Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Kirill A. Shutemov]

On Mon, Jul 16, 2018 at 07:40:42PM +0200, Michal Hocko wrote:
> On Mon 16-07-18 17:47:39, Kirill A. Shutemov wrote:
> > On Mon, Jul 16, 2018 at 04:22:45PM +0200, Michal Hocko wrote:
> > > On Mon 16-07-18 17:04:41, Kirill A. Shutemov wrote:
> > > > On Mon, Jul 16, 2018 at 01:30:28PM +0000, Michal Hocko wrote:
> > > > > On Tue 10-07-18 13:48:58, Andrew Morton wrote:
> > > > > > On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > > > > > 
> > > > > > > vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> > > > > > > VMA. This is unreliable as ->mmap may not set ->vm_ops.
> > > > > > > 
> > > > > > > False-positive vma_is_anonymous() may lead to crashes:
> > > > > > > 
> > > > > > > ...
> > > > > > > 
> > > > > > > This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> > > > > > > on it being NULL.
> > > > > > > 
> > > > > > > If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> > > > > > > dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
> > > > > > 
> > > > > > Is there a smaller, simpler fix which we can use for backporting
> > > > > > purposes and save the larger rework for development kernels?
> > > > > 
> > > > > Why cannot we simply keep anon vma with null vm_ops and set dummy_vm_ops
> > > > > for all users who do not initialize it in their mmap callbacks?
> > > > > Basically have a sanity check&fixup in call_mmap?
> > > > 
> > > > As I said, there's a corner case of MAP_PRIVATE of /dev/zero.
> > > 
> > > This is really creative. I really didn't think about that. I am
> > > wondering whether this really has to be handled as a private anonymous
> > > mapping implicitly. Why does vma_is_anonymous has to succeed for these
> > > mappings? Why cannot we simply handle it as any other file backed
> > > PRIVATE mapping?
> > 
> > Because it's established way to create anonymous mappings in Linux.
> > And we cannot break the semantics.
> 
> How exactly would semantic break? You would still get zero pages on read
> faults and anonymous pages on CoW. So basically the same thing as for
> any other file backed MAP_PRIVATE mapping.

You are wrong about zero page. And you won't get THP. And I'm sure there's
more differences. Just grep for vma_is_anonymous().

-- 
 Kirill A. Shutemov