Date: Wed, 6 Jan 1999 14:59:19 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Terry Lambert <tlambert@primenet.com> Cc: dyson@iquest.net, pfgiffun@bachue.usc.unal.edu.co, freebsd-hackers@FreeBSD.ORG Subject: Re: questions/problems with vm_fault() in Stable Message-ID: <199901062259.OAA25909@apollo.backplane.com>
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:
: system calls NFS server VFS
: ------------ ---------- ---
: VFS VFS VFS
:
:In the provider case, where the VFS consumer is, you have three
:cases as well:
:
: VFS VFS VFS
: --- ---------- ---
: VM NFS client VFS
:
:Again, in the -current code, only the first one works correctly in
I think you have to get away from thinking about 'consumers' and
'providers'. It is precisely this sort of thinking that screwed
up the existing VFS design.
The best way to abstract a VFS layer is consider that each VFS layer
has a 'frontside' and 'backside'. The VFS layer should make no
assumptions whatsoever as to who attaches to it on the frontside,
and who it is attached to on the backside.
If you really want, you could consider a 'consumer' to be the VFS
layer's backside and a 'provider' to be the SAME VFS layer's frontside.
So a VFS layer's backside 'consumer' is linked to another VFS layer's
frontside 'provider'. And so forth. But don't try to 'type' a VFS
layer -- it doesn't work. It was precisely that sort of thinking
that required something like the MFS filesystem, which blurs distinctions,
to be a major hack in existing kernels.
The only way to do cache coherency through a multi-layered VFS design
is to extend the vm_object model. You *cannot* require that a VM
system use VOP_GETPAGES or VOP_PUTPAGES whenever it wants to verify
the validity of a page it already has in the cache. If a page is sitting
in the cache accessible to someone, that someone should be able to use
the page immediately. This is why a two-way cache coherency protocol
is so necessary, so things that effect coherency can be propogated
back up through the layers rather then through hacks. Requiring the
GET/PUTPAGES interface to be used in a cache case destroys the efficiency
of the cache and, also, makes it virtually impossible to implement async
I/O. The VFS layer, as it stands, cannot do async I/O - the struct buf
mechanisms 'sorta' does it, but it isn't really async due to the huge
number of places where the system can block even before it returns a bp.
An extended vm_object and cache coherency model would, for example,
allow something like MFS, VN, or VINUM to be implemented almost trivially
and definitely more efficiently, even unto having filesystems relocate
underlying storage on the fly.
-Matt
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