Re: XIP

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

On Fri, Nov 05, 2010 at 05:44:55PM +0000, Mindaugas Rasiukevicius wrote:
> Masao Uebayashi <uebayasi%tombi.co.jp@localhost> wrote:
> > > > Those mechanical vm_page -> vm_page_md changes done in pmaps have
> > > > a valid point by itself.  Passing around vm_page pointer across
> > > > pmap functions is unnecessary.  I'd rather say wrong.  All pmap
> > > > needs is vm_page_md.
> > > > 
> > > > I'd propose to do this vm_page -> vm_page_md change in pmaps first
> > > > in HEAD and sync the branch with it, rather than revert it.
> > > 
> > > that seems a bit premature, don't you think?
> > > since you're already talking about redesigning it?
> > > 
> > > ... apparently not, you already checked it in.
> > 
> > It's not premature.  It clarifies that passing struct vm_page * to
> > pmap is unnecessary at all.  We'll need to move those MD PV data
> > structures to MI anyway.
> 
> There are ideas to move P->V tracking to MI, right.  However, you are
> starting to re-design the pmap abstraction here - that is really out of
> XIP project scope, and it definitely needs rmind-uvmplock merge first.
> 
> > > is your desire to control whether the mount accesses the device via
> > > mappings, or just to be able to see whether or not the device is being
> > > accessed via mappings?
> > 
> > Both.
> > 
> > My understanding is that mount options change only its internal behavior.
> > I don't see how MNT_LOG and MNT_XIP differ.  Mount is done only by
> > admins who know internals.  There may be cases where an XIP device
> > is much slower than RAM, and page cache is wanted.
> > 
> > I also think that mount option is the only way to configure per-mount
> > behavior.
> 
> Mount option seems quite useful to me.  Speaking of them.. I would also
> like to add MNT_DIRECTIO. :)
> 
> > > I've thought about loaning and XIP again quite a bit, but in the
> > > interest of actually sending this mail today, I'll leave this part for
> > > later too.
> > 
> > I spent a little time for this...
> > 
> > Now uvm_loan() returns to a loaner an array of pointers to struct
> > vm_page.  Which means that those struct vm_page's are shared among
> > loaner and loanee.  The owner of those pages are recorded in
> > vm_page::uanon and vm_page::uobject.
> > 
> > I'm thinking this design is broken.  At least A->A loan is impossible
> > for no reason.  I think loanee should allocate (by kmem_alloc(9))
> > a new *alias* vm_page, to keep loaning state.  Alias vm_page has
> > a pointer to its new owner, and with a pointer to the *real* vm_page
> > too.
> > 
> > uanon/uobject is also bad in that it obscures the 1:1 relationship
> > of pages and owners.  These two members should be merged into a single
> > void *pg_owner.  vm_page::loan_count is only for loaner.  Loaners
> > of read-only, wired pages don't need to remember anything.
> 
> I do not think loaning is "broken", but I see where you are coming from.
> "Owner-less" state i.e. when locker needs to resolve orphaned page brings
> some obscurity.  Also, O->A loaning where we do trylocks (since we cannot
> hold a reference on the object) is very messy.
> 
> Your idea about allocating extra meta-data in order to do the loaning
> seems counter-productive, given the whole concept of loaning.

I believe I understand loaning *now*.  I thought loaning has
direction.  It's not; someone who writes to it is penalized.

I also understand why O->O doesn't exist.

> 
> So again.. I would say it is something to revisit after XIP merge.

I'm only trying to show that my XIP is not going in a totally wrong
direction in big pictures.  Unfortunately Chuck seems suspecting yet...