Re: [PATCH RFC] extent mapped page cache

[Anton Altaparmakov]

On 26 Jul 2007, at 03:36, Nick Piggin wrote:

> On Wed, Jul 25, 2007 at 10:10:07PM -0400, Chris Mason wrote:
> > On Thu, 26 Jul 2007 03:37:28 +0200
> > Nick Piggin <npiggin@xxxxxxx> wrote:
> >
> > > > One advantage to the state tree is that it separates the state from
> > > > the memory being described, allowing a simple kmap style interface
> > > > that covers subpages, highmem and superpages.
> > >
> > > I suppose so, although we should have added those interfaces long
> > > ago ;) The variants in fsblock are pretty good, and you could always
> > > do an arbitrary extent (rather than block) based API using the
> > > pagecache tree if it would be helpful.
> >
> > Yes, you could use fsblock for the state bits and make a separate API
> > to map the actual pages.
> >
> > > > It also more naturally matches the way we want to do IO, making for
> > > > easy clustering.
> > >
> > > Well the pagecache tree is used to reasonable effect for that now.
> > > OK the code isn't beautiful ;). Granted, this might be an area where
> > > the seperate state tree ends up being better. We'll see.
> >
> > One thing it gains us is finding the start of the cluster.  Even if
> > called by kswapd, the state tree allows writepage to find the  
> > start of
> > the cluster and send down a big bio (provided I implement trylock to
> > avoid various deadlocks).
>
> That's very true, we could potentially also do that with the block  
> extent
> tree that I want to try with fsblock.
>
> I'm looking at "cleaning up" some of these aops APIs so hopefully  
> most of
> the deadlock problems go away. Should be useful to both our  
> efforts. Will
> post patches hopefully when I get time to finish the draft this  
> weekend.
>
>
> > > > O_DIRECT becomes a special case of readpages and writepages....the
> > > > memory used for IO just comes from userland instead of the page
> > > > cache.
> > >
> > > Could be, although you'll probably also need to teach the mm about
> > > the state tree and/or still manipulate the pagecache tree to prevent
> > > concurrency?
> >
> > Well, it isn't coded yet, but I should be able to do it from the FS
> > specific ops.
>
> Probably, if you invalidate all the pagecache in the range beforehand
> you should be able to do it (and I guess you want to do the invalidate
> anyway). Although, below deadlock issues might still bite somehwere...
>
>
> > > But isn't the main aim of O_DIRECT to do as little locking and
> > > synchronisation with the pagecache as possible? I thought this is
> > > why your race fixing patches got put on the back burner (although
> > > they did look fairly nice from a correctness POV).
> >
> > I put the placeholder patches on hold because handling a corner case
> > where userland did O_DIRECT from a mmap'd region of the same file  
> > (Linus
> > pointed it out to me).  Basically my patches had to work in 64k  
> > chunks
> > to avoid a deadlock in get_user_pages.  With the state tree, I can
> > allow the page to be faulted in but still properly deal with it.
>
> Oh right, I didn't think of that one. Would you still have similar
> issues with the external state tree? I mean, the filesystem doesn't
> really know why the fault is taken. O_DIRECT read from a file into
> mmapped memory of the same block in the file is almost hopeless I
> think.
>
>
> > > Well I'm kind of handwaving when it comes to O_DIRECT ;) It does  
> > > look
> > > like this might be another advantage of the state tree (although you
> > > aren't allowed to slow down buffered IO to achieve the locking ;)).
> >
> > ;) The O_DIRECT benefit is a fringe thing.  I've long wanted to help
> > clean up that code, but the real point of the patch is to make  
> > general
> > usage faster and less complex.  If I can't get there, the O_DIRECT
> > stuff doesn't matter.
>
> Sure, although unifying code is always a plus so I like that you've
> got that in mind.
>
>
> > > > The ability to put in additional tracking info like the process  
> > > > that
> > > > first dirtied a range is also significant.  So, I think it is worth
> > > > trying.
> > >
> > > Definitely, and I'm glad you are. You haven't converted me yet, but
> > > I look forward to finding the best ideas from our two approaches  
> > > when
> > > the patches are further along (ext2 port of fsblock coming along, so
> > > we'll be able to have races soon :P).
> >
> > I'm sure we can find some river in Cambridge, winner gets to throw
> > Axboe in.
>
> Very noble of you to donate your colleage to such a worthy cause.

Cambridge = Cam + Bridge = Bridge over the river Cam

The Cam is a bit muddy though so it is not a very enjoyable  
experience falling/being thrown into it.

Nice for punting on though when the weather is nice.  (-:

Best regards,

	Anton
--
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer, http://www.linux-ntfs.org/


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