Re: [PATCH V2] mm/gup: Clear the LRU flag of a page before adding to LRU batch

[Ge Yang]

在 2024/7/30 17:41, David Hildenbrand 写道:
> On 30.07.24 11:36, Ge Yang wrote:
> > 在 2024/7/30 15:45, David Hildenbrand 写道:
> > > > > Looking at this in more detail, I wonder if we can turn that to
> > > > >
> > > > > if (!folio_test_clear_lru(folio))
> > > > >        return;
> > > > > folio_get(folio);
> > > > >
> > > > > In all cases? The caller must hold a reference, so this should be 
> > > > > fine.
> > > >
> > > > Seems the caller madvise_free_pte_range(...), calling
> > > > folio_mark_lazyfree(...), doesn't hold a reference on folio.
> > >
> > > If that would be the case and the folio could get freed concurrently,
> > > the folio_get(folio) would be completely broken.
> > >
> > > In madvise_free_pte_range() we hold the PTL, so the folio cannot get
> > > freed concurrently.
> >
> > Right.
> >
> > > folio_get() is only allowed when we are sure the folio cannot get freed
> > > concurrently, because we know there is a reference that cannot go away.
> >
> > When cpu0 runs folio_activate(), and cpu1 runs folio_put() concurrently,
> > a possible bad scenario would like:
> >
> > cpu0                                           cpu1
> >
> >                                              folio_put_testzero(folio)
> > if (!folio_test_clear_lru(folio))// Seems folio shouldn't be accessed
> >
> >          return;
> > folio_get(folio);
> >                                               __folio_put(folio)
> >                                               __folio_clear_lru(folio)
> >
> >
> > Seems we should use folio_try_get(folio) instead of folio_get(folio).
>
> In which case is folio_activate() called without the PTL on a mapped 
> page or without a raised refcount?

No such case has been found. But, folio_put() can be run at anytime, so 
folio_activate() may access a folio with a reference count of 0.