Re: [PATCH v2 01/14] mm: Batch-copy PTE ranges during fork()

Ryan Roberts
2023-11-28T11:00:57Z

On 28/11/2023 00:11, Barry Song wrote:
> On Mon, Nov 27, 2023 at 10:24 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 27/11/2023 05:54, Barry Song wrote:
>>>> +copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>>>> + pte_t *dst_pte, pte_t *src_pte,
>>>> + unsigned long addr, unsigned long end,
>>>> + int *rss, struct folio **prealloc)
>>>> {
>>>> struct mm_struct *src_mm = src_vma->vm_mm;
>>>> unsigned long vm_flags = src_vma->vm_flags;
>>>> pte_t pte = ptep_get(src_pte);
>>>> struct page *page;
>>>> struct folio *folio;
>>>> + int nr = 1;
>>>> + bool anon;
>>>> + bool any_dirty = pte_dirty(pte);
>>>> + int i;
>>>>
>>>> page = vm_normal_page(src_vma, addr, pte);
>>>> - if (page)
>>>> + if (page) {
>>>> folio = page_folio(page);
>>>> - if (page && folio_test_anon(folio)) {
>>>> - /*
>>>> - * If this page may have been pinned by the parent process,
>>>> - * copy the page immediately for the child so that we'll always
>>>> - * guarantee the pinned page won't be randomly replaced in the
>>>> - * future.
>>>> - */
>>>> - folio_get(folio);
>>>> - if (unlikely(page_try_dup_anon_rmap(page, false, src_vma))) {
>>>> - /* Page may be pinned, we have to copy. */
>>>> - folio_put(folio);
>>>> - return copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
>>>> - addr, rss, prealloc, page);
>>>> + anon = folio_test_anon(folio);
>>>> + nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr,
>>>> + end, pte, &any_dirty);
>>>
>>> in case we have a large folio with 16 CONTPTE basepages, and userspace
>>> do madvise(addr + 4KB * 5, DONTNEED);
>>
>> nit: if you are offsetting by 5 pages from addr, then below I think you mean
>> page0~page4 and page6~15?
>>
>>>
>>> thus, the 4th basepage of PTE becomes PTE_NONE and folio_nr_pages_cont_mapped()
>>> will return 15. in this case, we should copy page0~page3 and page5~page15.
>>
>> No I don't think folio_nr_pages_cont_mapped() will return 15; that's certainly
>> not how its intended to work. The function is scanning forwards from the current
>> pte until it finds the first pte that does not fit in the batch - either because
>> it maps a PFN that is not contiguous, or because the permissions are different
>> (although this is being relaxed a bit; see conversation with DavidH against this
>> same patch).
>>
>> So the first time through this loop, folio_nr_pages_cont_mapped() will return 5,
>> (page0~page4) then the next time through the loop we will go through the
>> !present path and process the single swap marker. Then the 3rd time through the
>> loop folio_nr_pages_cont_mapped() will return 10.
>
> one case we have met by running hundreds of real phones is as below,
>
>
> static int
> copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
> pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
> unsigned long end)
> {
> ...
> dst_pte = pte_alloc_map_lock(dst_mm, dst_pmd, addr, &dst_ptl);
> if (!dst_pte) {
> ret = -ENOMEM;
> goto out;
> }
> src_pte = pte_offset_map_nolock(src_mm, src_pmd, addr, &src_ptl);
> if (!src_pte) {
> pte_unmap_unlock(dst_pte, dst_ptl);
> /* ret == 0 */
> goto out;
> }
> spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
> orig_src_pte = src_pte;
> orig_dst_pte = dst_pte;
> arch_enter_lazy_mmu_mode();
>
> do {
> /*
> * We are holding two locks at this point - either of them
> * could generate latencies in another task on another CPU.
> */
> if (progress >= 32) {
> progress = 0;
> if (need_resched() ||
> spin_needbreak(src_ptl) || spin_needbreak(dst_ptl))
> break;
> }
> ptent = ptep_get(src_pte);
> if (pte_none(ptent)) {
> progress++;
> continue;
> }
>
> the above iteration can break when progress > =32. for example, at the
> beginning,
> if all PTEs are none, we break when progress >=32, and we break when we
> are in the 8th pte of 16PTEs which might become CONTPTE after we release
> PTL.
>
> since we are releasing PTLs, next time when we get PTL, those pte_none() might
> become pte_cont(), then are you going to copy CONTPTE from 8th pte,
> thus, immediately
> break the consistent CONPTEs rule of hardware?
>
> pte0 - pte_none
> pte1 - pte_none
> ...
> pte7 - pte_none
>
> pte8 - pte_cont
> ...
> pte15 - pte_cont
>
> so we did some modification to avoid a break in the middle of PTEs
> which can potentially
> become CONTPE.
> do {
> /*
> * We are holding two locks at this point - either of them
> * could generate latencies in another task on another CPU.
> */
> if (progress >= 32) {
> progress = 0;
> #ifdef CONFIG_CONT_PTE_HUGEPAGE
> /*
> * XXX: don't release ptl at an unligned address as
> cont_pte might form while
> * ptl is released, this causes double-map
> */
> if (!vma_is_chp_anonymous(src_vma) ||
> (vma_is_chp_anonymous(src_vma) && IS_ALIGNED(addr,
> HPAGE_CONT_PTE_SIZE)))
> #endif
> if (need_resched() ||
> spin_needbreak(src_ptl) || spin_needbreak(dst_ptl))
> break;
> }
>
> We could only reproduce the above issue by running thousands of phones.
>
> Does your code survive from this problem?

Yes I'm confident my code is safe against this; as I said before, the CONT_PTE
bit is not blindly "copied" from parent to child pte. As far as the core-mm is
concerned, there is no CONT_PTE bit; they are just regular PTEs. So the code
will see some pte_none() entries followed by some pte_present() entries. And
when calling set_ptes() on the child, the arch code will evaluate the current
state of the pgtable along with the new set_ptes() request and determine where
it should insert the CONT_PTE bit.

>
>>
>> Thanks,
>> Ryan
>>
>>>
>>> but the current code is copying page0~page14, right? unless we are immediatly
>>> split_folio to basepages in zap_pte_range(), we will have problems?
>>>
>>>> +
>>>> + for (i = 0; i < nr; i++, page++) {
>>>> + if (anon) {
>>>> + /*
>>>> + * If this page may have been pinned by the
>>>> + * parent process, copy the page immediately for
>>>> + * the child so that we'll always guarantee the
>>>> + * pinned page won't be randomly replaced in the
>>>> + * future.
>>>> + */
>>>> + if (unlikely(page_try_dup_anon_rmap(
>>>> + page, false, src_vma))) {
>>>> + if (i != 0)
>>>> + break;
>>>> + /* Page may be pinned, we have to copy. */
>>>> + return copy_present_page(
>>>> + dst_vma, src_vma, dst_pte,
>>>> + src_pte, addr, rss, prealloc,
>>>> + page);
>>>> + }
>>>> + rss[MM_ANONPAGES]++;
>>>> + VM_BUG_ON(PageAnonExclusive(page));
>>>> + } else {
>>>> + page_dup_file_rmap(page, false);
>>>> + rss[mm_counter_file(page)]++;
>>>> + }
>>>
>
> Thanks
> Barry