[PATCH v14 05/10] mm: introduce memfd_secret system call to create "secret" memory areas

[Matthew Wilcox]

On Thu, Dec 03, 2020 at 08:29:44AM +0200, Mike Rapoport wrote:
> +static vm_fault_t secretmem_fault(struct vm_fault *vmf)
> +{
> +	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
> +	struct inode *inode = file_inode(vmf->vma->vm_file);
> +	pgoff_t offset = vmf->pgoff;
> +	vm_fault_t ret = 0;
> +	unsigned long addr;
> +	struct page *page;
> +	int err;
> +
> +	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
> +		return vmf_error(-EINVAL);
> +
> +	page = find_get_page(mapping, offset);
> +	if (!page) {
> +
> +		page = secretmem_alloc_page(vmf->gfp_mask);
> +		if (!page)
> +			return vmf_error(-ENOMEM);

Just use VM_FAULT_OOM directly.

> +		err = add_to_page_cache(page, mapping, offset, vmf->gfp_mask);
> +		if (unlikely(err))
> +			goto err_put_page;

What if the error is EEXIST because somebody else raced with you to add
a new page to the page cache?

> +		err = set_direct_map_invalid_noflush(page, 1);
> +		if (err)
> +			goto err_del_page_cache;

Does this work correctly if somebody else has a reference to the page
in the meantime?

> +		addr = (unsigned long)page_address(page);
> +		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> +
> +		__SetPageUptodate(page);

Once you've added it to the cache, somebody else can come along and try
to lock it.  They will set PageWaiter.  Now you call __SetPageUptodate
and wipe out their PageWaiter bit.  So you won't wake them up when you
unlock.

You can call __SetPageUptodate before adding it to the page cache,
but once it's visible to another thread, you can't do that.

> +		ret = VM_FAULT_LOCKED;
> +	}
> +
> +	vmf->page = page;

You're supposed to return the page locked, so use find_lock_page() instead
of find_get_page().

> +	return ret;
> +
> +err_del_page_cache:
> +	delete_from_page_cache(page);
> +err_put_page:
> +	put_page(page);
> +	return vmf_error(err);
> +}