Re: CVS commit: src/sys/uvm

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

On Thu, Nov 25, 2010 at 11:32:39PM +0000, YAMAMOTO Takashi wrote:
> [ adding cc: tech-kern@ ]
> 
> hi,
> 
> > On Wed, Nov 24, 2010 at 11:26:39PM -0800, Matt Thomas wrote:
> >> 
> >> On Nov 24, 2010, at 10:47 PM, Masao Uebayashi wrote:
> >> 
> >> > On Thu, Nov 25, 2010 at 05:44:21AM +0000, YAMAMOTO Takashi wrote:
> >> >> hi,
> >> >> 
> >> >>> On Thu, Nov 25, 2010 at 04:18:25AM +0000, YAMAMOTO Takashi wrote:
> >> >>>> hi,
> >> >>>> 
> >> >>>>> Hi, thanks for review.
> >> >>>>> 
> >> >>>>> On Thu, Nov 25, 2010 at 01:58:04AM +0000, YAMAMOTO Takashi wrote:
> >> >>>>>> hi,
> >> >>>>>> 
> >> >>>>>> - what's VM_PHYSSEG_OP_PG?
> >> >>>>> 
> >> >>>>> It's to lookup vm_physseg by "struct vm_page *", relying on that
> >> >>>>> "struct vm_page *[]" is allocated linearly.  It'll be used to remove
> >> >>>>> vm_page::phys_addr as we talked some time ago.
> >> >>>> 
> >> >>>> i'm not sure if commiting this unused uncommented code now helps it.
> >> >>>> some try-and-benchmark cycles might be necessary given that
> >> >>>> vm_page <-> paddr conversion could be performace critical.
> >> >>> 
> >> >>> If you really care performance, we can directly pass "struct vm_page
> >> >>> *" to pmap_enter().
> >> >>> 
> >> >>> We're doing "struct vm_page *" -> "paddr_t" just before pmap_enter(),
> >> >>> then doing "paddr_t" -> "vm_physseg" reverse lookup again in
> >> >>> pmap_enter() to check if a given PA is managed.  What is really
> >> >>> needed here is, to lookup "struct vm_page *" -> "vm_physseg" once
> >> >>> and you'll know both paddr_t and managed or not.
> >> >> 
> >> >> i agree that the current code is not ideal in that respect.
> >> >> otoh, i'm not sure if passing vm_physseg around is a good idea.
> >> > 
> >> > It's great you share the interest.
> >> > 
> >> > I chose vm_physseg, because it was there.  I'm open to alternatives,
> >> > but I don't think you have many options...
> >> 
> >> Passing vm_page * doesn't work if the page isn't managed since there
> >> won't be a vm_page for the paddr_t.
> >> 
> >> Now passing both paddr_t and vm_page * would work and if the pointer
> >> to the vm_page, it would be an unmanaged mapping.  This also gives the
> >> access to mdpg without another lookup.
> > 
> > What if XIP'ed md(4), where physical pages are in .data (or .rodata)?
> > 
> > And don't forget that you're the one who first pointed out that
> > allocating vm_pages for XIP is a pure waste of memory. ;)
> 
> i guess matt meant "if the pointer to the vm_page is NULL,".
> 
> > 
> > I'm allocating vm_pages, only because of phys_addr and loan_count.
> > I believe vm_pages is unnecessary for read-only XIP segments.
> > Because they're read-only, and stateless.
> > 
> > I've already concluded that the current "managed or not" model
> > doesn't work for XIP.  I'm pretty sure that my vm_physseg + off_t
> > model can explain everything.  I'm rather waiting for a counter
> > example how vm_physseg doesn't work...
> 
> i guess your suggestion is too vague.
> where do you want to use vm_physseg * + off_t instead of vm_page * ?
> getpages, pmap_enter, and?  how their function prototypes would be?

The basic idea is straightforward; always allocate vm_physseg for
memories/devices.  If a vm_physseg is used as general purpose
memory, you allocate vm_page[] (as vm_physseg::pgs).  If it's
potentially mapped as cached, you allocate pvh (as vm_physseg:pvh).

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.

> any valid paddr_t value will belong to exactly one vm_phsseg?

That's the idea.  This would clarify mem(4) backend too.

Note that allocating vm_physseg for device segments is cheap.