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. -aneesh
