Message-ID: <ZmORqYFhE73AdQB6@google.com>
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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