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Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jul 01, 2025 at 09:23:05AM +0100, Ryan Roberts wrote: > >>>>> +#ifndef modify_prot_commit_ptes > >>>>> +static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, > >>>>> unsigned long addr, > >>>>> +        pte_t *ptep, pte_t old_pte, pte_t pte, unsigned int nr) > >>>>> +{ > >>>>> +    int i; > >>>>> + > >>>>> +    for (i = 0; i < nr; ++i) { > >>>>> +        ptep_modify_prot_commit(vma, addr, ptep, old_pte, pte); > >>>>> +        ptep++; > >>>> Weird place to put this increment, maybe just stick it in the for loop. > >>>> > >>>>> +        addr += PAGE_SIZE; > >>>> Same comment here. > >>> > >>> Sure. > >>> > >>>> > >>>>> +        old_pte = pte_next_pfn(old_pte); > >>>> Could be: > >>>> > >>>>         old_pte = pte; > >>>> > >>>> No? > >>> > >>> We will need to update old_pte also since that > >>> is used by powerpc in radix__ptep_modify_prot_commit(). > >> > >> I think perhaps Lorenzo has the model in his head where old_pte is the previous > >> pte in the batch. That's not the case. old_pte is the value of the pte in the > >> current position of the batch before any changes were made. pte is the new value > >> for the pte. So we need to expliticly advance the PFN in both old_pte and pte > >> each iteration round the loop. > > > > Yeah, you're right, apologies, I'd misinterpreted. > > > > I really, really, really hate how all this is implemented. This is obviously an > > mprotect() and legacy thing but it's almost designed for confusion. Not the > > fault of this series, and todo++ on improving mprotect as a whole (been on my > > list for a while...) > > Agreed. I struggled for a long time with some of the pgtable helper abstractions > to the arch and all the assumptions they make. But ultimately all Dev is trying > to do here is make some incremental improvements, following the established > patterns. Hopefully you agree that cleanups on a larger scale should be reserved > for a systematic, focussed series. Totally agree, when I mention my distaste for existing logic, see those as asides, I'm not asking the series to be blocked until that's fixed :) I'm happy for us to take Dev's changes obviously once review issues are resolved. I think my suggestion below helps get us to a good compromise (modulo mm's beautifully overloaded/confusing use of terminology :>) > > > > > So we're ultimately updating ptep (this thing that we update, of course, is > > buried in the middle of the function invocation) in: > > > > ptep_modify_prot_commit(vma, addr, ptep, old_pte, pte); > > > > We are setting *ptep++ = pte essentially (roughly speaking) right? > > Yeah, pretty much. > > The API was originally created for Xen IIRC. The problem is that the HW can > update the A/D bits asynchronously if the PTE is valid (from the HW perspective) > so the previous approach was to get_and_clear (atomic), modify, write. But that > required 2 Xen hypervisor calls per PTE. This start/commit approach allows Xen > to both avoid the get_and_clear() and batch the writes for all PTEs in a lazy > mmu batch. So hypervisor calls are reduced from 2 per PTE to 1 per lazy mmu > batch. TBH I'm no Xen expert; some of those details may be off, but big picture > is correct. Yeah, here we go again with some horror show stuff on Xen's behalf. I've played Half Life, so I already know to fear Xen ;) I believe David has _thoughts_ on this :) Again this is aside stuff. > > Anyway, arm64 doesn't care about any of that, but it does override > ptep_modify_prot_start() / ptep_modify_prot_commit() to implement an erratum > workaround. And it can benefit substantially from batching. Ack. And of course, the batching part is why we're all here! > > > > > And the arch needs to know about any bits that have changed I guess hence > > providing old_pte as well right? > > > > OK so yeah, I get it now, we're not actually advancing through ptes here, we're > > just advancing the PFN and applying the same 'template'. > > > > How about something like: > > > > static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned long addr, > > pte_t *ptep, pte_t old_pte, pte_t pte, unsigned int nr) > > { > > int i; > > > > for (i = 0; i < nr; i++, ptep++, addr += PAGE_SIZE) { > > ptep_modify_prot_commit(vma, addr, ptep, old_pte, pte); > > > > /* Advance PFN only, set same flags. */ > > old_pte = pte_next_pfn(old_pte); > > pte = pte_next_pfn(pte); > > } > > } > > > > Neatens it up a bit and makes it clear that we're effectively propagating the > > flags here. > > Yes, except we don't usually refer to the non-pfn parts of a pte as "flags". We > normally call them pgprot or prot. God knows why... Ah of course we love to do this kind of thing to oursevles :>) Dev - suggestion above then but s/flags/prot/.