Re: [PATCH v4] mm: fix is_pinnable_page against on cma page

Paul E. McKenney
2022-05-12T00:49:59Z

On Wed, May 11, 2022 at 05:34:52PM -0700, John Hubbard wrote:
> On 5/11/22 17:26, Minchan Kim wrote:
> > > > Let me try to say this more clearly: I don't think that the following
> > > > __READ_ONCE() statement can actually help anything, given that
> > > > get_pageblock_migratetype() is non-inlined:
> > > >
> > > > + int __mt = get_pageblock_migratetype(page);
> > > > + int mt = __READ_ONCE(__mt);
> > > > +
> > > > + if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE))
> > > > + return false;
> > > >
> > > >
> > > > Am I missing anything here?
> > >
> > > In the absence of future aggression from link-time optimizations (LTO),
> > > you are missing nothing.
> >
> > A thing I want to note is Android kernel uses LTO full mode.
>
> Thanks Paul for explaining the state of things.
>
> Minchan, how about something like very close to your original draft,
> then, but with a little note, and the "&" as well:
>
> int __mt = get_pageblock_migratetype(page);
>
> /*
> * Defend against future compiler LTO features, or code refactoring
> * that inlines the above function, by forcing a single read. Because, this
> * routine races with set_pageblock_migratetype(), and we want to avoid
> * reading zero, when actually one or the other flags was set.
> */
> int mt = __READ_ONCE(__mt);
>
> if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE))
> return false;
>
>
> ...which should make everyone comfortable and protected from the
> future sins of the compiler and linker teams? :)

This would work, but it would force a store to the stack and an immediate
reload. Which might be OK on this code path.

But using READ_ONCE() in (I think?) __get_pfnblock_flags_mask()
would likely generate the same code that is produced today.

word = READ_ONCE(bitmap[word_bitidx]);

But I could easily have missed a turn in that cascade of functions. ;-)

Or there might be some code path that really hates a READ_ONCE() in
that place.

Thanx, Paul