Re: [External] Re: [PATCH v20 6/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

[Muchun Song <songmuchun@bytedance.com>]

On Tue, Apr 20, 2021 at 7:20 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 4/15/21 1:40 AM, Muchun Song wrote:
> > When we free a HugeTLB page to the buddy allocator, we need to allocate
> > the vmemmap pages associated with it. However, we may not be able to
> > allocate the vmemmap pages when the system is under memory pressure. In
> > this case, we just refuse to free the HugeTLB page. This changes behavior
> > in some corner cases as listed below:
> >
> >  1) Failing to free a huge page triggered by the user (decrease nr_pages).
> >
> >     User needs to try again later.
> >
> >  2) Failing to free a surplus huge page when freed by the application.
> >
> >     Try again later when freeing a huge page next time.
> >
> >  3) Failing to dissolve a free huge page on ZONE_MOVABLE via
> >     offline_pages().
> >
> >     This can happen when we have plenty of ZONE_MOVABLE memory, but
> >     not enough kernel memory to allocate vmemmmap pages.  We may even
> >     be able to migrate huge page contents, but will not be able to
> >     dissolve the source huge page.  This will prevent an offline
> >     operation and is unfortunate as memory offlining is expected to
> >     succeed on movable zones.  Users that depend on memory hotplug
> >     to succeed for movable zones should carefully consider whether the
> >     memory savings gained from this feature are worth the risk of
> >     possibly not being able to offline memory in certain situations.
> >
> >  4) Failing to dissolve a huge page on CMA/ZONE_MOVABLE via
> >     alloc_contig_range() - once we have that handling in place. Mainly
> >     affects CMA and virtio-mem.
> >
> >     Similar to 3). virito-mem will handle migration errors gracefully.
> >     CMA might be able to fallback on other free areas within the CMA
> >     region.
> >
> > Vmemmap pages are allocated from the page freeing context. In order for
> > those allocations to be not disruptive (e.g. trigger oom killer)
> > __GFP_NORETRY is used. hugetlb_lock is dropped for the allocation
> > because a non sleeping allocation would be too fragile and it could fail
> > too easily under memory pressure. GFP_ATOMIC or other modes to access
> > memory reserves is not used because we want to prevent consuming
> > reserves under heavy hugetlb freeing.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  Documentation/admin-guide/mm/hugetlbpage.rst    |  8 +++
> >  Documentation/admin-guide/mm/memory-hotplug.rst | 13 ++++
> >  include/linux/hugetlb.h                         |  3 +
> >  include/linux/mm.h                              |  2 +
> >  mm/hugetlb.c                                    | 85 ++++++++++++++++++++-----
> >  mm/hugetlb_vmemmap.c                            | 34 ++++++++++
> >  mm/hugetlb_vmemmap.h                            |  6 ++
> >  mm/sparse-vmemmap.c                             | 75 +++++++++++++++++++++-
> >  8 files changed, 210 insertions(+), 16 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
> > index f7b1c7462991..6988895d09a8 100644
> > --- a/Documentation/admin-guide/mm/hugetlbpage.rst
> > +++ b/Documentation/admin-guide/mm/hugetlbpage.rst
> > @@ -60,6 +60,10 @@ HugePages_Surp
> >          the pool above the value in ``/proc/sys/vm/nr_hugepages``. The
> >          maximum number of surplus huge pages is controlled by
> >          ``/proc/sys/vm/nr_overcommit_hugepages``.
> > +     Note: When the feature of freeing unused vmemmap pages associated
> > +     with each hugetlb page is enabled, the number of surplus huge pages
> > +     may be temporarily larger than the maximum number of surplus huge
> > +     pages when the system is under memory pressure.
> >  Hugepagesize
> >       is the default hugepage size (in Kb).
> >  Hugetlb
> > @@ -80,6 +84,10 @@ returned to the huge page pool when freed by a task.  A user with root
> >  privileges can dynamically allocate more or free some persistent huge pages
> >  by increasing or decreasing the value of ``nr_hugepages``.
> >
> > +Note: When the feature of freeing unused vmemmap pages associated with each
> > +hugetlb page is enabled, we can fail to free the huge pages triggered by
> > +the user when ths system is under memory pressure.  Please try again later.
> > +
> >  Pages that are used as huge pages are reserved inside the kernel and cannot
> >  be used for other purposes.  Huge pages cannot be swapped out under
> >  memory pressure.
> > diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
> > index 05d51d2d8beb..c6bae2d77160 100644
> > --- a/Documentation/admin-guide/mm/memory-hotplug.rst
> > +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
> > @@ -357,6 +357,19 @@ creates ZONE_MOVABLE as following.
> >     Unfortunately, there is no information to show which memory block belongs
> >     to ZONE_MOVABLE. This is TBD.
> >
> > +   Memory offlining can fail when dissolving a free huge page on ZONE_MOVABLE
> > +   and the feature of freeing unused vmemmap pages associated with each hugetlb
> > +   page is enabled.
> > +
> > +   This can happen when we have plenty of ZONE_MOVABLE memory, but not enough
> > +   kernel memory to allocate vmemmmap pages.  We may even be able to migrate
> > +   huge page contents, but will not be able to dissolve the source huge page.
> > +   This will prevent an offline operation and is unfortunate as memory offlining
> > +   is expected to succeed on movable zones.  Users that depend on memory hotplug
> > +   to succeed for movable zones should carefully consider whether the memory
> > +   savings gained from this feature are worth the risk of possibly not being
> > +   able to offline memory in certain situations.
> > +
> >  .. note::
> >     Techniques that rely on long-term pinnings of memory (especially, RDMA and
> >     vfio) are fundamentally problematic with ZONE_MOVABLE and, therefore, memory
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index 0abed7e766b8..6e970a7d3480 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -525,6 +525,7 @@ unsigned long hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
> >   *   code knows it has only reference.  All other examinations and
> >   *   modifications require hugetlb_lock.
> >   * HPG_freed - Set when page is on the free lists.
> > + * HPG_vmemmap_optimized - Set when the vmemmap pages of the page are freed.
> >   *   Synchronization: hugetlb_lock held for examination and modification.
>
> I like the per-page flag.  In previous versions of the series, you just
> checked the free_vmemmap_pages_per_hpage() to determine if vmemmmap
> should be allocated.  Is there any change in functionality that makes is
> necessary to set the flag in each page, or is it mostly for flexibility
> going forward?

Actually, only the routine of dissolving the page cares whether
the page is on the buddy free list when update_and_free_page
returns. But we cannot change the return type of the
update_and_free_page (e.g. change return type from 'void' to 'int').
Why? If the hugepage is freed through a kworker, we cannot
know the return value when update_and_free_page returns.
So adding a return value seems odd.

In the dissolving routine, We can allocate vmemmap pages first,
if it is successful, then we can make sure that
update_and_free_page can successfully free page. So I need
some stuff to mark the page which does not need to allocate
vmemmap pages.

On the surface, we seem to have a straightforward method
to do this.

Add a new parameter 'alloc_vmemmap' to update_and_free_page() to
indicate that the caller is already allocated the vmemmap pages.
update_and_free_page() do not need to allocate. Just like below.

   void update_and_free_page(struct hstate *h, struct page *page, bool atomic,
           bool alloc_vmemmap)
   {
       if (alloc_vmemmap)
           // allocate vmemmap pages
   }

But if the page is freed through a kworker. How to pass
'alloc_vmemmap' to the kworker? We can embed this
information into the per-page flag. So if we introduce
HPG_vmemmap_optimized, the parameter of
alloc_vmemmap is also necessary.

So it seems that introducing HPG_vmemmap_optimized is
a good choice.

>
> >   */
> >  enum hugetlb_page_flags {
> > @@ -532,6 +533,7 @@ enum hugetlb_page_flags {
> >       HPG_migratable,
> >       HPG_temporary,
> >       HPG_freed,
> > +     HPG_vmemmap_optimized,
> >       __NR_HPAGEFLAGS,
> >  };
> >
> > @@ -577,6 +579,7 @@ HPAGEFLAG(RestoreReserve, restore_reserve)
> >  HPAGEFLAG(Migratable, migratable)
> >  HPAGEFLAG(Temporary, temporary)
> >  HPAGEFLAG(Freed, freed)
> > +HPAGEFLAG(VmemmapOptimized, vmemmap_optimized)
> >
> >  #ifdef CONFIG_HUGETLB_PAGE
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index a4d160ddb749..d0854828bb9c 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -3048,6 +3048,8 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
> >
> >  void vmemmap_remap_free(unsigned long start, unsigned long end,
> >                       unsigned long reuse);
> > +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> > +                     unsigned long reuse, gfp_t gfp_mask);
> >
> >  void *sparse_buffer_alloc(unsigned long size);
> >  struct page * __populate_section_memmap(unsigned long pfn,
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index eeb8f5480170..1c37f0098e00 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1376,6 +1376,34 @@ static void remove_hugetlb_page(struct hstate *h, struct page *page,
> >       h->nr_huge_pages_node[nid]--;
> >  }
> >
> > +static void add_hugetlb_page(struct hstate *h, struct page *page,
> > +                          bool adjust_surplus)
> > +{
>
> We need to be a bit careful with hugepage specific flags that may be
> set.  The routine remove_hugetlb_page which is called for 'page' before
> this routine will not clear any of the hugepage specific flags.  If the
> calling path goes through free_huge_page, most but not all flags are
> cleared.
>
> We had a discussion about clearing the page->private field in Oscar's
> series.  In the case of 'new' pages we can assume page->private is
> cleared, but perhaps we should not make that assumption here.  Since we
> hope to rarely call this routine, it might be safer to do something
> like:
>
>         set_page_private(page, 0);
>         SetHPageVmemmapOptimized(page);

Agree. Thanks for your reminder. I will fix this.

>
> > +     int nid = page_to_nid(page);
> > +
> > +     lockdep_assert_held(&hugetlb_lock);
> > +
> > +     INIT_LIST_HEAD(&page->lru);
> > +     h->nr_huge_pages++;
> > +     h->nr_huge_pages_node[nid]++;
> > +
> > +     if (adjust_surplus) {
> > +             h->surplus_huge_pages++;
> > +             h->surplus_huge_pages_node[nid]++;
> > +     }
> > +
> > +     set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> > +
> > +     /*
> > +      * The refcount can possibly be increased by memory-failure or
> > +      * soft_offline handlers.
> > +      */
> > +     if (likely(put_page_testzero(page))) {
>
> In the existing code there is no such test.  Is the need for the test
> because of something introduced in the new code?

No.

> Or, should this test be in the existing code?

Yes. gather_surplus_pages should be fixed. I can fix it
in a separate patch.

The possible bad scenario:

CPU0:                           CPU1:
                                set_compound_page_dtor(HUGETLB_PAGE_DTOR);
memory_failure_hugetlb
  get_hwpoison_page
    __get_hwpoison_page
      get_page_unless_zero
                                put_page_testzero()

  put_page(page)


More details and discussion can refer to:

https://lore.kernel.org/linux-doc/CAMZfGtVRSBkKe=tKAKLY8dp_hywotq3xL+EJZNjXuSKt3HK3bQ@mail.gmail.com/

>
> Sorry, I am not seeing why this is needed.
>
> > +             arch_clear_hugepage_flags(page);
> > +             enqueue_huge_page(h, page);
> > +     }
> > +}
> > +
> >  static void __update_and_free_page(struct hstate *h, struct page *page)
> >  {
> >       int i;
> > @@ -1384,6 +1412,18 @@ static void __update_and_free_page(struct hstate *h, struct page *page)
> >       if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> >               return;
> >
> > +     if (alloc_huge_page_vmemmap(h, page)) {
> > +             spin_lock_irq(&hugetlb_lock);
> > +             /*
> > +              * If we cannot allocate vmemmap pages, just refuse to free the
> > +              * page and put the page back on the hugetlb free list and treat
> > +              * as a surplus page.
> > +              */
> > +             add_hugetlb_page(h, page, true);
> > +             spin_unlock_irq(&hugetlb_lock);
> > +             return;
> > +     }
> > +
> >       for (i = 0; i < pages_per_huge_page(h);
> >            i++, subpage = mem_map_next(subpage, page, i)) {
> >               subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
> > @@ -1444,7 +1484,7 @@ static inline void flush_free_hpage_work(struct hstate *h)
> >  static void update_and_free_page(struct hstate *h, struct page *page,
> >                                bool atomic)
> >  {
> > -     if (!free_vmemmap_pages_per_hpage(h) || !atomic) {
> > +     if (!HPageVmemmapOptimized(page) || !atomic) {
> >               __update_and_free_page(h, page);
> >               return;
> >       }
>
> When update_and_free_pages_bulk was added it was done to avoid
> lock/unlock cycles with each page.  At the time, I thought about the
> addition of code to allocate vmmemmap, and the possibility that those
> allocations could fail.  I thought it might make sense to perhaps
> process the pages one at a time so that we could quit at the first
> allocation failure.  After more thought, I think it is best to leave the
> code to do bulk operations as you have done above.  Why?
> - Just because one allocation fails does not mean the next will fail.
>   It is possible the allocations could be from different nodes/zones.
> - We will still need to put the requested number of pages into surplus
>   state.
>
> I am not suggesting you change anything.  Just wanted to share my
> thoughts in case someone thought otherwise.
>
> > @@ -1790,10 +1830,14 @@ static struct page *remove_pool_huge_page(struct hstate *h,
> >   * nothing for in-use hugepages and non-hugepages.
> >   * This function returns values like below:
> >   *
> > - *  -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> > - *          (allocated or reserved.)
> > - *       0: successfully dissolved free hugepages or the page is not a
> > - *          hugepage (considered as already dissolved)
> > + *  -ENOMEM: failed to allocate vmemmap pages to free the freed hugepages
> > + *           when the system is under memory pressure and the feature of
> > + *           freeing unused vmemmap pages associated with each hugetlb page
> > + *           is enabled.
> > + *  -EBUSY:  failed to dissolved free hugepages or the hugepage is in-use
> > + *           (allocated or reserved.)
> > + *       0:  successfully dissolved free hugepages or the page is not a
> > + *           hugepage (considered as already dissolved)
> >   */
> >  int dissolve_free_huge_page(struct page *page)
> >  {
> > @@ -1835,19 +1879,30 @@ int dissolve_free_huge_page(struct page *page)
> >                       goto retry;
> >               }
> >
> > -             /*
> > -              * Move PageHWPoison flag from head page to the raw error page,
> > -              * which makes any subpages rather than the error page reusable.
> > -              */
> > -             if (PageHWPoison(head) && page != head) {
> > -                     SetPageHWPoison(page);
> > -                     ClearPageHWPoison(head);
> > -             }
> >               remove_hugetlb_page(h, page, false);
> >               h->max_huge_pages--;
> >               spin_unlock_irq(&hugetlb_lock);
> > -             update_and_free_page(h, head, false);
> > -             return 0;
> > +
> > +             rc = alloc_huge_page_vmemmap(h, page);
> > +             if (!rc) {
> > +                     /*
> > +                      * Move PageHWPoison flag from head page to the raw
> > +                      * error page, which makes any subpages rather than
> > +                      * the error page reusable.
> > +                      */
> > +                     if (PageHWPoison(head) && page != head) {
> > +                             SetPageHWPoison(page);
> > +                             ClearPageHWPoison(head);
> > +                     }
> > +                     update_and_free_page(h, head, false);
> > +             } else {
> > +                     spin_lock_irq(&hugetlb_lock);
> > +                     add_hugetlb_page(h, page, false);
> > +                     h->max_huge_pages++;
> > +                     spin_unlock_irq(&hugetlb_lock);
> > +             }
> > +
> > +             return rc;
> >       }
> >  out:
> >       spin_unlock_irq(&hugetlb_lock);
>
> Changes in the files below have not changed in any significant way
> since the previous version.  The code looks good to me, but I would
> like to see if there are comments from others.

Thanks for your review. :-)

>
> Thanks,
> --
> Mike Kravetz
>
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index cb28c5b6c9ff..a897c7778246 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -185,6 +185,38 @@ static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> >       return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> >  }
> >
> > +/*
> > + * Previously discarded vmemmap pages will be allocated and remapping
> > + * after this function returns zero.
> > + */
> > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> > +{
> > +     int ret;
> > +     unsigned long vmemmap_addr = (unsigned long)head;
> > +     unsigned long vmemmap_end, vmemmap_reuse;
> > +
> > +     if (!HPageVmemmapOptimized(head))
> > +             return 0;
> > +
> > +     vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> > +     vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> > +     vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> > +     /*
> > +      * The pages which the vmemmap virtual address range [@vmemmap_addr,
> > +      * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> > +      * the range is mapped to the page which @vmemmap_reuse is mapped to.
> > +      * When a HugeTLB page is freed to the buddy allocator, previously
> > +      * discarded vmemmap pages must be allocated and remapping.
> > +      */
> > +     ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> > +                               GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
> > +
> > +     if (!ret)
> > +             ClearHPageVmemmapOptimized(head);
> > +
> > +     return ret;
> > +}
> > +
> >  void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> >  {
> >       unsigned long vmemmap_addr = (unsigned long)head;
> > @@ -203,4 +235,6 @@ void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> >        * which the range [@vmemmap_addr, @vmemmap_end] is mapped to.
> >        */
> >       vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse);
> > +
> > +     SetHPageVmemmapOptimized(head);
> >  }
> > diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> > index 01f8637adbe0..a37771b0b82a 100644
> > --- a/mm/hugetlb_vmemmap.h
> > +++ b/mm/hugetlb_vmemmap.h
> > @@ -11,6 +11,7 @@
> >  #include <linux/hugetlb.h>
> >
> >  #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head);
> >  void free_huge_page_vmemmap(struct hstate *h, struct page *head);
> >
> >  /*
> > @@ -25,6 +26,11 @@ static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> >       return 0;
> >  }
> >  #else
> > +static inline int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> > +{
> > +     return 0;
> > +}
> > +
> >  static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> >  {
> >  }
> > diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> > index 7d40b5bd7046..693de0aec7a8 100644
> > --- a/mm/sparse-vmemmap.c
> > +++ b/mm/sparse-vmemmap.c
> > @@ -40,7 +40,8 @@
> >   * @remap_pte:               called for each lowest-level entry (PTE).
> >   * @reuse_page:              the page which is reused for the tail vmemmap pages.
> >   * @reuse_addr:              the virtual address of the @reuse_page page.
> > - * @vmemmap_pages:   the list head of the vmemmap pages that can be freed.
> > + * @vmemmap_pages:   the list head of the vmemmap pages that can be freed
> > + *                   or is mapped from.
> >   */
> >  struct vmemmap_remap_walk {
> >       void (*remap_pte)(pte_t *pte, unsigned long addr,
> > @@ -224,6 +225,78 @@ void vmemmap_remap_free(unsigned long start, unsigned long end,
> >       free_vmemmap_page_list(&vmemmap_pages);
> >  }
> >
> > +static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
> > +                             struct vmemmap_remap_walk *walk)
> > +{
> > +     pgprot_t pgprot = PAGE_KERNEL;
> > +     struct page *page;
> > +     void *to;
> > +
> > +     BUG_ON(pte_page(*pte) != walk->reuse_page);
> > +
> > +     page = list_first_entry(walk->vmemmap_pages, struct page, lru);
> > +     list_del(&page->lru);
> > +     to = page_to_virt(page);
> > +     copy_page(to, (void *)walk->reuse_addr);
> > +
> > +     set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
> > +}
> > +
> > +static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
> > +                                gfp_t gfp_mask, struct list_head *list)
> > +{
> > +     unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
> > +     int nid = page_to_nid((struct page *)start);
> > +     struct page *page, *next;
> > +
> > +     while (nr_pages--) {
> > +             page = alloc_pages_node(nid, gfp_mask, 0);
> > +             if (!page)
> > +                     goto out;
> > +             list_add_tail(&page->lru, list);
> > +     }
> > +
> > +     return 0;
> > +out:
> > +     list_for_each_entry_safe(page, next, list, lru)
> > +             __free_pages(page, 0);
> > +     return -ENOMEM;
> > +}
> > +
> > +/**
> > + * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
> > + *                    to the page which is from the @vmemmap_pages
> > + *                    respectively.
> > + * @start:   start address of the vmemmap virtual address range that we want
> > + *           to remap.
> > + * @end:     end address of the vmemmap virtual address range that we want to
> > + *           remap.
> > + * @reuse:   reuse address.
> > + * @gpf_mask:        GFP flag for allocating vmemmap pages.
> > + */
> > +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> > +                     unsigned long reuse, gfp_t gfp_mask)
> > +{
> > +     LIST_HEAD(vmemmap_pages);
> > +     struct vmemmap_remap_walk walk = {
> > +             .remap_pte      = vmemmap_restore_pte,
> > +             .reuse_addr     = reuse,
> > +             .vmemmap_pages  = &vmemmap_pages,
> > +     };
> > +
> > +     /* See the comment in the vmemmap_remap_free(). */
> > +     BUG_ON(start - reuse != PAGE_SIZE);
> > +
> > +     might_sleep_if(gfpflags_allow_blocking(gfp_mask));
> > +
> > +     if (alloc_vmemmap_page_list(start, end, gfp_mask, &vmemmap_pages))
> > +             return -ENOMEM;
> > +
> > +     vmemmap_remap_range(reuse, end, &walk);
> > +
> > +     return 0;
> > +}
> > +
> >  /*
> >   * Allocate a block of memory to be used to back the virtual memory map
> >   * or to back the page tables that are used to create the mapping.
> >