Could you ack this discussion?
On Tue, Dec 07, 2010 at 01:19:46AM +0900, Masao Uebayashi wrote: > 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.
