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

Mike Rapoport
2021-01-20T15:05:29Z

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 ;-)

> > + 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.

Will fix.

> > + 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().

Ok/

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

--
Sincerely yours,
Mike.