Re: [PATCH v3] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

[David Hildenbrand]

On 24.08.23 15:59, Zach O'Keefe wrote:
> On Thu, Aug 24, 2023 at 12:39 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 22.08.23 01:48, Zach O'Keefe wrote:
> > > The 6.0 commits:
> > >
> > > commit 9fec51689ff6 ("mm: thp: kill transparent_hugepage_active()")
> > > commit 7da4e2cb8b1f ("mm: thp: kill __transhuge_page_enabled()")
> > >
> > > merged "can we have THPs in this VMA?" logic that was previously done
> > > separately by fault-path, khugepaged, and smaps "THPeligible" checks.
> > >
> > > During the process, the semantics of the fault path check changed in two
> > > ways:
> > >
> > > 1) A VM_NO_KHUGEPAGED check was introduced (also added to smaps path).
> > > 2) We no longer checked if non-anonymous memory had a vm_ops->huge_fault
> > >      handler that could satisfy the fault.  Previously, this check had been
> > >      done in create_huge_pud() and create_huge_pmd() routines, but after
> > >      the changes, we never reach those routines.
> > >
> > > During the review of the above commits, it was determined that in-tree
> > > users weren't affected by the change; most notably, since the only relevant
> > > user (in terms of THP) of VM_MIXEDMAP or ->huge_fault is DAX, which is
> > > explicitly approved early in approval logic.  However, there is at least
> > > one occurrence where an out-of-tree driver that used
> > > VM_HUGEPAGE|VM_MIXEDMAP with a vm_ops->huge_fault handler, was broken.
> >
> > ... so all we did is break an arbitrary out-of-tree driver? Sorry to
> > say, but why should we care?
> >
> > Is there any in-tree code affected and needs a "Fixes:" ?
>
> The in-tree code was taken care of during the rework .. but I didn't
> know about the possibility of a driver hooking in here.

And that's the problem of the driver, no? It's not the job of the kernel 
developers to be aware of each out-of-tree driver to not accidentally 
break something in there.

> I don't know what the normal policy / stance here is, but I figured
> the change was simple enough that it was worth helping out.

If you decide to be out-of-tree, then you have be prepared to only 
support tested kernels and fix your driver when something changes 
upstream -- like upstream developers would do for you when it would be 
in-tree.

So why can't the out-of-tree driver be fixed, similarly to how we would 
have fixed it if it would be in-tree?

--
Cheers,

David / dhildenb