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

[Matthew Wilcox]

On Wed, Jan 20, 2021 at 05:05:10PM +0200, Mike Rapoport wrote:
> On Tue, Jan 19, 2021 at 08:22:13PM +0000, Matthew Wilcox wrote:
> > 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.
>  
> Ok.
> 
> > > +		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?
> 
> Right, for -EEXIST I need a retry here, thanks.
> 
> > > +		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?
> 
> Yes, it does. If somebody else won the race that page was dropped from the
> direct map and this call would be essentially a nop. And anyway, the very
> next patch changes the way pages are removed from the direct map ;-)

What I'm thinking is:

thread A page faults
doesn't find page
allocates page
adds page to page cache
				thread B page faults
				does find page in page cache
set direct map invalid fails
deletes from page cache
				... ?