vm/fs meetup in september?

vm/fs meetup in september?

[Nick Piggin]

I'd just like to take the chance also to ask about a VM/FS meetup some
time around kernel summit (maybe take a big of time during UKUUG or so).

I was thinking about trying to arrange a proper mini summit thing, but
it's a bit difficult and we could talk this year about doing it for
subsequent years. If there is a bit of interest, we could probably find
a small room somewhere this year on pretty short notice or do it as a
BOF or something.

I don't want to do it in the VM summit, because that kind of alienates
the filesystem guys. What I want to talk about is anything and everything
that the VM can do better to help the fs and vice versa.  I'd like to
stay away from memory management where not too applicable to the fs.

A few things I'd like to talk about are:

- the address space operations APIs, and their page based nature. I think
  it would be nice to generally move toward offset,length based ones as
  much as possible because it should give more efficiency and flexibility
  in the filesystem.

- write_begin API if it is still an issue by that date. Hope not :)

- truncate races

- fsblock if it hasn't been shot down by then

- how to make complex API changes without having to fix most things
  yourself.


Anyway, if you will be in the area and are interested, let me know (off
list) and we can work out time and place.

Thanks,
Nick

Re: vm/fs meetup in september?

[Christoph Hellwig]

On Sun, Jun 24, 2007 at 06:23:45AM +0200, Nick Piggin wrote:
> I'd just like to take the chance also to ask about a VM/FS meetup some
> time around kernel summit (maybe take a big of time during UKUUG or so).

I won't be around until a day or two before KS, so I'd prefer to have it
after KS if possible.

> I don't want to do it in the VM summit, because that kind of alienates
> the filesystem guys. What I want to talk about is anything and everything
> that the VM can do better to help the fs and vice versa.  I'd like to
> stay away from memory management where not too applicable to the fs.

As more of a filesystem person I wouldn't mind it being attached to a VM
conf.  In the worst case we'll just rename it VM/FS conference.  When and
where is it scheduled?

> - the address space operations APIs, and their page based nature. I think
>   it would be nice to generally move toward offset,length based ones as
>   much as possible because it should give more efficiency and flexibility
>   in the filesystem.
> 
> - write_begin API if it is still an issue by that date. Hope not :)
> 
> - truncate races
> 
> - fsblock if it hasn't been shot down by then

Don't forget high order pagecache please.

> - how to make complex API changes without having to fix most things
>   yourself.

More issues:

 - aio once again
 - refactoring the dio code to separate locking down user VM and doing
   the actual page based I/O.  I've seen valid requests from kernel
   initiated direct I/O from a few real world linux users.
 - generic code for delayed allocation and writeout using efficient
   multi-page allocator calls.  I'll hopefully have an example (lifted XFS
   code) by then
 - what to do about reads/writes from kernelspace.  Currently we have
   some places (loop mostly) calling directly into ->prepare_write / 
   ->commit_write which is completely wrong from the layerin perspective
   and a locking nightmare for distributed or generally more complex
   filesystems.  And we have a lot of places using set_fs/set_ds and
   calling into ->write.  The first category could probably be covered
   by using the splice infrastructure, but for the latter we'd want
   something more optimal and less hacky, especially given all the overhead
   related avoiding deadlocks involing the user address space in the
   generic write path.  Maybe it's time for generic_file_kernel_write?

Re: vm/fs meetup in september?

[Zach Brown]

> > I'd just like to take the chance also to ask about a VM/FS meetup some
> > time around kernel summit (maybe take a big of time during UKUUG or so).

Yeah, I'd be interested.

> More issues:

- chris mason's patches to normalize buffered and direct locking

- z

Re: vm/fs meetup in september?

[Nick Piggin]

Christoph Hellwig wrote:
> On Sun, Jun 24, 2007 at 06:23:45AM +0200, Nick Piggin wrote:
> 
>>I'd just like to take the chance also to ask about a VM/FS meetup some
>>time around kernel summit (maybe take a big of time during UKUUG or so).
> 
> 
> I won't be around until a day or two before KS, so I'd prefer to have it
> after KS if possible.

I'd like to see you there, so I hope we can find a date that most
people are happy with. I'll try to start working that out after we
have a rough idea of who's interested.


>>I don't want to do it in the VM summit, because that kind of alienates
>>the filesystem guys. What I want to talk about is anything and everything
>>that the VM can do better to help the fs and vice versa.  I'd like to
>>stay away from memory management where not too applicable to the fs.
> 
> 
> As more of a filesystem person I wouldn't mind it being attached to a VM
> conf.  In the worst case we'll just rename it VM/FS conference.  When and
> where is it scheduled?

I'll just cc Martin, however the VM conference I think is pretty short
on filesystem people. I'd also like to avoid a lot of VM topics and
hopefully have enough time for a topic of interest or so from each fs
maintainer who has something to talk about.

But I'm open to ideas that will make it work better. FWIW, Anton has
offered to try arranging conference facilities at the university, so
I think we should be covered there.


>>- the address space operations APIs, and their page based nature. I think
>>  it would be nice to generally move toward offset,length based ones as
>>  much as possible because it should give more efficiency and flexibility
>>  in the filesystem.
>>
>>- write_begin API if it is still an issue by that date. Hope not :)
>>
>>- truncate races
>>
>>- fsblock if it hasn't been shot down by then
> 
> 
> Don't forget high order pagecache please.


Leaving my opinion of higher order pagecache aside, this _may_ be an
example of something that doesn't need a lot of attention, because it
should be fairly uncontroversial from a filesystem's POV? (eg. it is
more a relevant item to memory management and possibly block layer).
OTOH if it is discussed in the context of "large blocks in the buffer
layer is crap because we can do it with higher order pagecache", then
that might be interesting :)

Anyway, I won't say no to any proposal, so keep the ideas coming. We
can talk about whatever we find interesting on the day.


>>- how to make complex API changes without having to fix most things
>>  yourself.
> 
> 
> More issues:

Thanks Christoph, sounds good.

-- 
SUSE Labs, Novell Inc.

Re: vm/fs meetup in september?

[Andreas Dilger]

On Jun 26, 2007  12:35 +1000, Nick Piggin wrote:
> Leaving my opinion of higher order pagecache aside, this _may_ be an
> example of something that doesn't need a lot of attention, because it
> should be fairly uncontroversial from a filesystem's POV? (eg. it is
> more a relevant item to memory management and possibly block layer).
> OTOH if it is discussed in the context of "large blocks in the buffer
> layer is crap because we can do it with higher order pagecache", then
> that might be interesting :)

FWIW, being able to have large (8-64kB) blocksize would be great for
ext2/3/4.  We'd sort of been betting on this by limiting the on-disk
extent format to 48-bit physical block numbers, and to have 2 patches
to implement this in as many weeks is excellent.

To me the mechanism doesn't matter, whether through fsblock or high-order
PAGE_SIZE.  I'll let the rest of you duke it out as long as at least one
of them makes it into the kernel.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.

Re: vm/fs meetup in september?

[Chris Mason]

On Tue, Jun 26, 2007 at 12:35:09PM +1000, Nick Piggin wrote:
> Christoph Hellwig wrote:
> >On Sun, Jun 24, 2007 at 06:23:45AM +0200, Nick Piggin wrote:
> >
> >>I'd just like to take the chance also to ask about a VM/FS meetup some
> >>time around kernel summit (maybe take a big of time during UKUUG or so).
> >
> >
> >I won't be around until a day or two before KS, so I'd prefer to have it
> >after KS if possible.
> 
> I'd like to see you there, so I hope we can find a date that most
> people are happy with. I'll try to start working that out after we
> have a rough idea of who's interested.

I'm game, but won't be staying past the end of KS (I'll arrive Sept 2nd
or so though).  Given debates so far, it probably makes sense to talk
about things at KS too.

-chris

Re: vm/fs meetup in september?

[Christoph Hellwig]

On Tue, Jun 26, 2007 at 12:35:09PM +1000, Nick Piggin wrote:
> I'd like to see you there, so I hope we can find a date that most
> people are happy with. I'll try to start working that out after we
> have a rough idea of who's interested.

Do we have any data preferences yet?

Re: vm/fs meetup in september?

[Martin J. Bligh]

Christoph Hellwig wrote:
> On Tue, Jun 26, 2007 at 12:35:09PM +1000, Nick Piggin wrote:
>   
>> I'd like to see you there, so I hope we can find a date that most
>> people are happy with. I'll try to start working that out after we
>> have a rough idea of who's interested.
>>     
>
> Do we have any data preferences yet?
>   

You mean date?


VM is arranged for the 3rd, IIRC Kernel summit doesn't
start until the 5th, so there's a gap on the 4th if you want
to sort out the fs stuff then? Not 100% sure on the dates.

M.

Re: vm/fs meetup in september?

[Nick Piggin]

On Sat, Jun 30, 2007 at 05:35:04AM -0700, Martin J. Bligh wrote:
> Christoph Hellwig wrote:
> >On Tue, Jun 26, 2007 at 12:35:09PM +1000, Nick Piggin wrote:
> >  
> >>I'd like to see you there, so I hope we can find a date that most
> >>people are happy with. I'll try to start working that out after we
> >>have a rough idea of who's interested.
> >>    
> >
> >Do we have any data preferences yet?
> >  
> 
> You mean date?
> 
> 
> VM is arranged for the 3rd, IIRC Kernel summit doesn't
> start until the 5th, so there's a gap on the 4th if you want
> to sort out the fs stuff then? Not 100% sure on the dates.

Well it says kernel summit is 4-6th, but also that it is a two
day event, so I think you're right that it starts on 5th.

I'd like to see what people's preferences are, whether we try to
do this on the 4th, or after KS (which could be 7th or 8th)? I
know some can't make it before KS and some can't make it after,
but if you absolutely won't come on any of these dates can you let
me know? And let me know preferences or other ideas too.

Regarding numbers, there are about a dozen so far which is good
but not as many filesystem maintainers as I had hoped (do they
tend not to get invited to KS?). We may get a few more people yet
so I think if we try to get a room to fit 20-25 people it would
be ideal: I don't want to turn anyone away ;)

Thanks,
Nick

Re: vm/fs meetup in september?

[Jörn Engel]

On Mon, 2 July 2007 08:19:44 +0200, Nick Piggin wrote:
> 
> Regarding numbers, there are about a dozen so far which is good
> but not as many filesystem maintainers as I had hoped (do they
> tend not to get invited to KS?). We may get a few more people yet
> so I think if we try to get a room to fit 20-25 people it would
> be ideal: I don't want to turn anyone away ;)

I'm interested.

My particular pet subject would be sync behaviour.  LogFS benefits from
writing data in a particular order - data first, then indirect blocks,
doubly indirect, triply, etc.  Reason is that indirect blocks get
dirtied when data is written.

The current solution is to write indirect blocks immediatly, causing
quite bad performance.

Jörn

-- 
Measure. Don't tune for speed until you've measured, and even then
don't unless one part of the code overwhelms the rest.
-- Rob Pike
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Re: vm/fs meetup in september?

[Jared Hulbert]

> A few things I'd like to talk about are:
>
> - the address space operations APIs, and their page based nature. I think
>   it would be nice to generally move toward offset,length based ones as
>   much as possible because it should give more efficiency and flexibility
>   in the filesystem.
>
> - write_begin API if it is still an issue by that date. Hope not :)
>
> - truncate races
>
> - fsblock if it hasn't been shot down by then
>
> - how to make complex API changes without having to fix most things
>   yourself.

I'd like to add:

-revamping filemap_xip.c

-memory mappable swap file (I'm not sure if this one is appropriate
for the proposed meeting)

Re: vm/fs meetup in september?

[Christoph Hellwig]

On Mon, Jun 25, 2007 at 05:08:02PM -0700, Jared Hulbert wrote:
> -memory mappable swap file (I'm not sure if this one is appropriate
> for the proposed meeting)

Please explain what this is supposed to mean.

Re: vm/fs meetup in september?

[Jared Hulbert]

On 6/25/07, Christoph Hellwig <hch@infradead.org> wrote:
> On Mon, Jun 25, 2007 at 05:08:02PM -0700, Jared Hulbert wrote:
> > -memory mappable swap file (I'm not sure if this one is appropriate
> > for the proposed meeting)
>
> Please explain what this is supposed to mean.

If you have a large array of a non-volatile semi-writeable memory such
as a highspeed NOR Flash or some of the similar emerging technologies
in a system.  It would be useful to use that memory as an extension of
RAM.  One of the ways you could do that is allow pages to be swapped
out to this memory.  Once there these pages could be read directly,
but would require a COW procedure on a write access.  The reason why I
think this may be a vm/fs topic is that the hardware makes writing to
this memory efficiently a non-trivial operation that requires
management just like a filesystem.  Also it seems to me that there are
probably overlaps between this topic and the recent filemap_xip.c
discussions.

Re: vm/fs meetup in september?

[Christoph Hellwig]

On Tue, Jun 26, 2007 at 10:07:24AM -0700, Jared Hulbert wrote:
> If you have a large array of a non-volatile semi-writeable memory such
> as a highspeed NOR Flash or some of the similar emerging technologies
> in a system.  It would be useful to use that memory as an extension of
> RAM.  One of the ways you could do that is allow pages to be swapped
> out to this memory.  Once there these pages could be read directly,
> but would require a COW procedure on a write access.  The reason why I
> think this may be a vm/fs topic is that the hardware makes writing to
> this memory efficiently a non-trivial operation that requires
> management just like a filesystem.  Also it seems to me that there are
> probably overlaps between this topic and the recent filemap_xip.c
> discussions.

So what you mean is "swap on flash" ?  Defintively sounds like an
interesting topic, although I'm not too sure it's all that
filesystem-related.

Re: vm/fs meetup in september?

[peter]

>>>>> "Christoph" == Christoph Hellwig <hch@infradead.org> writes:

Christoph> On Tue, Jun 26, 2007 at 10:07:24AM -0700, Jared Hulbert
Christoph> wrote:
>> If you have a large array of a non-volatile semi-writeable memory
>> such as a highspeed NOR Flash or some of the similar emerging
>> technologies in a system.  It would be useful to use that memory as
>> an extension of RAM.  One of the ways you could do that is allow
>> pages to be swapped out to this memory.  Once there these pages
>> could be read directly, but would require a COW procedure on a
>> write access.  The reason why I think this may be a vm/fs topic is
>> that the hardware makes writing to this memory efficiently a
>> non-trivial operation that requires management just like a
>> filesystem.  Also it seems to me that there are probably overlaps
>> between this topic and the recent filemap_xip.c discussions.

Christoph> So what you mean is "swap on flash" ?  Defintively sounds
Christoph> like an interesting topic, although I'm not too sure it's
Christoph> all that filesystem-related.


You need either a block translation layer, or a (swap) filesystem that
understands flash peculiarities in order to make such a thing work.
The standard Linux swap format will not work.

--
Dr Peter Chubb  http://www.gelato.unsw.edu.au  peterc AT gelato.unsw.edu.au
http://www.ertos.nicta.com.au           ERTOS within National ICT Australia

Re: vm/fs meetup in september?

[Christoph Hellwig]

On Sat, Jun 30, 2007 at 06:02:44AM -0400, peter@chubb.wattle.id.au wrote:
> You need either a block translation layer, or a (swap) filesystem that
> understands flash peculiarities in order to make such a thing work.
> The standard Linux swap format will not work.

Yes, it basically needs an ftl.  

Re: vm/fs meetup in september?

[Al Boldi]

peter@chubb.wattle.id.au wrote:
> >>>>> "Christoph" == Christoph Hellwig <hch@infradead.org> writes:
>
> Christoph> On Tue, Jun 26, 2007 at 10:07:24AM -0700, Jared Hulbert
>
> Christoph> wrote:
> >> If you have a large array of a non-volatile semi-writeable memory
> >> such as a highspeed NOR Flash or some of the similar emerging
> >> technologies in a system.  It would be useful to use that memory as
> >> an extension of RAM.  One of the ways you could do that is allow
> >> pages to be swapped out to this memory.  Once there these pages
> >> could be read directly, but would require a COW procedure on a
> >> write access.  The reason why I think this may be a vm/fs topic is
> >> that the hardware makes writing to this memory efficiently a
> >> non-trivial operation that requires management just like a
> >> filesystem.  Also it seems to me that there are probably overlaps
> >> between this topic and the recent filemap_xip.c discussions.
>
> Christoph> So what you mean is "swap on flash" ?  Defintively sounds
> Christoph> like an interesting topic, although I'm not too sure it's
> Christoph> all that filesystem-related.

I wouldn't want to call it swap, as this carries with it block-io 
connotations.  It's really mmap on flash.

> You need either a block translation layer,

Are you suggesting to go through the block layer to reach the flash?

> or a (swap) filesystem that
> understands flash peculiarities in order to make such a thing work.
> The standard Linux swap format will not work.

Correct.

BTW, you may want to have a look at my "[RFC] VM: I have a dream..." thread.

Here is an excerpt:

"What's more, there is no more swap.
Apps are executed inplace, as if already loaded.
Physical RAM is used to cache slower storage RAM, much the same as the CPU 
cache RAM caches slower physical RAM."

The thread ended with this conclusion:

Alan Cox wrote:
> On Iau, 2006-02-02 at 21:59 +0300, Al Boldi wrote:
> > So w/ 1GB RAM, no swap, and 1TB disk mmap'd, could this mmap'd space be
> > added to the total memory available to the OS, as is done w/ swap?
>
> Yes in theory. It would be harder to manage.
>
> > And if that's possible, why not replace swap w/ mmap'd disk-space?
>
> Swap is just somewhere to stick data that isnt file backed, you could
> build a swapless mmap based OS but it wouldn't be quite the same as
> Unix/Linux are.


Thanks!

--
Al

Re: vm/fs meetup in september?

[Jared Hulbert]

> > Christoph> So what you mean is "swap on flash" ?  Defintively sounds
> > Christoph> like an interesting topic, although I'm not too sure it's
> > Christoph> all that filesystem-related.
>
> I wouldn't want to call it swap, as this carries with it block-io
> connotations.  It's really mmap on flash.

Yes it is really mmap on flash.  But you are "swapping" pages from RAM
to be mmap'ed on flash.  Also the flash-io complexities are similar to
the block-io layer.  I think "swap on flash" is fair.  Though that
might be confused with making swap work on a NAND flash, which is very
much like the current block-io approach.  "Mmappable swap on flash" is
more exact, I suppose.

> > You need either a block translation layer,
>
> Are you suggesting to go through the block layer to reach the flash?

Well the obvious route would be to have this management layer use the
MTD, I can't see anything wrong with that.

> > or a (swap) filesystem that
> > understands flash peculiarities in order to make such a thing work.
> > The standard Linux swap format will not work.
>
> Correct.
>
> BTW, you may want to have a look at my "[RFC] VM: I have a dream..." thread.

Interesting.  This idea does allow for swap to be access directly.

Re: vm/fs meetup in september?

[Jared Hulbert]

> So what you mean is "swap on flash" ?  Defintively sounds like an
> interesting topic, although I'm not too sure it's all that
> filesystem-related.

Maybe not. Yet, it would be a very useful place to store data from a
file as a non-volatile page cache.

Also it is something that I believe would benefit from a VFS-like API.
 I mean there is a consistent interface a management layer like this
could use, yet the algorithms used to order the data and the interface
to the physical media may vary.  There is no single right way to do
the management layer, much like filesystems.

Given the page orientation of the current VFS seems to me like there
might be a nice way to use it for this purpose.

Or maybe the real experts on this stuff can tell me how wrong that is
and where it should go :)

Re: vm/fs meetup in september?

[Jörn Engel]

On Mon, 2 July 2007 10:44:00 -0700, Jared Hulbert wrote:
> 
> >So what you mean is "swap on flash" ?  Defintively sounds like an
> >interesting topic, although I'm not too sure it's all that
> >filesystem-related.
> 
> Maybe not. Yet, it would be a very useful place to store data from a
> file as a non-volatile page cache.
> 
> Also it is something that I believe would benefit from a VFS-like API.
> I mean there is a consistent interface a management layer like this
> could use, yet the algorithms used to order the data and the interface
> to the physical media may vary.  There is no single right way to do
> the management layer, much like filesystems.
> 
> Given the page orientation of the current VFS seems to me like there
> might be a nice way to use it for this purpose.
> 
> Or maybe the real experts on this stuff can tell me how wrong that is
> and where it should go :)

I don't believe anyone has implemented this before, so any experts would
be self-appointed.

Maybe this should be turned into a filesystem subject after all.  The
complexity comes from combining XIP with writes on the same chip.  So
solving your problem should be identical to solving the rw XIP
filesystem problem.

If there is interest in the latter, I'd offer my self-appointed
expertise.

Jörn

-- 
Joern's library part 5:
http://www.faqs.org/faqs/compression-faq/part2/section-9.html
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Re: vm/fs meetup in september?

[Jared Hulbert]

On 7/2/07, Jörn Engel <joern@logfs.org> wrote:
> On Mon, 2 July 2007 10:44:00 -0700, Jared Hulbert wrote:
> >
> > >So what you mean is "swap on flash" ?  Defintively sounds like an
> > >interesting topic, although I'm not too sure it's all that
> > >filesystem-related.
> >
> > Maybe not. Yet, it would be a very useful place to store data from a
> > file as a non-volatile page cache.
> >
> > Also it is something that I believe would benefit from a VFS-like API.
> > I mean there is a consistent interface a management layer like this
> > could use, yet the algorithms used to order the data and the interface
> > to the physical media may vary.  There is no single right way to do
> > the management layer, much like filesystems.
> >
> > Given the page orientation of the current VFS seems to me like there
> > might be a nice way to use it for this purpose.
> >
> > Or maybe the real experts on this stuff can tell me how wrong that is
> > and where it should go :)
>
> I don't believe anyone has implemented this before, so any experts would
> be self-appointed.
>
> Maybe this should be turned into a filesystem subject after all.  The
> complexity comes from combining XIP with writes on the same chip.  So
> solving your problem should be identical to solving the rw XIP
> filesystem problem.
>
> If there is interest in the latter, I'd offer my self-appointed
> expertise.

Right, the solution to swap problem is identical to the rw XIP
filesystem problem.    Jörn, that's why you're the self-appointed
subject matter expert!
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Re: vm/fs meetup in september?

[Jörn Engel]

On Mon, 2 July 2007 17:46:40 -0700, Jared Hulbert wrote:
> 
> Right, the solution to swap problem is identical to the rw XIP
> filesystem problem.    Jörn, that's why you're the self-appointed
> subject matter expert!

All right.  I'll try to make an important face whenever the subject
comes up.

Nick, do you have a problem if LogFS occupies two brainslots at the
meeting?

Jörn

-- 
Eighty percent of success is showing up.
-- Woody Allen
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Re: vm/fs meetup in september?

[Dongjun Shin]

I'd like to reference a paper titled "FASS : A Flash-Aware Swap System".
(http://kernel.kaist.ac.kr/~jinsoo/publication/iwssps05.pdf)

The paper describes a technique that uses NAND flash as a swap device
without FTL (Flash Translation Layer) or filesystem.

It is not related with XIP, however.

On 7/3/07, Jörn Engel <joern@logfs.org> wrote:
> On Mon, 2 July 2007 17:46:40 -0700, Jared Hulbert wrote:
> >
> > Right, the solution to swap problem is identical to the rw XIP
> > filesystem problem.    Jörn, that's why you're the self-appointed
> > subject matter expert!
>
> All right.  I'll try to make an important face whenever the subject
> comes up.
>
> Nick, do you have a problem if LogFS occupies two brainslots at the
> meeting?
>
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