On Sat, Nov 19, 2016 at 01:27:28AM +0530, Aneesh Kumar K.V wrote: > Jérôme Glisse <jgli...@redhat.com> writes: > > > [...] > > >+ > > +static int hmm_collect_walk_pmd(pmd_t *pmdp, > > + unsigned long start, > > + unsigned long end, > > + struct mm_walk *walk) > > +{ > > + struct hmm_migrate *migrate = walk->private; > > + struct mm_struct *mm = walk->vma->vm_mm; > > + unsigned long addr = start; > > + spinlock_t *ptl; > > + hmm_pfn_t *pfns; > > + int pages = 0; > > + pte_t *ptep; > > + > > +again: > > + if (pmd_none(*pmdp)) > > + return 0; > > + > > + split_huge_pmd(walk->vma, pmdp, addr); > > + if (pmd_trans_unstable(pmdp)) > > + goto again; > > + > > + pfns = &migrate->pfns[(addr - migrate->start) >> PAGE_SHIFT]; > > + ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl); > > + arch_enter_lazy_mmu_mode(); > > + > > + for (; addr < end; addr += PAGE_SIZE, pfns++, ptep++) { > > + unsigned long pfn; > > + swp_entry_t entry; > > + struct page *page; > > + hmm_pfn_t flags; > > + bool write; > > + pte_t pte; > > + > > + pte = ptep_get_and_clear(mm, addr, ptep); > > + if (!pte_present(pte)) { > > + if (pte_none(pte)) > > + continue; > > + > > + entry = pte_to_swp_entry(pte); > > + if (!is_device_entry(entry)) { > > + set_pte_at(mm, addr, ptep, pte); > > + continue; > > + } > > + > > + flags = HMM_PFN_DEVICE | HMM_PFN_UNADDRESSABLE; > > + page = device_entry_to_page(entry); > > + write = is_write_device_entry(entry); > > + pfn = page_to_pfn(page); > > + > > + if (!(page->pgmap->flags & MEMORY_MOVABLE)) { > > + set_pte_at(mm, addr, ptep, pte); > > + continue; > > + } > > + > > + } else { > > + pfn = pte_pfn(pte); > > + page = pfn_to_page(pfn); > > + write = pte_write(pte); > > + flags = is_zone_device_page(page) ? HMM_PFN_DEVICE : 0; > > + } > > + > > + /* FIXME support THP see hmm_migrate_page_check() */ > > + if (PageTransCompound(page)) > > + continue; > > + > > + *pfns = hmm_pfn_from_pfn(pfn) | HMM_PFN_MIGRATE | flags; > > + *pfns |= write ? HMM_PFN_WRITE : 0; > > + migrate->npages++; > > + get_page(page); > > + > > + if (!trylock_page(page)) { > > + set_pte_at(mm, addr, ptep, pte); > > + } else { > > + pte_t swp_pte; > > + > > + *pfns |= HMM_PFN_LOCKED; > > + > > + entry = make_migration_entry(page, write); > > + swp_pte = swp_entry_to_pte(entry); > > + if (pte_soft_dirty(pte)) > > + swp_pte = pte_swp_mksoft_dirty(swp_pte); > > + set_pte_at(mm, addr, ptep, swp_pte); > > + > > + page_remove_rmap(page, false); > > + put_page(page); > > + pages++; > > + } > > Can you explain this. What does a failure to lock means here. Also why > convert the pte to migration entries here ? We do that in try_to_unmap right ?
This an optimization for the usual case where the memory is only use in one process and that no concurrent migration/memory event is happening. Basicly if we can lock the page without waiting then we unmap it and the later call to try_to_unmap() is a no op. This is purely to optimize this common case. In short it is doing try_to_unmap() work ahead of time. Cheers, Jérôme