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

[Kirill A. Shutemov]

On Tue, Jul 17, 2018 at 09:00:53AM +0000, Michal Hocko wrote:
> On Mon 16-07-18 23:38:46, Kirill A. Shutemov wrote:
> > 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.
> 
> Well, if we redirect ->fault to do_anonymous_page and

Yeah. And it will make write fault to allocate *two* pages. One in
do_anonymous_page() and one in do_cow_fault(). Just no.

We have a reason why anon VMAs handled separately. It's possible to unify
them, but it requires substantial ground work.

> > And you won't get THP.
> 
> huge_fault to do_huge_pmd_anonymous_page then we should emulate the
> standard anonymous mapping.
> 
> > And I'm sure there's more differences. Just grep for
> > vma_is_anonymous().
> 
> I am sorry to push on this but if we have one odd case I would rather
> handle it and have a simple _rule_ that every mmap provide _has_ to
> provide vm_ops and have a trivial fix up at a single place rather than
> patch a subtle placeholders you were proposing.
> 
> I will not insist of course but this looks less fragile to me.

You propose quite a big redesign on how we handle anonymous VMAs.
Feel free to propose the patch(set). But I don't think it would fly for
stable@.

-- 
 Kirill A. Shutemov