On 9/5/22 2:08 PM, David Hildenbrand wrote: > On 04.09.22 18:52, Aneesh Kumar K V wrote: >> On 9/2/22 12:02 PM, David Hildenbrand wrote: >>> On 01.09.22 20:35, Yang Shi wrote: >>>> On Thu, Sep 1, 2022 at 11:07 AM Peter Xu <peterx@xxxxxxxxxx> wrote: >>>>> >>>>> On Thu, Sep 01, 2022 at 10:50:48AM -0700, Yang Shi wrote: >>>>>> Yeah, because THP collapse does copy the data before clearing pte. If >>>>>> we want to remove pmdp_collapse_flush() by just clearing pmd, we >>>>>> should clear *AND* flush pte before copying the data IIRC. >>>>> >>>>> Yes tlb flush is still needed. IIUC the generic pmdp_collapse_flush() will >>>>> still be working (with the pte level flushing there) but it should just >>>>> start to work for all archs, so potentially we could drop the arch-specific >>>>> pmdp_collapse_flush()s, mostly the ppc impl. >>>> >>>> I'm don't know why powperpc needs to have its specific >>>> pmdp_collapse_flush() in the first place, not only the mandatory IPI >>>> broadcast, but also the specific implementation of pmd tlb flush. But >>>> anyway the IPI broadcast could be removed at least IMO. >>>> >>> >>> pmdp_collapse_flush() is overwritten on book3s only. It either translates >>> to radix__pmdp_collapse_flush() or hash__pmdp_collapse_flush(). >>> >>> >>> radix__pmdp_collapse_flush() has a comment explaining the situation: >>> >>> >>> + /* >>> + * pmdp collapse_flush need to ensure that there are no parallel gup >>> + * walk after this call. This is needed so that we can have stable >>> + * page ref count when collapsing a page. We don't allow a collapse page >>> + * if we have gup taken on the page. We can ensure that by sending IPI >>> + * because gup walk happens with IRQ disabled. >>> + */ >>> >>> >>> The comment for hash__pmdp_collapse_flush() is a bit more involved: >>> >>> /* >>> * Wait for all pending hash_page to finish. This is needed >>> * in case of subpage collapse. When we collapse normal pages >>> * to hugepage, we first clear the pmd, then invalidate all >>> * the PTE entries. The assumption here is that any low level >>> * page fault will see a none pmd and take the slow path that >>> * will wait on mmap_lock. But we could very well be in a >>> * hash_page with local ptep pointer value. Such a hash page >>> * can result in adding new HPTE entries for normal subpages. >>> * That means we could be modifying the page content as we >>> * copy them to a huge page. So wait for parallel hash_page >>> * to finish before invalidating HPTE entries. We can do this >>> * by sending an IPI to all the cpus and executing a dummy >>> * function there. >>> */ >>> >>> I'm not sure if that implies that the IPI is needed for some other hash-magic. >>> >>> Maybe Aneesh can clarify. >>> >> >> We still need the IPI for the hash. Another reason for architecture to override that >> function is to help them use the right page size when flushing the TLB. > > Thanks for clarifying. So the radix variant wouldn't need the IPI anymore, once GUP-fast is handled differently, correct? > yes. With this patch https://lkml.kernel.org/r/20220901222707.477402-1-shy828301@xxxxxxxxx we can remove the serialize_against_pte_lookup(vma->vm_mm); in radix__pmdp_collapse_flush() -aneesh
