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SubjectRe: [HMM v13 16/18] mm/hmm/migrate: new memory migration helper for use with device memory
On Sat, Nov 19, 2016 at 01:27:28AM +0530, Aneesh Kumar K.V wrote:
> Jérôme Glisse <jglisse@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

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