Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20200327170601.18563-1-kirill.shutemov@linux.intel.com>
 <20200327170601.18563-7-kirill.shutemov@linux.intel.com> <CAHbLzkqvev+kRopVuLAgoAbv1-06gB5Cczo+yC+POqzrX9CnBA@mail.gmail.com>
 <20200328004352.ghjjm7tefkcstbup@box>
In-Reply-To: <20200328004352.ghjjm7tefkcstbup@box>
From:   Yang Shi <shy828301@gmail.com>
Date:   Fri, 27 Mar 2020 18:30:20 -0700
Message-ID: <CAHbLzkq+ine1k2YhDv+3dyZDX19OYrKBhjkdFzrGVjY=1Xprhw@mail.gmail.com>
Subject: Re: [PATCH 6/7] thp: Change CoW semantics for anon-THP
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Mar 27, 2020 at 5:43 PM Kirill A. Shutemov <kirill@shutemov.name> w=
rote:
>
> 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 faul=
t,
> > > 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 wa=
nt
> > > to unshare full PMD range from the parent on the first access. This i=
s
> > > 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.

Please see the below splat:

[ 7417.871422] INFO: task /home/staragent:9088 blocked for more than
1200 seconds.
[ 7417.880501]       Tainted: G        W         4.19.91 #1
[ 7417.889015] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[ 7417.897373] /home/staragent D    0  9088      1 0x00000000
[ 7417.905017] Call Trace:
[ 7417.907898]  ? __schedule+0x3cf/0x660
[ 7417.911988]  ? core_sys_select+0x1d2/0x270
[ 7417.916562]  schedule+0x33/0xc0
[ 7417.920205]  rwsem_down_read_failed+0x130/0x190
[ 7417.925153]  ? call_rwsem_down_read_failed+0x14/0x30
[ 7417.930648]  call_rwsem_down_read_failed+0x14/0x30
[ 7417.935894]  down_read+0x1c/0x30
[ 7417.939603]  __do_page_fault+0x435/0x4d0
[ 7417.943984]  do_page_fault+0x32/0x110
[ 7417.949315]  ? page_fault+0x8/0x30
[ 7417.953189]  page_fault+0x1e/0x30
[ 7417.956974] RIP: 0033:0x8063b0
[ 7417.960582] Code: Bad RIP value.

***  the task got current->mm->mmap_sem: 0xffff88a65ed02f80
crash> bt ffff88a65ed02f80
PID: 102341  TASK: ffff88a65ed02f80  CPU: 64  COMMAND: "/home/staragent"
 #0 [ffffc90021bd73e8] __schedule at ffffffff818436af
 #1 [ffffc90021bd7480] schedule at ffffffff81843973
 #2 [ffffc90021bd7498] rwsem_down_read_failed at ffffffff818472b0
 #3 [ffffc90021bd7540] call_rwsem_down_read_failed at ffffffff818393d4
 #4 [ffffc90021bd7588] down_read at ffffffff8184662c
 #5 [ffffc90021bd7598] page_lock_anon_vma_read at ffffffff81242d9c
 #6 [ffffc90021bd75c0] rmap_walk_anon at ffffffff8124154f
 #7 [ffffc90021bd7620] page_referenced at ffffffff8124350e
 #8 [ffffc90021bd7690] shrink_page_list at ffffffff8120b2f4
 #9 [ffffc90021bd7760] shrink_inactive_list at ffffffff8120c4c6
#10 [ffffc90021bd7808] shrink_node_memcg at ffffffff8120cffa
#11 [ffffc90021bd7910] shrink_node at ffffffff8120d534
#12 [ffffc90021bd79a8] do_try_to_free_pages at ffffffff8120dac4
#13 [ffffc90021bd7a10] try_to_free_pages at ffffffff8120dec4
#14 [ffffc90021bd7aa8] __alloc_pages_slowpath at ffffffff811fa367
#15 [ffffc90021bd7bd0] __alloc_pages_nodemask at ffffffff811fb116
#16 [ffffc90021bd7c30] __do_page_cache_readahead at ffffffff812013c4
#17 [ffffc90021bd7cc8] filemap_fault at ffffffff811ef652
#18 [ffffc90021bd7d98] ext4_filemap_fault at ffffffffc030c48c [ext4]
#19 [ffffc90021bd7db0] __do_fault at ffffffff81234cf0
#20 [ffffc90021bd7dd0] __handle_mm_fault at ffffffff812337d8
#21 [ffffc90021bd7e80] handle_mm_fault at ffffffff81233adc
#22 [ffffc90021bd7eb0] __do_page_fault at ffffffff8106f084
#23 [ffffc90021bd7f20] do_page_fault at ffffffff8106f342
#24 [ffffc90021bd7f50] page_fault at ffffffff81a0112e
    RIP: 00007f67de084e72  RSP: 00007f67677fddb8  RFLAGS: 00010206
    RAX: 00007f67de084e72  RBX: 0000000002802310  RCX: 0000000000080000
    RDX: 00007f67680008c0  RSI: 0000000000080000  RDI: 00007f67680008c0
    RBP: 00007f67677fddf0   R8: 0000000000000000   R9: 00007f6768086460
    R10: 00007f67677fdde0  R11: 00000000009b3ca8  R12: 0000000002802120
    R13: 0000000002802768  R14: 0000000000000003  R15: 00007f67677fe700
    ORIG_RAX: ffffffffffffffff  CS: 0033  SS: 002b

*** staragent held mmap_sem and was doing memory reclaim, and was
trying to acquire anon_vma lock.

*** the anon_vma->root->rwsem is held by khugepaged
crash> struct task_struct 0xffff88fe6f5dc740
comm =3D =E2=80=9Ckhugepaged\000\000\000\000\000=E2=80=9D
crash> bt 0xffff88fe6f5dc740
PID: 504    TASK: ffff88fe6f5dc740  CPU: 34  COMMAND: "khugepaged"
 #0 [ffffc9000da2f630] __schedule at ffffffff818436af
 #1 [ffffc9000da2f6c8] _cond_resched at ffffffff81843c0d
 #2 [ffffc9000da2f6d8] rmap_walk_anon at ffffffff81241465
 #3 [ffffc9000da2f738] remove_migration_ptes at ffffffff8127129d
 #4 [ffffc9000da2f770] remap_page at ffffffff8127504f
 #5 [ffffc9000da2f788] split_huge_page_to_list at ffffffff8127a82a
 #6 [ffffc9000da2f810] shrink_page_list at ffffffff8120b3fc
 #7 [ffffc9000da2f8e0] shrink_inactive_list at ffffffff8120c4c6
 #8 [ffffc9000da2f988] shrink_node_memcg at ffffffff8120cffa
 #9 [ffffc9000da2fa90] shrink_node at ffffffff8120d534
#10 [ffffc9000da2fb28] do_try_to_free_pages at ffffffff8120dac4
#11 [ffffc9000da2fb90] try_to_free_pages at ffffffff8120dec4
#12 [ffffc9000da2fc28] __alloc_pages_slowpath at ffffffff811fa367
#13 [ffffc9000da2fd50] __alloc_pages_nodemask at ffffffff811fb116
#14 [ffffc9000da2fdb0] khugepaged at ffffffff8127e478
#15 [ffffc9000da2ff10] kthread at ffffffff810bcb75
#16 [ffffc9000da2ff50] ret_from_fork at ffffffff81a001cf

The system was overloaded and experiencing memory pressure. Releasing
mmap_sem before doing any blocking operations could mitigate the
problem, but the underlying priority inversion caused by khugepaged
still exists.

> > 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.

I don't have a stable reproducer.

>
> > 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 =3D vmf->vma;
> > > -       unsigned long haddr =3D vmf->address & HPAGE_PMD_MASK;
> > > -       struct mem_cgroup *memcg;
> > > -       pgtable_t pgtable;
> > > -       pmd_t _pmd;
> > > -       int i;
> > > -       vm_fault_t ret =3D 0;
> > > -       struct page **pages;
> > > -       struct mmu_notifier_range range;
> > > -
> > > -       pages =3D kmalloc_array(HPAGE_PMD_NR, sizeof(struct page *),
> > > -                             GFP_KERNEL);
> > > -       if (unlikely(!pages)) {
> > > -               ret |=3D VM_FAULT_OOM;
> > > -               goto out;
> > > -       }
> > > -
> > > -       for (i =3D 0; i < HPAGE_PMD_NR; i++) {
> > > -               pages[i] =3D 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], vm=
a->vm_mm,
> > > -                                    GFP_KERNEL, &memcg, false))) {
> > > -                       if (pages[i])
> > > -                               put_page(pages[i]);
> > > -                       while (--i >=3D 0) {
> > > -                               memcg =3D (void *)page_private(pages[=
i]);
> > > -                               set_page_private(pages[i], 0);
> > > -                               mem_cgroup_cancel_charge(pages[i], me=
mcg,
> > > -                                               false);
> > > -                               put_page(pages[i]);
> > > -                       }
> > > -                       kfree(pages);
> > > -                       ret |=3D VM_FAULT_OOM;
> > > -                       goto out;
> > > -               }
> > > -               set_page_private(pages[i], (unsigned long)memcg);
> > > -       }
> > > -
> > > -       for (i =3D 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 =3D 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 he=
re as
> > > -        * concurrent CPU thread might write to new page before the c=
all 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 =3D pgtable_trans_huge_withdraw(vma->vm_mm, vmf->pmd)=
;
> > > -       pmd_populate(vma->vm_mm, &_pmd, pgtable);
> > > -
> > > -       for (i =3D 0; i < HPAGE_PMD_NR; i++, haddr +=3D PAGE_SIZE) {
> > > -               pte_t entry;
> > > -               entry =3D mk_pte(pages[i], vma->vm_page_prot);
> > > -               entry =3D maybe_mkwrite(pte_mkdirty(entry), vma);
> > > -               memcg =3D (void *)page_private(pages[i]);
> > > -               set_page_private(pages[i], 0);
> > > -               page_add_new_anon_rmap(pages[i], vmf->vma, haddr, fal=
se);
> > > -               mem_cgroup_commit_charge(pages[i], memcg, false, fals=
e);
> > > -               lru_cache_add_active_or_unevictable(pages[i], vma);
> > > -               vmf->pte =3D 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() ca=
llback as
> > > -        * the above pmdp_huge_clear_flush_notify() did already call =
it.
> > > -        */
> > > -       mmu_notifier_invalidate_range_only_end(&range);
> > > -
> > > -       ret |=3D VM_FAULT_WRITE;
> > > -       put_page(page);
> > > -
> > > -out:
> > > -       return ret;
> > > -
> > > -out_free_pages:
> > > -       spin_unlock(vmf->ptl);
> > > -       mmu_notifier_invalidate_range_end(&range);
> > > -       for (i =3D 0; i < HPAGE_PMD_NR; i++) {
> > > -               memcg =3D (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 =3D vmf->vma;
> > > -       struct page *page =3D NULL, *new_page;
> > > -       struct mem_cgroup *memcg;
> > > +       struct page *page;
> > >         unsigned long haddr =3D vmf->address & HPAGE_PMD_MASK;
> > > -       struct mmu_notifier_range range;
> > > -       gfp_t huge_gfp;                 /* for allocation and charge =
*/
> > > -       vm_fault_t ret =3D 0;
> > >
> > >         vmf->ptl =3D 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 =3D 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 pa=
ge 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 pa=
ge or it's
> > > +        * part.
> > > +        */
> > >         if (reuse_swap_page(page, NULL)) {
> > >                 pmd_t entry;
> > >                 entry =3D pmd_mkyoung(orig_pmd);
> > >                 entry =3D 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 |=3D 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 =3D alloc_hugepage_direct_gfpmask(vma);
> > > -               new_page =3D alloc_hugepage_vma(huge_gfp, vma, haddr,=
 HPAGE_PMD_ORDER);
> > > -       } else
> > > -               new_page =3D NULL;
> > > -
> > > -       if (likely(new_page)) {
> > > -               prep_transhuge_page(new_page);
> > > -       } else {
> > > -               if (!page) {
> > > -                       split_huge_pmd(vma, vmf->pmd, vmf->address);
> > > -                       ret |=3D VM_FAULT_FALLBACK;
> > > -               } else {
> > > -                       ret =3D do_huge_pmd_wp_page_fallback(vmf, ori=
g_pmd, page);
> > > -                       if (ret & VM_FAULT_OOM) {
> > > -                               split_huge_pmd(vma, vmf->pmd, vmf->ad=
dress);
> > > -                               ret |=3D 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 |=3D 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 =3D mk_huge_pmd(new_page, vma->vm_page_prot);
> > > -               entry =3D 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, HPAG=
E_PMD_NR);
> > > -               } else {
> > > -                       VM_BUG_ON_PAGE(!PageHead(page), page);
> > > -                       page_remove_rmap(page, true);
> > > -                       put_page(page);
> > > -               }
> > > -               ret |=3D 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() ca=
llback 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