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

Matthew Wilcox
2021-01-20T16:04:38Z

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
... ?