Re: [RESEND PATCH v2 1/1] mm/rmap: add CONFIG_MM_ID guard for folio_test_large_maybe_mapped_shared()

["Lance Yang" <lance.yang@linux.dev>]

April 21, 2025 at 3:17 PM, "David Hildenbrand" <david@redhat.com> wrote:



> 
> On 21.04.25 07:16, Lance Yang wrote:
> 
> > 
> > April 21, 2025 at 1:13 PM, "Lance Yang" <lance.yang@linux.dev> wrote:
> > 
> >  > >>
> > 
> > > 
> > > April 21, 2025 at 7:29 AM, "Andrew Morton" <akpm@linux-foundation.org> wrote:
> > > 
> > 
> >  On Fri, 18 Apr 2025 23:22:28 +0800 Lance Yang <ioworker0@gmail.com> wrote:
> > 
> >  >>>
> > 
> >  >>>
> > 
> >  From: Lance Yang <lance.yang@linux.dev>
> > 
> >  >>>
> > 
> >  >>>
> > 
> >  >>>
> > 
> >  To prevent folio_test_large_maybe_mapped_shared() from being used without
> > 
> >  >>>
> > 
> >  CONFIG_MM_ID, we add a compile-time check rather than wrapping it in
> > 
> >  >>>
> > 
> >  '#ifdef', avoiding even more #ifdef in callers that already use
> > 
> >  >>>
> > 
> >  IS_ENABLED(CONFIG_MM_ID).
> > 
> >  >>>
> > 
> >  >>>
> > 
> >  >>>
> > 
> >  Also, we used plenty of IS_ENABLED() on purpose to keep the code free of
> > 
> >  >>>
> > 
> >  '#ifdef' mess.
> > 
> >  >>>
> > 
> >  >>>
> > 
> >  I dunno, this just seems really whacky.
> > 
> > > 
> > > I'd hope David could leave some comments on that.
> > > 
> > 
> >  --- a/include/linux/page-flags.h
> > 
> >  >>>
> > 
> >  +++ b/include/linux/page-flags.h
> > 
> >  >>>
> > 
> >  @@ -1232,6 +1232,8 @@ static inline int folio_has_private(const struct folio *folio)
> > 
> >  >>>
> > 
> >  >>>
> > 
> >  >>>
> > 
> >  static inline bool folio_test_large_maybe_mapped_shared(const struct folio *folio)
> > 
> >  >>>
> > 
> >  {
> > 
> >  >>>
> > 
> >  + /* This function should never be called without CONFIG_MM_ID enabled. */
> > 
> >  >>>
> > 
> >  >>>
> > 
> >  A correcter comment would be "This function should never be compiled
> > 
> >  >>>
> > 
> >  without CONFIG_MM_ID enabled".
> > 
> > > 
> > > Yes, that is more exact ;)
> > > 
> > 
> >  Which lets the cat out of the bag. Why the heck is it being compiled
> > 
> >  >>>
> > 
> >  with CONFIG_MM_ID=n?? We have tools to prevent that.
> > 
> >  >>>
> > 
> >  Can we just slap "#ifdef CONFIG_MM_ID" around the whole function? It
> > 
> >  >>>
> > 
> >  should have no callers, right? If the linker ends up complaining then
> > 
> >  >>>
> > 
> >  something went wrong.
> > 
> > > 
> > > The reason we can't simply add #ifdef CONFIG_MM_ID around folio_test_large_maybe_mapped_shared()
> > > 
> > >  is because its caller folio_maybe_mapped_shared() relies on IS_ENABLED(CONFIG_MM_ID).
> > > 
> > 
> >  static inline bool folio_maybe_mapped_shared(struct folio *folio)
> > 
> >  {
> > 
> >  [...]
> > 
> >  if (!IS_ENABLED(CONFIG_MM_ID))
> > 
> >  return true;
> > 
> >  [...]
> > 
> >  return folio_test_large_maybe_mapped_shared(folio);
> > 
> >  }
> > 
> >  folio_maybe_mapped_shared() is always available - doesn't depend on
> > 
> >  CONFIG_MM_ID or CONFIG_TRANSPARENT_HUGEPAGE.
> > 
> 
> We could #ifdef in folio_maybe_mapped_shared(), which I find rather suboptimal ...
> 
> or simply inline it into the 4 callers.
> 
> That might be the best approach, given that only selected user should be using the low-level primitive and everybody else should be using folio_maybe_mapped_shared().

Yep, I tend to prefer the second approach as well, given there are only
four callers. Andrew, wdyt?

Thanks,
Lance

> 
> -- Cheers,
> 
> David / dhildenb
>