Re: [LSF/MM TOPIC] Un-addressable device memory and block/fs implications

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2016-12-13 at 13:55 -0500, Jerome Glisse wrote:
> On Tue, Dec 13, 2016 at 10:20:52AM -0800, James Bottomley wrote:
> > On Tue, 2016-12-13 at 13:15 -0500, Jerome Glisse wrote:
> > > I would like to discuss un-addressable device memory in the
> > > context 
> > > of filesystem and block device. Specificaly how to handle write
> > > -back,
> > > read, ... when a filesystem page is migrated to device memory
> > > that 
> > > CPU can not access.
> > > 
> > > I intend to post a patchset leveraging the same idea as the
> > > existing
> > > block bounce helper (block/bounce.c) to handle this. I believe
> > > this 
> > > is worth discussing during summit see how people feels about such
> > > plan and if they have better ideas.
> > 
> > Isn't this pretty much what the transcendent memory interfaces we
> > currently have are for?  It's current use cases seem to be
> > compressed
> > swap and distributed memory, but there doesn't seem to be any
> > reason in
> > principle why you can't use the interface as well.
> > 
> 
> I am not a specialist of tmem or cleancache

Well, that makes two of us; I just got to sit through Dan Magenheimer's
talks and some stuff stuck.

>  but my understand is that there is no way to allow for file back 
> page to be dirtied while being in this special memory.

Unless you have some other definition of dirtied, I believe that's what
an exclusive tmem get in frontswap actually does.  It marks the page
dirty when it comes back because it may have been modified.

> In my case when you migrate a page to the device it might very well 
> be so that the device can write something in it (results of some sort 
> of computation). So page might migrate to device memory as clean but
> return from it in dirty state.
> 
> Second aspect is that even if memory i am dealing with is un
> -addressable i still have struct page for it and i want to be able to 
> use regular page migration.

Tmem keeps a struct page ... what's the problem with page migration?
the fact that tmem locks the page when it's not addressable and you
want to be able to migrate the page even when it's not addressable?

> So given my requirement i didn't thought that cleancache was the way
> to address them. Maybe i am wrong.

I'm not saying it is, I just asked if you'd considered it, since the
requirements look similar.

James


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