Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

Hyeonggon Yoo
2022-10-05T11:07:34Z

On Tue, Oct 04, 2022 at 03:40:36PM +0100, Matthew Wilcox wrote:
> On Tue, Oct 04, 2022 at 11:26:33PM +0900, Hyeonggon Yoo wrote:
> > > It's the acquisition of
> > > the refcount which stabilises the slab flag, not holding the lock.
> >
> > But can you please elaborate how this prevents race between
> > allocation & initialization of a slab and isolate_movable_page()?
> >
> > Or maybe we can handle it with frozen folio as Vlastimil suggested? ;-)
>
> Yes, we discussed that a little yesterday. I'm hoping to have a
> refreshed patchset for frozen folios out today. Some of this patch
> is still needed, even if we go that route.

Good to hear that.
With that, everyting looks sane to me.

> > > @@ -91,8 +99,8 @@ int isolate_movable_page(struct page *page, isolate_mode_t mode)
> > > * lets be sure we have the page lock
> > > * before proceeding with the movable page isolation steps.
> > > */
> > > - if (unlikely(!trylock_page(page)))
> > > - goto out_putpage;
> > > + if (unlikely(!folio_trylock(folio)))
> > > + goto out_put;
> >
> > I don't know much about callers that this is trying to avoid race aginst...
> >
> > But for this to make sense, I think *every users* that doing their stuff with
> > sub-page of a compound page should acquire folio lock and not page lock
> > of sub-page, right?
>
> There is no page lock per se. If you try to acquire the lock on a tail
> page, it acquires the lock on its head page. It's been that way for a
> very long time. A lot of people are confused by this, which was part of
> the motivation for making it explicit with folios.

You are right! Reading the code, too bad
I even assumed that there was sub-page lock.

--
Thanks,
Hyeonggon