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

From: Mike Rapoport <rppt@kernel•org>
To: Matthew Wilcox <willy@infradead•org>
Cc: Mark Rutland <mark.rutland@arm•com>,
	David Hildenbrand <david@redhat•com>,
	Peter Zijlstra <peterz@infradead•org>,
	Catalin Marinas <catalin.marinas@arm•com>,
	Dave Hansen <dave.hansen@linux•intel.com>,
	linux-mm@kvack•org, linux-kselftest@vger•kernel.org,
	"H. Peter Anvin" <hpa@zytor•com>,
	Christopher Lameter <cl@linux•com>, Shuah Khan <shuah@kernel•org>,
	Thomas Gleixner <tglx@linutronix•de>,
	Elena Reshetova <elena.reshetova@intel•com>,
	linux-arch@vger•kernel.org, Tycho Andersen <tycho@tycho•ws>,
	linux-nvdimm@lists•01.org, Will Deacon <will@kernel•org>,
	x86@kernel•org, linux-riscv@lists•infradead.org,
	Mike Rapoport <rppt@linux•ibm.com>,
	Ingo Molnar <mingo@redhat•com>,
	Michael Kerrisk <mtk.manpages@gmail•com>,
	Arnd Bergmann <arnd@arndb•de>,
	James Bottomley <jejb@linux•ibm.com>,
	Hagen Paul Pfeifer <hagen@jauu•net>,
	Borislav Petkov <bp@alien8•de>,
	Alexander Viro <viro@zeniv•linux.org.uk>,
	Andy Lutomirski <luto@kernel•org>,
	Paul Walmsley <paul.walmsley@sifive•com>,
	"Kirill A. Shutemov" <kirill@shutemov•name>,
	Dan Williams <dan.j.williams@intel•com>,
	linux-arm-kernel@lists•infradead.org, linux-api@vger•kernel.org,
	linux-kernel@vger•kernel.org, Palmer Dabbelt <palmer@dabbelt•com>,
	linux-fsdevel@vger•kernel.org, Shakeel Butt <shakeelb@google•com>,
	Andrew Morton <akpm@linux-foundation•org>,
	Rick Edgecombe <rick.p.edgecombe@intel•com>,
	Roman Gushchin <guro@fb•com>
Subject: Re: [PATCH v14 05/10] mm: introduce memfd_secret system call to create "secret" memory areas
Date: Wed, 20 Jan 2021 17:05:10 +0200	[thread overview]
Message-ID: <20210120150510.GO1106298@kernel.org> (raw)
In-Reply-To: <20210119202213.GI2260413@casper.infradead.org>

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.

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