrelevant here.
> 
> What we are talking about here is that some filesystems can only do
> certain operations through buffered IO, such as block allocation or
> file extension, and so silently fall back to doing them via buffered
> IO even when O_DIRECT is specified. The old direct IO code used to
> be full of conditionals to allow this - I think DIO_SKIP_HOLES is
> only one remaining:
> 
>                 /*
>                  * For writes that could fill holes inside i_size on a
>                  * DIO_SKIP_HOLES filesystem we forbid block creations: only
>                  * overwrites are permitted. We will return early to the caller
>                  * once we see an unmapped buffer head returned, and the caller
>                  * will fall back to buffered I/O.
>                  *
>                  * Otherwise the decision is left to the get_blocks method,
>                  * which may decide to handle it or also return an unmapped
>                  * buffer head.
>                  */
>                 create = dio->op == REQ_OP_WRITE;
>                 if (dio->flags & DIO_SKIP_HOLES) {
>                         if (fs_startblk <= ((i_size_read(dio->inode) - 1) >>
>                                                         i_blkbits))
>                                 create = 0;
>                 }
> 
> Other cases like file extension cases are caught by the filesystems
> before calling into the DIO code itself, so there's multiple avenues
> for O_DIRECT transparently falling back to buffered IO.
> 
> This means the applications don't fail just because the filesystem
> can't do a specific operation via O_DIRECT. The data writes still
> succeed because they fall back to buffered IO, and the application
> is blissfully unaware that the filesystem behaved that way.
> 
> > When filesystem can not use O_DIRECT and it uses page cache instead,
> > then system uses more memory resource than user's expectation.
> 
> That's far better than failing unexpectedly because the app
> unexpectedly came across a hole in the file (e.g. someone ran
> sparsify across the filesystem).
> 
> > So, there is a side effect, and it may cause other trouble.
> > (memory pressure, expected performance can not be gained, and so on ..)
> 
> Which is why people are supposed to test their systems before they
> put them into production.
> 
> I've lost count of the number of times I've heard "but O_DIRECT is
> supposed to make things faster!" because people don't understand
> exactly what it does or means. Bypassing the page cache does not
> magically make applications go faster - it puts the responsibility
> for doing optimal IO on the application, not the kernel.
> 
> MAP_DIRECT will be no different. It's no guarantee that it will make
> things faster, or that everything will just work as users expect
> them to. It specifically places the responsibility for performing IO
> in an optimal fashion on the application and the user for making
> sure that it is fit for their purposes. Like O_DIRECT, using
> MAP_DIRECT means "I, the application, know exactly what I'm doing,
> so get out of the way as much as possible because I'm taking
> responsibility for issuing IO in the most optimal manner now".
> 
> > In such case its administrator (or technical support engineer) needs to struggle to
> > investigate what is the reason.
> 
> That's no different to performance problems that arise from
> inappropriate use of O_DIRECT. It requires a certain level of
> expertise to be able to understand and diagnose such issues.
> 
> > So, I would like to know in MAP_DIRECT case, what is the reasons?
> > I think it will be helpful for users.
> > Only splice?
> 
> The filesystem can ignore MAP_DIRECT for any reason it needs to. I'm
> certain that filesystem developers will try to maintain MAP_DIRECT
> semantics as much as possible, but it's not going to be possible in
> /all situations/ on XFS and ext4 because they simply haven't been
> designed with DAX in mind. Filesystems designed specifically for
> pmem and DAX might be able to provide MAP_DIRECT in all situations,
> but those filesystems don't really exist yet.
> 
> This is no different to the early days of O_DIRECT. e.g.  ext3
> couldn't do O_DIRECT for all operations when it was first
> introduced, but over time the functionality improved as the
> underlying issues were solved. If O_DIRECT was a guarantee, then
> ext3 would have never supported O_DIRECT at all...

Hmm, Ok. I see.
Thank you very much for your detail explanation.

> 
> > (Maybe such document will be necessary....)
> 
> The semantics will need to be documented in the relevant man pages.

I agree.

Thanks, again.
----
Yasunori Goto

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