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Date: Fri, 7 Jun 2024 16:03:05 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Isaku Yamahata <isaku.yamahata@...el.com>, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org, michael.roth@....com, isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH 09/11] KVM: guest_memfd: Add interface for populating gmem
pages with user data
SNP folks and/or Paolo, what's the plan for this? I don't see how what's sitting
in kvm/next can possibly be correct without conditioning population on the folio
being !uptodate.
On Fri, Apr 26, 2024, Sean Christopherson wrote:
> On Fri, Apr 26, 2024, Paolo Bonzini wrote:
> > On Thu, Apr 25, 2024 at 6:00 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > > On Thu, Apr 25, 2024, Paolo Bonzini wrote:
> > > > On Thu, Apr 25, 2024 at 3:12 AM Isaku Yamahata <isaku.yamahata@...el.com> wrote:
> > > > > > > get_user_pages_fast(source addr)
> > > > > > > read_lock(mmu_lock)
> > > > > > > kvm_tdp_mmu_get_walk_private_pfn(vcpu, gpa, &pfn);
> > > > > > > if the page table doesn't map gpa, error.
> > > > > > > TDH.MEM.PAGE.ADD()
> > > > > > > TDH.MR.EXTEND()
> > > > > > > read_unlock(mmu_lock)
> > > > > > > put_page()
> > > > > >
> > > > > > Hmm, KVM doesn't _need_ to use invalidate_lock to protect against guest_memfd
> > > > > > invalidation, but I also don't see why it would cause problems.
> > > >
> > > > The invalidate_lock is only needed to operate on the guest_memfd, but
> > > > it's a rwsem so there are no risks of lock inversion.
> > > >
> > > > > > I.e. why not
> > > > > > take mmu_lock() in TDX's post_populate() implementation?
> > > > >
> > > > > We can take the lock. Because we have already populated the GFN of guest_memfd,
> > > > > we need to make kvm_gmem_populate() not pass FGP_CREAT_ONLY. Otherwise we'll
> > > > > get -EEXIST.
> > > >
> > > > I don't understand why TDH.MEM.PAGE.ADD() cannot be called from the
> > > > post-populate hook. Can the code for TDH.MEM.PAGE.ADD be shared
> > > > between the memory initialization ioctl and the page fault hook in
> > > > kvm_x86_ops?
> > >
> > > Ah, because TDX is required to pre-fault the memory to establish the S-EPT walk,
> > > and pre-faulting means guest_memfd()
> > >
> > > Requiring that guest_memfd not have a page when initializing the guest image
> > > seems wrong, i.e. I don't think we want FGP_CREAT_ONLY. And not just because I
> > > am a fan of pre-faulting, I think the semantics are bad.
> >
> > Ok, fair enough. I wanted to do the once-only test in common code but since
> > SEV code checks for the RMP I can remove that. One less headache.
>
> I definitely don't object to having a check in common code, and I'd be in favor
> of removing the RMP checks if possible, but tracking needs to be something more
> explicit in guest_memfd.
>
> *sigh*
>
> I even left behind a TODO for this exact thing, and y'all didn't even wave at it
> as you flew by :-)
>
> /*
> * Use the up-to-date flag to track whether or not the memory has been
> * zeroed before being handed off to the guest. There is no backing
> * storage for the memory, so the folio will remain up-to-date until
> * it's removed.
> *
> * TODO: Skip clearing pages when trusted firmware will do it when <==========================
> * assigning memory to the guest.
> */
> if (!folio_test_uptodate(folio)) {
> unsigned long nr_pages = folio_nr_pages(folio);
> unsigned long i;
>
> for (i = 0; i < nr_pages; i++)
> clear_highpage(folio_page(folio, i));
>
> folio_mark_uptodate(folio);
> }
>
> if (prepare) {
> int r = kvm_gmem_prepare_folio(inode, index, folio);
> if (r < 0) {
> folio_unlock(folio);
> folio_put(folio);
> return ERR_PTR(r);
> }
> }
>
> Compile tested only (and not even fully as I didn't bother defining
> CONFIG_HAVE_KVM_GMEM_INITIALIZE), but I think this is the basic gist.
>
> 8< --------------------------------
>
> // SPDX-License-Identifier: GPL-2.0
> #include <linux/backing-dev.h>
> #include <linux/falloc.h>
> #include <linux/kvm_host.h>
> #include <linux/pagemap.h>
> #include <linux/anon_inodes.h>
>
> #include "kvm_mm.h"
>
> struct kvm_gmem {
> struct kvm *kvm;
> struct xarray bindings;
> struct list_head entry;
> };
>
> static int kvm_gmem_initialize_folio(struct kvm *kvm, struct folio *folio,
> pgoff_t index, void __user *src,
> void *opaque)
> {
> #ifdef CONFIG_HAVE_KVM_GMEM_INITIALIZE
> return kvm_arch_gmem_initialize(kvm, folio, index, src, opaque);
> #else
> unsigned long nr_pages = folio_nr_pages(folio);
> unsigned long i;
>
> if (WARN_ON_ONCE(src))
> return -EIO;
>
> for (i = 0; i < nr_pages; i++)
> clear_highpage(folio_file_page(folio, index + i));
> #endif
> return 0;
> }
>
>
> static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
> {
> return gfn - slot->base_gfn + slot->gmem.pgoff;
> }
>
>
> static inline struct file *kvm_gmem_get_file(struct kvm_memory_slot *slot)
> {
> /*
> * Do not return slot->gmem.file if it has already been closed;
> * there might be some time between the last fput() and when
> * kvm_gmem_release() clears slot->gmem.file, and you do not
> * want to spin in the meanwhile.
> */
> return get_file_active(&slot->gmem.file);
> }
>
> static struct folio *__kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
> {
> fgf_t fgp_flags = FGP_LOCK | FGP_ACCESSED | FGP_CREAT;
> struct folio *folio;
>
> /*
> * The caller is responsible for managing the up-to-date flag (or not),
> * as the memory doesn't need to be initialized until it's actually
> * mapped into the guest. Waiting to initialize memory is necessary
> * for VM types where the memory can be initialized exactly once.
> *
> * Ignore accessed, referenced, and dirty flags. The memory is
> * unevictable and there is no storage to write back to.
> *
> * TODO: Support huge pages.
> */
> folio = __filemap_get_folio(inode->i_mapping, index, fgp_flags,
> mapping_gfp_mask(inode->i_mapping));
>
> if (folio_test_hwpoison(folio)) {
> folio_unlock(folio);
> return ERR_PTR(-EHWPOISON);
> }
> return folio;
> }
>
> static struct folio *kvm_gmem_get_folio(struct file *file,
> struct kvm_memory_slot *slot,
> gfn_t gfn)
> {
> pgoff_t index = kvm_gmem_get_index(slot, gfn);
> struct kvm_gmem *gmem = file->private_data;
> struct inode *inode;
>
> if (file != slot->gmem.file) {
> WARN_ON_ONCE(slot->gmem.file);
> return ERR_PTR(-EFAULT);
> }
>
> gmem = file->private_data;
> if (xa_load(&gmem->bindings, index) != slot) {
> WARN_ON_ONCE(xa_load(&gmem->bindings, index));
> return ERR_PTR(-EIO);
> }
>
> inode = file_inode(file);
>
> /*
> * The caller is responsible for managing the up-to-date flag (or not),
> * as the memory doesn't need to be initialized until it's actually
> * mapped into the guest. Waiting to initialize memory is necessary
> * for VM types where the memory can be initialized exactly once.
> *
> * Ignore accessed, referenced, and dirty flags. The memory is
> * unevictable and there is no storage to write back to.
> *
> * TODO: Support huge pages.
> */
> return __kvm_gmem_get_folio(inode, index);
> }
>
> int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> gfn_t gfn, kvm_pfn_t *pfn, int *max_order)
> {
> pgoff_t index = kvm_gmem_get_index(slot, gfn);
> struct file *file = kvm_gmem_get_file(slot);
> struct folio *folio;
> struct page *page;
>
> if (!file)
> return -EFAULT;
>
> folio = kvm_gmem_get_folio(file, slot, gfn);
> if (IS_ERR(folio))
> goto out;
>
> if (!folio_test_uptodate(folio)) {
> kvm_gmem_initialize_folio(kvm, folio, index, NULL, NULL);
> folio_mark_uptodate(folio);
> }
>
> page = folio_file_page(folio, index);
>
> *pfn = page_to_pfn(page);
> if (max_order)
> *max_order = 0;
>
> out:
> fput(file);
> return IS_ERR(folio) ? PTR_ERR(folio) : 0;
> }
> EXPORT_SYMBOL_GPL(kvm_gmem_get_pfn);
>
> long kvm_gmem_populate(struct kvm *kvm, gfn_t base_gfn, void __user *base_src,
> long npages, void *opaque)
> {
> struct kvm_memory_slot *slot;
> struct file *file;
> int ret = 0, max_order;
> long i;
>
> lockdep_assert_held(&kvm->slots_lock);
> if (npages < 0)
> return -EINVAL;
>
> slot = gfn_to_memslot(kvm, base_gfn);
> if (!kvm_slot_can_be_private(slot))
> return -EINVAL;
>
> file = kvm_gmem_get_file(slot);
> if (!file)
> return -EFAULT;
>
> filemap_invalidate_lock(file->f_mapping);
>
> npages = min_t(ulong, slot->npages - (base_gfn - slot->base_gfn), npages);
> for (i = 0; i < npages; i += (1 << max_order)) {
> void __user *src = base_src + i * PAGE_SIZE;
> gfn_t gfn = base_gfn + i;
> pgoff_t index = kvm_gmem_get_index(slot, gfn);
> struct folio *folio;
>
> folio = kvm_gmem_get_folio(file, slot, gfn);
> if (IS_ERR(folio)) {
> ret = PTR_ERR(folio);
> break;
> }
>
> if (folio_test_uptodate(folio)) {
> folio_put(folio);
> ret = -EEXIST;
> break;
> }
>
> kvm_gmem_initialize_folio(kvm, folio, index, src, opaque);
> folio_unlock(folio);
> folio_put(folio);
> }
>
> filemap_invalidate_unlock(file->f_mapping);
>
> fput(file);
> return ret && !i ? ret : i;
> }
> EXPORT_SYMBOL_GPL(kvm_gmem_populate);
>
> static void kvm_gmem_invalidate_begin(struct kvm_gmem *gmem, pgoff_t start,
> pgoff_t end)
> {
> bool flush = false, found_memslot = false;
> struct kvm_memory_slot *slot;
> struct kvm *kvm = gmem->kvm;
> unsigned long index;
>
> xa_for_each_range(&gmem->bindings, index, slot, start, end - 1) {
> pgoff_t pgoff = slot->gmem.pgoff;
>
> struct kvm_gfn_range gfn_range = {
> .start = slot->base_gfn + max(pgoff, start) - pgoff,
> .end = slot->base_gfn + min(pgoff + slot->npages, end) - pgoff,
> .slot = slot,
> .may_block = true,
> };
>
> if (!found_memslot) {
> found_memslot = true;
>
> KVM_MMU_LOCK(kvm);
> kvm_mmu_invalidate_begin(kvm);
> }
>
> flush |= kvm_mmu_unmap_gfn_range(kvm, &gfn_range);
> }
>
> if (flush)
> kvm_flush_remote_tlbs(kvm);
>
> if (found_memslot)
> KVM_MMU_UNLOCK(kvm);
> }
>
> static void kvm_gmem_invalidate_end(struct kvm_gmem *gmem, pgoff_t start,
> pgoff_t end)
> {
> struct kvm *kvm = gmem->kvm;
>
> if (xa_find(&gmem->bindings, &start, end - 1, XA_PRESENT)) {
> KVM_MMU_LOCK(kvm);
> kvm_mmu_invalidate_end(kvm);
> KVM_MMU_UNLOCK(kvm);
> }
> }
>
> static long __kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
> {
> struct list_head *gmem_list = &inode->i_mapping->i_private_list;
> pgoff_t start = offset >> PAGE_SHIFT;
> pgoff_t end = (offset + len) >> PAGE_SHIFT;
> struct kvm_gmem *gmem;
> list_for_each_entry(gmem, gmem_list, entry)
> kvm_gmem_invalidate_begin(gmem, start, end);
>
> truncate_inode_pages_range(inode->i_mapping, offset, offset + len - 1);
>
> list_for_each_entry(gmem, gmem_list, entry)
> kvm_gmem_invalidate_end(gmem, start, end);
>
> return 0;
> }
>
> static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
> {
> int r;
>
> /*
> * Bindings must be stable across invalidation to ensure the start+end
> * are balanced.
> */
> filemap_invalidate_lock(inode->i_mapping);
> r = __kvm_gmem_punch_hole(inode, offset, len);
> filemap_invalidate_unlock(inode->i_mapping);
> return r;
> }
>
> static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len)
> {
> struct address_space *mapping = inode->i_mapping;
> pgoff_t start, index, end;
> int r;
>
> /* Dedicated guest is immutable by default. */
> if (offset + len > i_size_read(inode))
> return -EINVAL;
>
> filemap_invalidate_lock_shared(mapping);
>
> start = offset >> PAGE_SHIFT;
> end = (offset + len) >> PAGE_SHIFT;
>
> r = 0;
> for (index = start; index < end; ) {
> struct folio *folio;
>
> if (signal_pending(current)) {
> r = -EINTR;
> break;
> }
>
> folio = __kvm_gmem_get_folio(inode, index);
> if (IS_ERR(folio)) {
> r = PTR_ERR(folio);
> break;
> }
>
> index = folio_next_index(folio);
>
> folio_unlock(folio);
> folio_put(folio);
>
> /* 64-bit only, wrapping the index should be impossible. */
> if (WARN_ON_ONCE(!index))
> break;
>
> cond_resched();
> }
>
> filemap_invalidate_unlock_shared(mapping);
>
> return r;
> }
>
> static long kvm_gmem_fallocate(struct file *file, int mode, loff_t offset,
> loff_t len)
> {
> int ret;
>
> if (!(mode & FALLOC_FL_KEEP_SIZE))
> return -EOPNOTSUPP;
>
> if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
> return -EOPNOTSUPP;
>
> if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len))
> return -EINVAL;
>
> if (mode & FALLOC_FL_PUNCH_HOLE)
> ret = kvm_gmem_punch_hole(file_inode(file), offset, len);
> else
> ret = kvm_gmem_allocate(file_inode(file), offset, len);
>
> if (!ret)
> file_modified(file);
> return ret;
> }
>
> static int kvm_gmem_release(struct inode *inode, struct file *file)
> {
> struct kvm_gmem *gmem = file->private_data;
> struct kvm_memory_slot *slot;
> struct kvm *kvm = gmem->kvm;
> unsigned long index;
>
> /*
> * Prevent concurrent attempts to *unbind* a memslot. This is the last
> * reference to the file and thus no new bindings can be created, but
> * dereferencing the slot for existing bindings needs to be protected
> * against memslot updates, specifically so that unbind doesn't race
> * and free the memslot (kvm_gmem_get_file() will return NULL).
> */
> mutex_lock(&kvm->slots_lock);
>
> filemap_invalidate_lock(inode->i_mapping);
>
> xa_for_each(&gmem->bindings, index, slot)
> rcu_assign_pointer(slot->gmem.file, NULL);
>
> synchronize_rcu();
>
> /*
> * All in-flight operations are gone and new bindings can be created.
> * Zap all SPTEs pointed at by this file. Do not free the backing
> * memory, as its lifetime is associated with the inode, not the file.
> */
> kvm_gmem_invalidate_begin(gmem, 0, -1ul);
> kvm_gmem_invalidate_end(gmem, 0, -1ul);
>
> list_del(&gmem->entry);
>
> filemap_invalidate_unlock(inode->i_mapping);
>
> mutex_unlock(&kvm->slots_lock);
>
> xa_destroy(&gmem->bindings);
> kfree(gmem);
>
> kvm_put_kvm(kvm);
>
> return 0;
> }
>
> static struct file_operations kvm_gmem_fops = {
> .open = generic_file_open,
> .release = kvm_gmem_release,
> .fallocate = kvm_gmem_fallocate,
> };
>
> void kvm_gmem_init(struct module *module)
> {
> kvm_gmem_fops.owner = module;
> }
>
> static int kvm_gmem_migrate_folio(struct address_space *mapping,
> struct folio *dst, struct folio *src,
> enum migrate_mode mode)
> {
> WARN_ON_ONCE(1);
> return -EINVAL;
> }
>
> static int kvm_gmem_error_folio(struct address_space *mapping, struct folio *folio)
> {
> struct list_head *gmem_list = &mapping->i_private_list;
> struct kvm_gmem *gmem;
> pgoff_t start, end;
>
> filemap_invalidate_lock_shared(mapping);
>
> start = folio->index;
> end = start + folio_nr_pages(folio);
>
> list_for_each_entry(gmem, gmem_list, entry)
> kvm_gmem_invalidate_begin(gmem, start, end);
>
> /*
> * Do not truncate the range, what action is taken in response to the
> * error is userspace's decision (assuming the architecture supports
> * gracefully handling memory errors). If/when the guest attempts to
> * access a poisoned page, kvm_gmem_get_pfn() will return -EHWPOISON,
> * at which point KVM can either terminate the VM or propagate the
> * error to userspace.
> */
>
> list_for_each_entry(gmem, gmem_list, entry)
> kvm_gmem_invalidate_end(gmem, start, end);
>
> filemap_invalidate_unlock_shared(mapping);
>
> return MF_DELAYED;
> }
>
> #ifdef CONFIG_HAVE_KVM_GMEM_INVALIDATE
> static void kvm_gmem_free_folio(struct folio *folio)
> {
> struct page *page = folio_page(folio, 0);
> kvm_pfn_t pfn = page_to_pfn(page);
> int order = folio_order(folio);
>
> kvm_arch_gmem_invalidate(pfn, pfn + (1ul << order));
> }
> #endif
>
> static const struct address_space_operations kvm_gmem_aops = {
> .dirty_folio = noop_dirty_folio,
> .migrate_folio = kvm_gmem_migrate_folio,
> .error_remove_folio = kvm_gmem_error_folio,
> #ifdef CONFIG_HAVE_KVM_GMEM_INVALIDATE
> .free_folio = kvm_gmem_free_folio,
> #endif
> };
>
> static int kvm_gmem_getattr(struct mnt_idmap *idmap, const struct path *path,
> struct kstat *stat, u32 request_mask,
> unsigned int query_flags)
> {
> struct inode *inode = path->dentry->d_inode;
>
> generic_fillattr(idmap, request_mask, inode, stat);
> return 0;
> }
>
> static int kvm_gmem_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
> struct iattr *attr)
> {
> return -EINVAL;
> }
> static const struct inode_operations kvm_gmem_iops = {
> .getattr = kvm_gmem_getattr,
> .setattr = kvm_gmem_setattr,
> };
>
> static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
> {
> const char *anon_name = "[kvm-gmem]";
> struct kvm_gmem *gmem;
> struct inode *inode;
> struct file *file;
> int fd, err;
>
> fd = get_unused_fd_flags(0);
> if (fd < 0)
> return fd;
>
> gmem = kzalloc(sizeof(*gmem), GFP_KERNEL);
> if (!gmem) {
> err = -ENOMEM;
> goto err_fd;
> }
>
> file = anon_inode_create_getfile(anon_name, &kvm_gmem_fops, gmem,
> O_RDWR, NULL);
> if (IS_ERR(file)) {
> err = PTR_ERR(file);
> goto err_gmem;
> }
>
> file->f_flags |= O_LARGEFILE;
>
> inode = file->f_inode;
> WARN_ON(file->f_mapping != inode->i_mapping);
>
> inode->i_private = (void *)(unsigned long)flags;
> inode->i_op = &kvm_gmem_iops;
> inode->i_mapping->a_ops = &kvm_gmem_aops;
> inode->i_mapping->flags |= AS_INACCESSIBLE;
> inode->i_mode |= S_IFREG;
> inode->i_size = size;
> mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
> mapping_set_unmovable(inode->i_mapping);
> /* Unmovable mappings are supposed to be marked unevictable as well. */
> WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
>
> kvm_get_kvm(kvm);
> gmem->kvm = kvm;
> xa_init(&gmem->bindings);
> list_add(&gmem->entry, &inode->i_mapping->i_private_list);
>
> fd_install(fd, file);
> return fd;
>
> err_gmem:
> kfree(gmem);
> err_fd:
> put_unused_fd(fd);
> return err;
> }
>
> int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
> {
> loff_t size = args->size;
> u64 flags = args->flags;
> u64 valid_flags = 0;
>
> if (flags & ~valid_flags)
> return -EINVAL;
>
> if (size <= 0 || !PAGE_ALIGNED(size))
> return -EINVAL;
>
> return __kvm_gmem_create(kvm, size, flags);
> }
>
> int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
> unsigned int fd, loff_t offset)
> {
> loff_t size = slot->npages << PAGE_SHIFT;
> unsigned long start, end;
> struct kvm_gmem *gmem;
> struct inode *inode;
> struct file *file;
> int r = -EINVAL;
>
> BUILD_BUG_ON(sizeof(gfn_t) != sizeof(slot->gmem.pgoff));
>
> file = fget(fd);
> if (!file)
> return -EBADF;
>
> if (file->f_op != &kvm_gmem_fops)
> goto err;
>
> gmem = file->private_data;
> if (gmem->kvm != kvm)
> goto err;
>
> inode = file_inode(file);
>
> if (offset < 0 || !PAGE_ALIGNED(offset) ||
> offset + size > i_size_read(inode))
> goto err;
>
> filemap_invalidate_lock(inode->i_mapping);
>
> start = offset >> PAGE_SHIFT;
> end = start + slot->npages;
>
> if (!xa_empty(&gmem->bindings) &&
> xa_find(&gmem->bindings, &start, end - 1, XA_PRESENT)) {
> filemap_invalidate_unlock(inode->i_mapping);
> goto err;
> }
>
> /*
> * No synchronize_rcu() needed, any in-flight readers are guaranteed to
> * be see either a NULL file or this new file, no need for them to go
> * away.
> */
> rcu_assign_pointer(slot->gmem.file, file);
> slot->gmem.pgoff = start;
>
> xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL);
> filemap_invalidate_unlock(inode->i_mapping);
>
> /*
> * Drop the reference to the file, even on success. The file pins KVM,
> * not the other way 'round. Active bindings are invalidated if the
> * file is closed before memslots are destroyed.
> */
> r = 0;
> err:
> fput(file);
> return r;
> }
>
> void kvm_gmem_unbind(struct kvm_memory_slot *slot)
> {
> unsigned long start = slot->gmem.pgoff;
> unsigned long end = start + slot->npages;
> struct kvm_gmem *gmem;
> struct file *file;
>
> /*
> * Nothing to do if the underlying file was already closed (or is being
> * closed right now), kvm_gmem_release() invalidates all bindings.
> */
> file = kvm_gmem_get_file(slot);
> if (!file)
> return;
>
> gmem = file->private_data;
>
> filemap_invalidate_lock(file->f_mapping);
> xa_store_range(&gmem->bindings, start, end - 1, NULL, GFP_KERNEL);
> rcu_assign_pointer(slot->gmem.file, NULL);
> synchronize_rcu();
> filemap_invalidate_unlock(file->f_mapping);
>
> fput(file);
> }
>
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