shmem.c

		case SHMEM_HUGE_NEVER:
			return false;
		case SHMEM_HUGE_ALWAYS:
			return true;
		case SHMEM_HUGE_WITHIN_SIZE:
			off = round_up(vma->vm_pgoff, HPAGE_PMD_NR);
			i_size = round_up(i_size_read(inode), PAGE_SIZE);
			if (i_size >= HPAGE_PMD_SIZE &&
					i_size >> PAGE_SHIFT >= off)
				return true;
			/* fall through */
		case SHMEM_HUGE_ADVISE:
			/* TODO: implement fadvise() hints */
			return (vma->vm_flags & VM_HUGEPAGE);
		default:
			VM_BUG_ON(1);
			return false;
	}
}
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */

#else /* !CONFIG_SHMEM */

/*
 * tiny-shmem: simple shmemfs and tmpfs using ramfs code
 *
 * This is intended for small system where the benefits of the full
 * shmem code (swap-backed and resource-limited) are outweighed by
 * their complexity. On systems without swap this code should be
 * effectively equivalent, but much lighter weight.
 */

static struct file_system_type shmem_fs_type = {
	.name		= "tmpfs",
	.init_fs_context = ramfs_init_fs_context,
	.parameters	= ramfs_fs_parameters,
	.kill_sb	= kill_litter_super,
	.fs_flags	= FS_USERNS_MOUNT,
};

int __init shmem_init(void)
{
	BUG_ON(register_filesystem(&shmem_fs_type) != 0);

	shm_mnt = kern_mount(&shmem_fs_type);
	BUG_ON(IS_ERR(shm_mnt));

	return 0;
}

int shmem_unuse(unsigned int type, bool frontswap,
		unsigned long *fs_pages_to_unuse)
{
	return 0;
}

int shmem_lock(struct file *file, int lock, struct user_struct *user)
{
	return 0;
}

void shmem_unlock_mapping(struct address_space *mapping)
{
}

#ifdef CONFIG_MMU
unsigned long shmem_get_unmapped_area(struct file *file,
				      unsigned long addr, unsigned long len,
				      unsigned long pgoff, unsigned long flags)
{
	return current->mm->get_unmapped_area(file, addr, len, pgoff, flags);
}
#endif

void shmem_truncate_range(struct inode *inode, loff_t lstart, loff_t lend)
{
	truncate_inode_pages_range(inode->i_mapping, lstart, lend);
}
EXPORT_SYMBOL_GPL(shmem_truncate_range);

#define shmem_vm_ops				generic_file_vm_ops
#define shmem_file_operations			ramfs_file_operations
#define shmem_get_inode(sb, dir, mode, dev, flags)	ramfs_get_inode(sb, dir, mode, dev)
#define shmem_acct_size(flags, size)		0
#define shmem_unacct_size(flags, size)		do {} while (0)

#endif /* CONFIG_SHMEM */

/* common code */

static struct file *__shmem_file_setup(struct vfsmount *mnt, const char *name, loff_t size,
				       unsigned long flags, unsigned int i_flags)
{
	struct inode *inode;
	struct file *res;

	if (IS_ERR(mnt))
		return ERR_CAST(mnt);

	if (size < 0 || size > MAX_LFS_FILESIZE)
		return ERR_PTR(-EINVAL);

	if (shmem_acct_size(flags, size))
		return ERR_PTR(-ENOMEM);

	inode = shmem_get_inode(mnt->mnt_sb, NULL, S_IFREG | S_IRWXUGO, 0,
				flags);
	if (unlikely(!inode)) {
		shmem_unacct_size(flags, size);
		return ERR_PTR(-ENOSPC);
	}
	inode->i_flags |= i_flags;
	inode->i_size = size;
	clear_nlink(inode);	/* It is unlinked */
	res = ERR_PTR(ramfs_nommu_expand_for_mapping(inode, size));
	if (!IS_ERR(res))
		res = alloc_file_pseudo(inode, mnt, name, O_RDWR,
				&shmem_file_operations);
	if (IS_ERR(res))
		iput(inode);
	return res;
}

/**
 * shmem_kernel_file_setup - get an unlinked file living in tmpfs which must be
 * 	kernel internal.  There will be NO LSM permission checks against the
 * 	underlying inode.  So users of this interface must do LSM checks at a
 *	higher layer.  The users are the big_key and shm implementations.  LSM
 *	checks are provided at the key or shm level rather than the inode.
 * @name: name for dentry (to be seen in /proc/<pid>/maps
 * @size: size to be set for the file
 * @flags: VM_NORESERVE suppresses pre-accounting of the entire object size
 */
struct file *shmem_kernel_file_setup(const char *name, loff_t size, unsigned long flags)
{
	return __shmem_file_setup(shm_mnt, name, size, flags, S_PRIVATE);
}

/**
 * shmem_file_setup - get an unlinked file living in tmpfs
 * @name: name for dentry (to be seen in /proc/<pid>/maps
 * @size: size to be set for the file
 * @flags: VM_NORESERVE suppresses pre-accounting of the entire object size
 */
struct file *shmem_file_setup(const char *name, loff_t size, unsigned long flags)
{
	return __shmem_file_setup(shm_mnt, name, size, flags, 0);
}
EXPORT_SYMBOL_GPL(shmem_file_setup);

/**
 * shmem_file_setup_with_mnt - get an unlinked file living in tmpfs
 * @mnt: the tmpfs mount where the file will be created
 * @name: name for dentry (to be seen in /proc/<pid>/maps
 * @size: size to be set for the file
 * @flags: VM_NORESERVE suppresses pre-accounting of the entire object size
 */
struct file *shmem_file_setup_with_mnt(struct vfsmount *mnt, const char *name,
				       loff_t size, unsigned long flags)
{
	return __shmem_file_setup(mnt, name, size, flags, 0);
}
EXPORT_SYMBOL_GPL(shmem_file_setup_with_mnt);

/**
 * shmem_zero_setup - setup a shared anonymous mapping
 * @vma: the vma to be mmapped is prepared by do_mmap_pgoff
 */
int shmem_zero_setup(struct vm_area_struct *vma)
{
	struct file *file;
	loff_t size = vma->vm_end - vma->vm_start;

	/*
	 * Cloning a new file under mmap_sem leads to a lock ordering conflict
	 * between XFS directory reading and selinux: since this file is only
	 * accessible to the user through its mapping, use S_PRIVATE flag to
	 * bypass file security, in the same way as shmem_kernel_file_setup().
	 */
	file = shmem_kernel_file_setup("dev/zero", size, vma->vm_flags);
	if (IS_ERR(file))
		return PTR_ERR(file);

	if (vma->vm_file)
		fput(vma->vm_file);
	vma->vm_file = file;
	vma->vm_ops = &shmem_vm_ops;

	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
			((vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK) <
			(vma->vm_end & HPAGE_PMD_MASK)) {
		khugepaged_enter(vma, vma->vm_flags);
	}

	return 0;
}

/**
 * shmem_read_mapping_page_gfp - read into page cache, using specified page allocation flags.
 * @mapping:	the page's address_space
 * @index:	the page index
 * @gfp:	the page allocator flags to use if allocating
 *
 * This behaves as a tmpfs "read_cache_page_gfp(mapping, index, gfp)",
 * with any new page allocations done using the specified allocation flags.
 * But read_cache_page_gfp() uses the ->readpage() method: which does not
 * suit tmpfs, since it may have pages in swapcache, and needs to find those
 * for itself; although drivers/gpu/drm i915 and ttm rely upon this support.
 *
 * i915_gem_object_get_pages_gtt() mixes __GFP_NORETRY | __GFP_NOWARN in
 * with the mapping_gfp_mask(), to avoid OOMing the machine unnecessarily.
 */
struct page *shmem_read_mapping_page_gfp(struct address_space *mapping,
					 pgoff_t index, gfp_t gfp)
{
#ifdef CONFIG_SHMEM
	struct inode *inode = mapping->host;
	struct page *page;
	int error;

	BUG_ON(mapping->a_ops != &shmem_aops);
	error = shmem_getpage_gfp(inode, index, &page, SGP_CACHE,
				  gfp, NULL, NULL, NULL);
	if (error)
		page = ERR_PTR(error);
	else
		unlock_page(page);
	return page;
#else
	/*
	 * The tiny !SHMEM case uses ramfs without swap
	 */
	return read_cache_page_gfp(mapping, index, gfp);
#endif
}
EXPORT_SYMBOL_GPL(shmem_read_mapping_page_gfp);