Re: [PATCH 1/2] mm/mmu_notifier: avoid double notification when it is useless v2

From: Balbir Singh <bsingharora@gmail.com>
Date: 2017-10-19 10:53:16
Also in: linux-iommu, linux-mm, linux-next, lkml

On Thu, Oct 19, 2017 at 2:28 PM, Jerome Glisse [off-list ref] wrote:

On Thu, Oct 19, 2017 at 02:04:26PM +1100, Balbir Singh wrote:

quoted

On Mon, 16 Oct 2017 23:10:02 -0400
jglisse@redhat.com wrote:

quoted

From: J=C3=A9r=C3=B4me Glisse <redacted>

+           /*
+            * No need to call mmu_notifier_invalidate_range() as we a=

re

quoted

+            * downgrading page table protection not changing it to po=

int

quoted

+            * to a new page.
+            *
+            * See Documentation/vm/mmu_notifier.txt
+            */
            if (pmdp) {
 #ifdef CONFIG_FS_DAX_PMD
                    pmd_t pmd;

@@ -628,7 +635,6 @@ static void dax_mapping_entry_mkclean(struct addre=

ss_space *mapping,

quoted

                    pmd =3D pmd_wrprotect(pmd);
                    pmd =3D pmd_mkclean(pmd);
                    set_pmd_at(vma->vm_mm, address, pmdp, pmd);
-                   mmu_notifier_invalidate_range(vma->vm_mm, start, e=

nd);

quoted

Could the secondary TLB still see the mapping as dirty and propagate the=

 dirty bit back?

I am assuming hardware does sane thing of setting the dirty bit only
when walking the CPU page table when device does a write fault ie
once the device get a write TLB entry the dirty is set by the IOMMU
when walking the page table before returning the lookup result to the
device and that it won't be set again latter (ie propagated back
latter).

The other possibility is that the hardware things the page is writable
and already
marked dirty. It allows writes and does not set the dirty bit?

I should probably have spell that out and maybe some of the ATS/PASID
implementer did not do that.

quoted

 unlock_pmd:
                    spin_unlock(ptl);
 #endif

@@ -643,7 +649,6 @@ static void dax_mapping_entry_mkclean(struct addre=

ss_space *mapping,

quoted

                    pte =3D pte_wrprotect(pte);
                    pte =3D pte_mkclean(pte);
                    set_pte_at(vma->vm_mm, address, ptep, pte);
-                   mmu_notifier_invalidate_range(vma->vm_mm, start, e=

nd);

quoted

Ditto

quoted

 unlock_pte:
                    pte_unmap_unlock(ptep, ptl);
            }

diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier=

.h

quoted

index 6866e8126982..49c925c96b8a 100644

--- a/include/linux/mmu_notifier.h
+++ b/include/linux/mmu_notifier.h

@@ -155,7 +155,8 @@ struct mmu_notifier_ops {
     * shared page-tables, it not necessary to implement the
     * invalidate_range_start()/end() notifiers, as
     * invalidate_range() alread catches the points in time when an
-    * external TLB range needs to be flushed.
+    * external TLB range needs to be flushed. For more in depth
+    * discussion on this see Documentation/vm/mmu_notifier.txt
     *
     * The invalidate_range() function is called under the ptl
     * spin-lock and not allowed to sleep.

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index c037d3d34950..ff5bc647b51d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c

@@ -1186,8 +1186,15 @@ static int do_huge_pmd_wp_page_fallback(struct =

vm_fault *vmf, pmd_t orig_pmd,

quoted

            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 t=

quoted

+    * 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.txt
+    */
    pmdp_huge_clear_flush_notify(vma, haddr, vmf->pmd);
-   /* leave pmd empty until pte is filled */

    pgtable =3D pgtable_trans_huge_withdraw(vma->vm_mm, vmf->pmd);
    pmd_populate(vma->vm_mm, &_pmd, pgtable);

@@ -2026,8 +2033,15 @@ static void __split_huge_zero_page_pmd(struct v=

m_area_struct *vma,

quoted

    pmd_t _pmd;
    int i;

-   /* leave pmd empty until pte is filled */
-   pmdp_huge_clear_flush_notify(vma, haddr, pmd);
+   /*
+    * Leave pmd empty until pte is filled note that it is fine to del=

ay

quoted

+    * notification until mmu_notifier_invalidate_range_end() as we ar=

quoted

+    * replacing a zero pmd write protected page with a zero pte write
+    * protected page.
+    *
+    * See Documentation/vm/mmu_notifier.txt
+    */
+   pmdp_huge_clear_flush(vma, haddr, pmd);

Shouldn't the secondary TLB know if the page size changed?

It should not matter, we are talking virtual to physical on behalf
of a device against a process address space. So the hardware should
not care about the page size.

Does that not indicate how much the device can access? Could it try
to access more than what is mapped?

Moreover if any of the new 512 (assuming 2MB huge and 4K pages) zero
4K pages is replace by something new then a device TLB shootdown will
happen before the new page is set.

Only issue i can think of is if the IOMMU TLB (if there is one) or
the device TLB (you do expect that there is one) does not invalidate
TLB entry if the TLB shootdown is smaller than the TLB entry. That
would be idiotic but yes i know hardware bug.

quoted

    pgtable =3D pgtable_trans_huge_withdraw(mm, pmd);
    pmd_populate(mm, &_pmd, pgtable);

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 1768efa4c501..63a63f1b536c 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c

@@ -3254,9 +3254,14 @@ int copy_hugetlb_page_range(struct mm_struct *d=

st, struct mm_struct *src,

quoted

                    set_huge_swap_pte_at(dst, addr, dst_pte, entry, sz=

);

quoted

            } else {
                    if (cow) {
+                           /*
+                            * No need to notify as we are downgrading=

 page

quoted

+                            * table protection not changing it to poi=

nt

quoted

+                            * to a new page.
+                            *
+                            * See Documentation/vm/mmu_notifier.txt
+                            */
                            huge_ptep_set_wrprotect(src, addr, src_pte=

);

quoted

OK.. so we could get write faults on write accesses from the device.

quoted

-                           mmu_notifier_invalidate_range(src, mmun_st=

art,

quoted

-                                                              mmun_en=

d);

quoted

                    }
                    entry =3D huge_ptep_get(src_pte);
                    ptepage =3D pte_page(entry);

@@ -4288,7 +4293,12 @@ unsigned long hugetlb_change_protection(struct =

vm_area_struct *vma,

quoted

     * and that page table be reused and filled with junk.
     */
    flush_hugetlb_tlb_range(vma, start, end);
-   mmu_notifier_invalidate_range(mm, start, end);
+   /*
+    * No need to call mmu_notifier_invalidate_range() we are downgrad=

ing

quoted

+    * page table protection not changing it to point to a new page.
+    *
+    * See Documentation/vm/mmu_notifier.txt
+    */
    i_mmap_unlock_write(vma->vm_file->f_mapping);
    mmu_notifier_invalidate_range_end(mm, start, end);

diff --git a/mm/ksm.c b/mm/ksm.c
index 6cb60f46cce5..be8f4576f842 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c

@@ -1052,8 +1052,13 @@ static int write_protect_page(struct vm_area_st=

ruct *vma, struct page *page,

quoted

             * So we clear the pte and flush the tlb before the check
             * this assure us that no O_DIRECT can happen after the ch=

eck

quoted

             * or in the middle of the check.
+            *
+            * No need to notify as we are downgrading page table to r=

ead

quoted

+            * only not changing it to point to a new page.
+            *
+            * See Documentation/vm/mmu_notifier.txt
             */
-           entry =3D ptep_clear_flush_notify(vma, pvmw.address, pvmw.=

pte);

quoted

+           entry =3D ptep_clear_flush(vma, pvmw.address, pvmw.pte);
            /*
             * Check that no O_DIRECT or similar I/O is in progress on=

the

quoted

             * page

@@ -1136,7 +1141,13 @@ static int replace_page(struct vm_area_struct *=

vma, struct page *page,

quoted

    }

    flush_cache_page(vma, addr, pte_pfn(*ptep));
-   ptep_clear_flush_notify(vma, addr, ptep);
+   /*
+    * No need to notify as we are replacing a read only page with ano=

ther

quoted

+    * read only page with the same content.
+    *
+    * See Documentation/vm/mmu_notifier.txt
+    */
+   ptep_clear_flush(vma, addr, ptep);
    set_pte_at_notify(mm, addr, ptep, newpte);

    page_remove_rmap(page, false);

diff --git a/mm/rmap.c b/mm/rmap.c
index 061826278520..6b5a0f219ac0 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c

@@ -937,10 +937,15 @@ static bool page_mkclean_one(struct page *page, =

struct vm_area_struct *vma,

quoted

 #endif
            }

-           if (ret) {
-                   mmu_notifier_invalidate_range(vma->vm_mm, cstart, =

cend);

quoted

+           /*
+            * No need to call mmu_notifier_invalidate_range() as we a=

re

quoted

+            * downgrading page table protection not changing it to po=

int

quoted

+            * to a new page.
+            *
+            * See Documentation/vm/mmu_notifier.txt
+            */
+           if (ret)
                    (*cleaned)++;
-           }
    }

    mmu_notifier_invalidate_range_end(vma->vm_mm, start, end);

@@ -1424,6 +1429,10 @@ static bool try_to_unmap_one(struct page *page,=

 struct vm_area_struct *vma,

quoted

                    if (pte_soft_dirty(pteval))
                            swp_pte =3D pte_swp_mksoft_dirty(swp_pte);
                    set_pte_at(mm, pvmw.address, pvmw.pte, swp_pte);
+                   /*
+                    * No need to invalidate here it will synchronize =

on

quoted

+                    * against the special swap migration pte.
+                    */
                    goto discard;
            }

@@ -1481,6 +1490,9 @@ static bool try_to_unmap_one(struct page *page, =

struct vm_area_struct *vma,

quoted

                     * will take care of the rest.
                     */
                    dec_mm_counter(mm, mm_counter(page));
+                   /* We have to invalidate as we cleared the pte */
+                   mmu_notifier_invalidate_range(mm, address,
+                                                 address + PAGE_SIZE)=

quoted

            } else if (IS_ENABLED(CONFIG_MIGRATION) &&
                            (flags & (TTU_MIGRATION|TTU_SPLIT_FREEZE))=

) {

quoted

                    swp_entry_t entry;

@@ -1496,6 +1508,10 @@ static bool try_to_unmap_one(struct page *page,=

 struct vm_area_struct *vma,

quoted

                    if (pte_soft_dirty(pteval))
                            swp_pte =3D pte_swp_mksoft_dirty(swp_pte);
                    set_pte_at(mm, address, pvmw.pte, swp_pte);
+                   /*
+                    * No need to invalidate here it will synchronize =

on

quoted

+                    * against the special swap migration pte.
+                    */
            } else if (PageAnon(page)) {
                    swp_entry_t entry =3D { .val =3D page_private(subp=

age) };

quoted

                    pte_t swp_pte;

@@ -1507,6 +1523,8 @@ static bool try_to_unmap_one(struct page *page, =

struct vm_area_struct *vma,

quoted

                            WARN_ON_ONCE(1);
                            ret =3D false;
                            /* We have to invalidate as we cleared the=

 pte */

quoted

+                           mmu_notifier_invalidate_range(mm, address,
+                                                   address + PAGE_SIZ=

E);

quoted

                            page_vma_mapped_walk_done(&pvmw);
                            break;
                    }

@@ -1514,6 +1532,9 @@ static bool try_to_unmap_one(struct page *page, =

struct vm_area_struct *vma,

quoted

                    /* MADV_FREE page check */
                    if (!PageSwapBacked(page)) {
                            if (!PageDirty(page)) {
+                                   /* Invalidate as we cleared the pt=

e */

quoted

+                                   mmu_notifier_invalidate_range(mm,
+                                           address, address + PAGE_SI=

ZE);

quoted

                                    dec_mm_counter(mm, MM_ANONPAGES);
                                    goto discard;
                            }

@@ -1547,13 +1568,39 @@ static bool try_to_unmap_one(struct page *page=

, struct vm_area_struct *vma,

quoted

                    if (pte_soft_dirty(pteval))
                            swp_pte =3D pte_swp_mksoft_dirty(swp_pte);
                    set_pte_at(mm, address, pvmw.pte, swp_pte);
-           } else
+                   /* Invalidate as we cleared the pte */
+                   mmu_notifier_invalidate_range(mm, address,
+                                                 address + PAGE_SIZE)=

quoted

+           } else {
+                   /*
+                    * We should not need to notify here as we reach t=

his

quoted

+                    * case only from freeze_page() itself only call f=

rom

quoted

+                    * split_huge_page_to_list() so everything below m=

ust

quoted

+                    * be true:
+                    *   - page is not anonymous
+                    *   - page is locked
+                    *
+                    * So as it is a locked file back page thus it can=

not

quoted

+                    * be remove from the page cache and replace by a =

new

quoted

+                    * page before mmu_notifier_invalidate_range_end s=

o no

quoted

+                    * concurrent thread might update its page table t=

quoted

+                    * point at new page while a device still is using=

 this

quoted

+                    * page.
+                    *
+                    * See Documentation/vm/mmu_notifier.txt
+                    */
                    dec_mm_counter(mm, mm_counter_file(page));
+           }
 discard:
+           /*
+            * No need to call mmu_notifier_invalidate_range() it has =

be

quoted

+            * done above for all cases requiring it to happen under p=

age

quoted

+            * table lock before mmu_notifier_invalidate_range_end()
+            *
+            * See Documentation/vm/mmu_notifier.txt
+            */
            page_remove_rmap(subpage, PageHuge(page));
            put_page(page);
-           mmu_notifier_invalidate_range(mm, address,
-                                         address + PAGE_SIZE);
    }

    mmu_notifier_invalidate_range_end(vma->vm_mm, start, end);

Looking at the patchset, I understand the efficiency, but I am concerned
with correctness.

I am fine in holding this off from reaching Linus but only way to flush t=

his

issues out if any is to have this patch in linux-next or somewhere were t=

hey

get a chance of being tested.

Yep, I would like to see some additional testing around npu and get Alistai=
r
Popple to comment as well

Note that the second patch is always safe. I agree that this one might
not be if hardware implementation is idiotic (well that would be my
opinion and any opinion/point of view can be challenge :))


You mean the only_end variant that avoids shootdown after pmd/pte changes
that avoid the _start/_end and have just the only_end variant? That seemed
reasonable to me, but I've not tested it or evaluated it in depth

Balbir Singh.

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