Re: [PATCH v4] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

["Zach O'Keefe" <zokeefe@google.com>]

On Fri, Oct 6, 2023 at 10:50 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Mon, 25 Sep 2023 13:01:10 -0700 "Zach O'Keefe" <zokeefe@google.com> 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, this was a bad
> > assumption to make as it assumes the only reason to support ->huge_fault
> > was for DAX (which is not true in general).
> >
> > Remove the VM_NO_KHUGEPAGED check when not in collapse path and give
> > any ->huge_fault handler a chance to handle the fault.  Note that we
> > don't validate the file mode or mapping alignment, which is consistent
> > with the behavior before the aforementioned commits.
>
> It's unclear what are the userspace visible impacts of this change.
> Which makes it hard for others to determine whether -stable kernels
> should be patched.

IMO, I don't think this change is suitable for -stable; the only users
that would have been affected are those that maintain out-of-tree
drivers / code that hooked into ->huge_fault() or used VM_MIXEDMAP +
THP. No users of the in-tree kernel would have been affected. It's
still a good "fix" to make going forward (and certainly happy to be
able to help Saurabh out).

+ greg k-h for vis / to confirm.

> > Fixes: 7da4e2cb8b1f ("mm: thp: kill __transhuge_page_enabled()")
> > Reported-by: Saurabh Singh Sengar <ssengar@microsoft.com>
>
> It's nice to include a Closes: link after a Reported-by:.  Then readers
> are better able to answer the above question.

Ah, apologies, Andrew; I didn't know such a tag existed -- I'll be
sure to include it in the future.

> > Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> > Cc: Yang Shi <shy828301@gmail.com>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: David Hildenbrand <david@redhat.com>
>