Re: [PATCH v4] mm/mglru: fix cgroup OOM during MGLRU state switching

[Barry Song <21cnbao@gmail.com>]

On Wed, Mar 18, 2026 at 4:17 PM Leno Hou <lenohou@gmail.com> wrote:
>
> On 3/18/26 3:16 PM, Barry Song wrote:
> > On Wed, Mar 18, 2026 at 11:29 AM Leno Hou via B4 Relay
> > <devnull+lenohou.gmail.com@kernel.org> wrote:
> >>
> >> From: Leno Hou <lenohou@gmail.com>
> >
> > [...]
> >
> >>
> >> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> >> index ad50688d89db..1f6b19bf365b 100644
> >> --- a/include/linux/mm_inline.h
> >> +++ b/include/linux/mm_inline.h
> >> @@ -102,6 +102,12 @@ static __always_inline enum lru_list folio_lru_list(const struct folio *folio)
> >>
> >>   #ifdef CONFIG_LRU_GEN
> >>
> >> +static inline bool lru_gen_draining(void)
> >> +{
> >> +       DECLARE_STATIC_KEY_FALSE(lru_drain_core);
> >> +
> >> +       return static_branch_unlikely(&lru_drain_core);
> >> +}
> >
> > Can we name it lru_gen_switch() or lru_switch?
> > Since “drain” implies disabling MGLRU, the operation
> > could just as well be enabling it. Also, can we drop
> > the _core suffix?
>
> OK. Next V5 patch will be:
>
> +static inline bool lru_gen_switching(void)
> +{
> +       DECLARE_STATIC_KEY_FALSE(lru_switch);
> +
> +       return static_branch_unlikely(&lru_switch);
> +}
>
> >
> >
> >>   #ifdef CONFIG_LRU_GEN_ENABLED
> >>   static inline bool lru_gen_enabled(void)
> >>   {
> >> @@ -316,6 +322,11 @@ static inline bool lru_gen_enabled(void)
> >>          return false;
> >>   }
> >>
> >> +static inline bool lru_gen_draining(void)
> >
> > lru_gen_switching()? >
> >> +{
> >> +       return false;
> >> +}
> >> +
> >>   static inline bool lru_gen_in_fault(void)
> >>   {
> >>          return false;
> >> diff --git a/mm/rmap.c b/mm/rmap.c
> >> index 6398d7eef393..0b5f663f3062 100644
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -966,7 +966,7 @@ static bool folio_referenced_one(struct folio *folio,
> >>                          nr = folio_pte_batch(folio, pvmw.pte, pteval, max_nr);
> >>                  }
>
> OK. I'll be add following ducumentation that just you said.
> /* When LRU is switching, we don’t know where the surrounding folios
> are. —they could be on active/inactive lists or on MGLRU. So the
> simplest approach is to disable this look-around optimization.
> */
> >> -               if (lru_gen_enabled() && pvmw.pte) {
> >> +               if (lru_gen_enabled() && !lru_gen_draining() && pvmw.pte) {
> >
> > Ack. When LRU is switching, we don’t know where the
> > surrounding folios are—they could be on active/inactive
> > lists or on MGLRU. So the simplest approach is to
> > disable this look-around optimization.
> > But please add a comment here explaining it.
> >
> >
> >>                          if (lru_gen_look_around(&pvmw, nr))
> >>                                  referenced++;
> >>                  } else if (pvmw.pte) {
> >> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >> index 33287ba4a500..88b9db06e331 100644
> >> --- a/mm/vmscan.c
> >> +++ b/mm/vmscan.c
> >> @@ -886,7 +886,7 @@ static enum folio_references folio_check_references(struct folio *folio,
> >>          if (referenced_ptes == -1)
> >>                  return FOLIOREF_KEEP;
> >>
> >> -       if (lru_gen_enabled()) {
>
> documentation as following:
>
> /*
>   * During the MGLRU state transition (lru_gen_switching), we force
>   * folios to follow the traditional active/inactive reference checking.
>   *
>   * While MGLRU is switching,the generational state of folios is in flux.
>   * Falling back to the traditional logic (which relies on PG_referenced/
>   * PG_active flags that are consistent across both mechanisms) provides
>   * a stable, safe behavior for the folio until it is fully migrated back
>   * to the traditional LRU lists. This avoids relying on potentially
>   * inconsistent MGLRU generational metadata during the transition.
>   */
>
> >> +       if (lru_gen_enabled() && !lru_gen_draining()) {
> >
> > I’m curious what prompted you to do this.
> >
> > This feels a bit odd. I assume this effectively makes
> > folios on MGLRU, as well as those on active/inactive
> > lists, always follow the active/inactive logic.
> >
> > It might be fine, but it needs thorough documentation here.
> >
> > another approach would be:
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 33287ba4a500..91b60664b652 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -122,6 +122,9 @@ struct scan_control {
> >          /* Proactive reclaim invoked by userspace */
> >          unsigned int proactive:1;
> >
> > +       /* Are we reclaiming from MGLRU */
> > +       unsigned int lru_gen:1;
> > +
> >          /*
> >           * Cgroup memory below memory.low is protected as long as we
> >           * don't threaten to OOM. If any cgroup is reclaimed at
> > @@ -886,7 +889,7 @@ static enum folio_references
> > folio_check_references(struct folio *folio,
> >          if (referenced_ptes == -1)
> >                  return FOLIOREF_KEEP;
> >
> > -       if (lru_gen_enabled()) {
> > +       if (sc->lru_gen) {
> >                  if (!referenced_ptes)
> >                          return FOLIOREF_RECLAIM;
> >
> > This makes the logic perfectly correct (you know exactly
> > where your folios come from), but I’m not sure it’s worth it.
> >
> > Anyway, I’d like to understand why you always need to
> > use the active/inactive logic even for folios from MGLRU.
> > To me, it seems to work only by coincidence, which isn’t good.
> >
> > Thanks
> > Barry
>
> Hi Barry,
>
> I agree that using !lru_gen_draining() feels a bit like a fallback path.
> However, after considering your suggestion for sc->lru_gen, I’m
> concerned about the broad impact of modifying struct scan_control.Since
> lru_drain_core is a very transient state, I prefer a localized fix that
> doesn't propagate architectural changes throughout the entire reclaim stack.
>
> You mentioned that using the active/inactive logic feels like it works
> by 'coincidence'. To clarify, this is an intentional fallback: because
> the generational metadata in MGLRU becomes unreliable during draining,
> we intentionally downgrade these folios to the traditional logic. Since
> the PG_referenced and PG_active bits are maintained by the core VM and
> are consistent regardless of whether MGLRU is active, this fallback is
> technically sound and robust.
>
> I have added detailed documentation to the code to explain this design
> choice, clarifying that it's a deliberate transition strategy rather
> than a coincidence."

Nope. You still haven’t explained why the active/inactive LRU
logic makes it work. MGLRU and active/inactive use different
methods to determine whether a folio is hot or cold. You’re
forcing active/inactive logic to decide hot/cold for an MGLRU
folio. It’s not that simple—PG_referenced isn’t maintained
by the core; it’s specific to active/inactive. See folio_mark_accessed().

Best Regards
Barry