Re: [PATCH 6/7] thp: Change CoW semantics for anon-THP

Kirill A. Shutemov
2020-03-28T00:43:52Z

On Fri, Mar 27, 2020 at 01:07:34PM -0700, Yang Shi wrote:
> On Fri, Mar 27, 2020 at 10:06 AM Kirill A. Shutemov
> <kirill@shutemov.name> wrote:
> >
> > Currently we have different copy-on-write semantics for anon- and
> > file-THP. For anon-THP we try to allocate huge page on the write fault,
> > but on file-THP we split PMD and allocate 4k page.
>
> I agree this seems confusing.
>
> >
> > Arguably, file-THP semantics is more desirable: we don't necessary want
> > to unshare full PMD range from the parent on the first access. This is
> > the primary reason THP is unusable for some workloads, like Redis.
> >
> > The original THP refcounting didn't allow to have PTE-mapped compound
> > pages, so we had no options, but to allocate huge page on CoW (with
> > fallback to 512 4k pages).
> >
> > The current refcounting doesn't have such limitations and we can cut a
> > lot of complex code out of fault path.
> >
> > khugepaged is now able to recover THP from such ranges if the
> > configuration allows.
>
> It looks this patch would just split the PMD then fallback to handle
> it on PTE level, it definitely simplify the code a lot. However it
> makes the use of THP depend on the productivity of khugepaged. And the
> success rate of THP allocation in khugepaged depends on defrag. But by
> default khugepaged runs at very low priority, so khugepaged defrag may
> result in priority inversion easily.

If you have a workload that may be hurt by such change, please get some
numbers. It would be interesting to see.

> For example we saw THP split in reclaim path triggered by khugepaged
> held write anon_vma lock, but the other process which was doing
> reclaim too was blocked by anon_vma lock. Then the cond_resched() in
> rmap walk would make khugepaged preempted by all other processes
> easily so khugepaged may hold the anon_vma lock for indefinite time.
> Then we saw hung tasks.

Any chance you have a reproducer? I'm not sure I follow the problem, but
it's almost 4AM, so I'm slow.

> So we have khugepaged defrag disabled for some workloads to workaround
> the priority inversion problem. So, I'm concerned some processes may
> never get THP for some our workloads with this patch applied.
>
> The priority inversion caused by khugepaged was not very usual. Just
> my two cents.
>
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> > mm/huge_memory.c | 247 +++++------------------------------------------
> > 1 file changed, 24 insertions(+), 223 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index ef6a6bcb291f..15b7a9c86b7c 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1206,262 +1206,63 @@ void huge_pmd_set_accessed(struct vm_fault *vmf, pmd_t orig_pmd)
> > spin_unlock(vmf->ptl);
> > }
> >
> > -static vm_fault_t do_huge_pmd_wp_page_fallback(struct vm_fault *vmf,
> > - pmd_t orig_pmd, struct page *page)
> > -{
> > - struct vm_area_struct *vma = vmf->vma;
> > - unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> > - struct mem_cgroup *memcg;
> > - pgtable_t pgtable;
> > - pmd_t _pmd;
> > - int i;
> > - vm_fault_t ret = 0;
> > - struct page **pages;
> > - struct mmu_notifier_range range;
> > -
> > - pages = kmalloc_array(HPAGE_PMD_NR, sizeof(struct page *),
> > - GFP_KERNEL);
> > - if (unlikely(!pages)) {
> > - ret |= VM_FAULT_OOM;
> > - goto out;
> > - }
> > -
> > - for (i = 0; i < HPAGE_PMD_NR; i++) {
> > - pages[i] = alloc_page_vma_node(GFP_HIGHUSER_MOVABLE, vma,
> > - vmf->address, page_to_nid(page));
> > - if (unlikely(!pages[i] ||
> > - mem_cgroup_try_charge_delay(pages[i], vma->vm_mm,
> > - GFP_KERNEL, &memcg, false))) {
> > - if (pages[i])
> > - put_page(pages[i]);
> > - while (--i >= 0) {
> > - memcg = (void *)page_private(pages[i]);
> > - set_page_private(pages[i], 0);
> > - mem_cgroup_cancel_charge(pages[i], memcg,
> > - false);
> > - put_page(pages[i]);
> > - }
> > - kfree(pages);
> > - ret |= VM_FAULT_OOM;
> > - goto out;
> > - }
> > - set_page_private(pages[i], (unsigned long)memcg);
> > - }
> > -
> > - for (i = 0; i < HPAGE_PMD_NR; i++) {
> > - copy_user_highpage(pages[i], page + i,
> > - haddr + PAGE_SIZE * i, vma);
> > - __SetPageUptodate(pages[i]);
> > - cond_resched();
> > - }
> > -
> > - mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
> > - haddr, haddr + HPAGE_PMD_SIZE);
> > - mmu_notifier_invalidate_range_start(&range);
> > -
> > - vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> > - if (unlikely(!pmd_same(*vmf->pmd, orig_pmd)))
> > - goto out_free_pages;
> > - VM_BUG_ON_PAGE(!PageHead(page), page);
> > -
> > - /*
> > - * Leave pmd empty until pte is filled note we must notify here as
> > - * concurrent CPU thread might write to new page before the call to
> > - * mmu_notifier_invalidate_range_end() happens which can lead to a
> > - * device seeing memory write in different order than CPU.
> > - *
> > - * See Documentation/vm/mmu_notifier.rst
> > - */
> > - pmdp_huge_clear_flush_notify(vma, haddr, vmf->pmd);
> > -
> > - pgtable = pgtable_trans_huge_withdraw(vma->vm_mm, vmf->pmd);
> > - pmd_populate(vma->vm_mm, &_pmd, pgtable);
> > -
> > - for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
> > - pte_t entry;
> > - entry = mk_pte(pages[i], vma->vm_page_prot);
> > - entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> > - memcg = (void *)page_private(pages[i]);
> > - set_page_private(pages[i], 0);
> > - page_add_new_anon_rmap(pages[i], vmf->vma, haddr, false);
> > - mem_cgroup_commit_charge(pages[i], memcg, false, false);
> > - lru_cache_add_active_or_unevictable(pages[i], vma);
> > - vmf->pte = pte_offset_map(&_pmd, haddr);
> > - VM_BUG_ON(!pte_none(*vmf->pte));
> > - set_pte_at(vma->vm_mm, haddr, vmf->pte, entry);
> > - pte_unmap(vmf->pte);
> > - }
> > - kfree(pages);
> > -
> > - smp_wmb(); /* make pte visible before pmd */
> > - pmd_populate(vma->vm_mm, vmf->pmd, pgtable);
> > - page_remove_rmap(page, true);
> > - spin_unlock(vmf->ptl);
> > -
> > - /*
> > - * No need to double call mmu_notifier->invalidate_range() callback as
> > - * the above pmdp_huge_clear_flush_notify() did already call it.
> > - */
> > - mmu_notifier_invalidate_range_only_end(&range);
> > -
> > - ret |= VM_FAULT_WRITE;
> > - put_page(page);
> > -
> > -out:
> > - return ret;
> > -
> > -out_free_pages:
> > - spin_unlock(vmf->ptl);
> > - mmu_notifier_invalidate_range_end(&range);
> > - for (i = 0; i < HPAGE_PMD_NR; i++) {
> > - memcg = (void *)page_private(pages[i]);
> > - set_page_private(pages[i], 0);
> > - mem_cgroup_cancel_charge(pages[i], memcg, false);
> > - put_page(pages[i]);
> > - }
> > - kfree(pages);
> > - goto out;
> > -}
> > -
> > vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf, pmd_t orig_pmd)
> > {
> > struct vm_area_struct *vma = vmf->vma;
> > - struct page *page = NULL, *new_page;
> > - struct mem_cgroup *memcg;
> > + struct page *page;
> > unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> > - struct mmu_notifier_range range;
> > - gfp_t huge_gfp; /* for allocation and charge */
> > - vm_fault_t ret = 0;
> >
> > vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
> > VM_BUG_ON_VMA(!vma->anon_vma, vma);
> > +
> > if (is_huge_zero_pmd(orig_pmd))
> > - goto alloc;
> > + goto fallback;
> > +
> > spin_lock(vmf->ptl);
> > - if (unlikely(!pmd_same(*vmf->pmd, orig_pmd)))
> > - goto out_unlock;
> > +
> > + if (unlikely(!pmd_same(*vmf->pmd, orig_pmd))) {
> > + spin_unlock(vmf->ptl);
> > + return 0;
> > + }
> >
> > page = pmd_page(orig_pmd);
> > VM_BUG_ON_PAGE(!PageCompound(page) || !PageHead(page), page);
> > - /*
> > - * We can only reuse the page if nobody else maps the huge page or it's
> > - * part.
> > - */
> > +
> > + /* Lock page for reuse_swap_page() */
> > if (!trylock_page(page)) {
> > get_page(page);
> > spin_unlock(vmf->ptl);
> > lock_page(page);
> > spin_lock(vmf->ptl);
> > if (unlikely(!pmd_same(*vmf->pmd, orig_pmd))) {
> > + spin_unlock(vmf->ptl);
> > unlock_page(page);
> > put_page(page);
> > - goto out_unlock;
> > + return 0;
> > }
> > put_page(page);
> > }
> > +
> > + /*
> > + * We can only reuse the page if nobody else maps the huge page or it's
> > + * part.
> > + */
> > if (reuse_swap_page(page, NULL)) {
> > pmd_t entry;
> > entry = pmd_mkyoung(orig_pmd);
> > entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> > if (pmdp_set_access_flags(vma, haddr, vmf->pmd, entry, 1))
> > update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
> > - ret |= VM_FAULT_WRITE;
> > unlock_page(page);
> > - goto out_unlock;
> > - }
> > - unlock_page(page);
> > - get_page(page);
> > - spin_unlock(vmf->ptl);
> > -alloc:
> > - if (__transparent_hugepage_enabled(vma) &&
> > - !transparent_hugepage_debug_cow()) {
> > - huge_gfp = alloc_hugepage_direct_gfpmask(vma);
> > - new_page = alloc_hugepage_vma(huge_gfp, vma, haddr, HPAGE_PMD_ORDER);
> > - } else
> > - new_page = NULL;
> > -
> > - if (likely(new_page)) {
> > - prep_transhuge_page(new_page);
> > - } else {
> > - if (!page) {
> > - split_huge_pmd(vma, vmf->pmd, vmf->address);
> > - ret |= VM_FAULT_FALLBACK;
> > - } else {
> > - ret = do_huge_pmd_wp_page_fallback(vmf, orig_pmd, page);
> > - if (ret & VM_FAULT_OOM) {
> > - split_huge_pmd(vma, vmf->pmd, vmf->address);
> > - ret |= VM_FAULT_FALLBACK;
> > - }
> > - put_page(page);
> > - }
> > - count_vm_event(THP_FAULT_FALLBACK);
> > - goto out;
> > - }
> > -
> > - if (unlikely(mem_cgroup_try_charge_delay(new_page, vma->vm_mm,
> > - huge_gfp, &memcg, true))) {
> > - put_page(new_page);
> > - split_huge_pmd(vma, vmf->pmd, vmf->address);
> > - if (page)
> > - put_page(page);
> > - ret |= VM_FAULT_FALLBACK;
> > - count_vm_event(THP_FAULT_FALLBACK);
> > - goto out;
> > - }
> > -
> > - count_vm_event(THP_FAULT_ALLOC);
> > - count_memcg_events(memcg, THP_FAULT_ALLOC, 1);
> > -
> > - if (!page)
> > - clear_huge_page(new_page, vmf->address, HPAGE_PMD_NR);
> > - else
> > - copy_user_huge_page(new_page, page, vmf->address,
> > - vma, HPAGE_PMD_NR);
> > - __SetPageUptodate(new_page);
> > -
> > - mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
> > - haddr, haddr + HPAGE_PMD_SIZE);
> > - mmu_notifier_invalidate_range_start(&range);
> > -
> > - spin_lock(vmf->ptl);
> > - if (page)
> > - put_page(page);
> > - if (unlikely(!pmd_same(*vmf->pmd, orig_pmd))) {
> > spin_unlock(vmf->ptl);
> > - mem_cgroup_cancel_charge(new_page, memcg, true);
> > - put_page(new_page);
> > - goto out_mn;
> > - } else {
> > - pmd_t entry;
> > - entry = mk_huge_pmd(new_page, vma->vm_page_prot);
> > - entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> > - pmdp_huge_clear_flush_notify(vma, haddr, vmf->pmd);
> > - page_add_new_anon_rmap(new_page, vma, haddr, true);
> > - mem_cgroup_commit_charge(new_page, memcg, false, true);
> > - lru_cache_add_active_or_unevictable(new_page, vma);
> > - set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
> > - update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
> > - if (!page) {
> > - add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
> > - } else {
> > - VM_BUG_ON_PAGE(!PageHead(page), page);
> > - page_remove_rmap(page, true);
> > - put_page(page);
> > - }
> > - ret |= VM_FAULT_WRITE;
> > + return VM_FAULT_WRITE;
> > }
> > +
> > + unlock_page(page);
> > spin_unlock(vmf->ptl);
> > -out_mn:
> > - /*
> > - * No need to double call mmu_notifier->invalidate_range() callback as
> > - * the above pmdp_huge_clear_flush_notify() did already call it.
> > - */
> > - mmu_notifier_invalidate_range_only_end(&range);
> > -out:
> > - return ret;
> > -out_unlock:
> > - spin_unlock(vmf->ptl);
> > - return ret;
> > +fallback:
> > + __split_huge_pmd(vma, vmf->pmd, vmf->address, false, NULL);
> > + return VM_FAULT_FALLBACK;
> > }
> >
> > /*
> > --
> > 2.26.0
> >
> >

--
Kirill A. Shutemov