Re: [PATCH 7/7] mm/mglru: improve code readability and harden folio_inc_gen

[Baoquan He]

On 08/20/26 at 08:53am, Baoquan He wrote:
> On 08/18/26 at 01:38pm, Kairui Song via B4 Relay wrote:
> > From: Kairui Song <kasong@xxxxxxxxxxx>
> > 
> > The helper should never be called for an off-list folio, and it always
> > expects the folio to be in the oldest generation before doing any
> > cmpxchg. Add a sanity check for the off-list case: if it is ever
> > violated, bail out and keep the folio flags untouched to minimize the
> > damage, instead of silently treating the folio as if it were in the
> > oldest generation and promoting it updating the flags to an unexpected
> > status.
> > 
> > Also rename the variables to clearly distinguish the folio's current
> > gen from the oldest gen.
> > 
> > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> > ---
> >  mm/vmscan.c | 14 +++++++++-----
> >  1 file changed, 9 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 7169cac60869..7e3ae0c6cba3 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -3308,18 +3308,22 @@ static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio)
> >  {
> >  	int type = folio_is_file_lru(folio);
> >  	struct lru_gen_folio *lrugen = &lruvec->lrugen;
> > -	int new_gen, old_gen = lru_gen_from_seq(lrugen->min_seq[type]);
> > +	int new_gen, old_gen, min_gen = lru_gen_from_seq(lrugen->min_seq[type]);
> >  	unsigned long new_flags, old_flags = READ_ONCE(*folio_flags(folio, 0));
> >  
> >  	do {
> > -		new_gen = lru_gen_from_flags(old_flags);
> > +		old_gen = lru_gen_from_flags(old_flags);
> > +		/* This helper should never be called for off-list folios */
> > +		VM_WARN_ON_ONCE(old_gen < 0);
> > +		if (old_gen < 0)
> > +			return min_gen;
> 
> As Barry doubted, I think this change is wrong. old_gen < 0 in folio_inc_gen()
> could only happen inc_min_seq() call it. While inc_min_seq() call it
> because inc_max_seq() need increase max_gen to max_gen + 1 and found
> get_nr_gens(lruvec, type) == MAX_NR_GENS, it has to move the oldest gen to
                                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 2nd old oldest gen. Here returning min_gen for old_gen < 0 means it will
  ~~~~~~~~~~~~~~~~
Here, I mean it has to move folios from the oldest gen (min_gen) to the 2nd
oldest gen (min_gen + 1). The empty min_gen will become the new max_gen.

> be put in the lastest max_gen. It may not be expected.
> 
> >  
> >  		/* folio_update_gen() has promoted this page? */
> > -		if (new_gen >= 0 && new_gen != old_gen)
> > -			return new_gen;
> > +		if (old_gen != min_gen)
> > +			return old_gen;
> >  
> >  		new_flags = old_flags;
> > -		new_gen = (old_gen + 1) % MAX_NR_GENS;
> > +		new_gen = (min_gen + 1) % MAX_NR_GENS;
> >  		lru_gen_set_flags(&new_flags, new_gen);
> >  		lru_refs_set_flags(&new_flags, 0);
> >  	} while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
> > 
> > -- 
> > 2.55.0
> > 
> >