Re: CVS commit: src/sys/uvm

[Masao Uebayashi <uebayasi%tombi.co.jp@localhost>]

On Wed, Jan 05, 2011 at 05:58:34AM +0000, YAMAMOTO Takashi wrote:
> hi,
> 
> > I take silence as "no objection".
> 
> the silence in this case means was-busy-for-other-things-and-forgot.
> sorry.
> 
> >> I have no real code for this big picture at this moment.  Making
> >> vm_physseg available as reference is the first step.  This only
> >> changes uvm_page_physload() to return a pointer:
> >> 
> >>    -void uvm_page_physload();
> >>    +void *uvm_page_physload();
> >> 
> >> But this makes XIP pager MUCH cleaner.  The reason has been explained
> >> many times.
> 
> because the separate uvm_page_physload_device is no longer necessary,
> you mean?  i have no problem with the step.
> 
> >> Making fault handlers and pagers to use vm_physseg * + off_t is
> >> the next step, and I don't intend to work on it now.  I just want
> >> to explain the big picture.
> >> 
> >> > 
> >> > >> 
> >> > >> Keep vm_physseg * + off_t array on stack.  If UVM objects uses
> >> > >> vm_page (e.g. vnode), its pager looks up vm_page -> vm_physseg *
> >> > >> + off_t *once* and cache it on stack.
> >> > 
> >> > do you mean something like this?
> >> >  struct {
> >> >          vm_physseg *hoge;
> >> >          off_t fuga;
> >> >  } foo [16];
> >> 
> >> Yes.
> >> 
> >> Or cache vm_page * with it, like:
> >> 
> >>    struct vm_id {
> >>            vm_physseg *seg;
> >>            off_t off;
> >>            vm_page *pg;
> >>    };
> >> 
> >>    uvm_fault()
> >>    {
> >>            vm_id pgs[];
> >>            :
> >>    }
> >> 
> >> Vnode pager (genfs_getpages) takes vm_page's by looking up
> >> vnode::v_uobj's list, or uvn_findpages().
> >> 
> >> When it returns back to fault handler, we have to lookup vm_physseg
> >> for each page.  Then fill the "seg" slot above (assuming we'll
> >> remove vm_page::phys_addr soon).
> >> 
> >> Fault handler calls per-vm_page operations iff vm_page slot is filled.
> >> XIP pages are not pageq'ed.
> 
> pgo_get returns either seg+off or pg for each vm_id slots?

vm_id[] is passed to pgo_get, each pgo_get (uvn_get, udv_get, ...)
will fill seg + off and return the array back to uvm_fault.

Pagers that use vm_page (e.g. vnode) will get pg from vnode, then
lookup seg+off, fill them into the vm_id slot.

Those that don't use vm_page (e.g. cdev) will get seg+off from
underlying objects, fill them into the vm_id slot, and return.

> 
> >> XIP pages don't need vm_page, but
> >> cached because it's vnode.
> 
> can you explain this sentence?

Sorry, it was so unclear.

        XIP pages don't need vm_page (as a paging state variable),
        because the pages are read-only and never involve paging.

        XIP pages have to be mapped as cacheable at MMU level,
        because it's vnode.

I'm allocating the full vm_page for XIP'able pages, only because
chs@ wanted to support loaning, which needs vm_page::loan_count.

> 
> >> (Just in case, have you read my paper?)
> 
> which paper?  i guess no.

http://uebayasi.dyndns.org/~uebayasi/tmp/xip.pdf

> 
> YAMAMOTO Takashi

-- 
Masao Uebayashi / Tombi Inc. / Tel: +81-90-9141-4635