Re: [RFC PATCH v5 2/4] mm: distinguish large folio swap allocation failures

On Fri, Aug 7, 2026 at 4:29 PM Kairui Song <[email protected]> wrote:
>
> On Thu, Jul 30, 2026 at 08:23:02PM +0800, Xueyuan Chen wrote:
> > folio_alloc_swap() reports most allocation failures with a generic
> > negative error code. Reclaim cannot tell whether splitting a large folio
> > could make progress or whether there is no backing space at all.
> >
> > Keep the global free swap count and the remaining hierarchical memcg swap
> > margin as separate inputs. The memcg charge path reports only its own
> > margin; folio_alloc_swap() combines the two layers when classifying an
> > allocation failure.
> >
> > Return -E2BIG for large folios when a smaller allocation might still fit,
> > -ENOSPC when no global swap space is available, and -ENOMEM when the
> > failure is not helped by splitting.
> >
> > For early large-folio rejections, check global and memcg swap availability
> > instead of returning -E2BIG unconditionally. On a memcg charge failure,
> > swap slot allocation has already succeeded, so use the remaining memcg
> > margin to decide whether a smaller charge might fit.
> >
> > This only refines folio_alloc_swap() return codes. The reclaim callers are
> > updated separately.
> >
> > Suggested-by: Barry Song <[email protected]>
> > Suggested-by: Youngjun Park <[email protected]>
> > Signed-off-by: Xueyuan Chen <[email protected]>
> > ---
> >  include/linux/swap.h | 16 ++++++++++++----
> >  mm/memcontrol.c      | 32 +++++++++++++++++++++++++++++++-
> >  mm/swapfile.c        | 32 ++++++++++++++++++++++++--------
> >  3 files changed, 67 insertions(+), 13 deletions(-)
> >
>
> Hello Xueyuan,
>
> Thanks for the patch!
>
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 0544b2ec4c56..7d12058174ae 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -509,12 +509,13 @@ static inline void folio_throttle_swaprate(struct folio *folio, gfp_t gfp)
> >  #endif
> >
> >  #if defined(CONFIG_MEMCG) && defined(CONFIG_SWAP)
> > -int __mem_cgroup_try_charge_swap(struct folio *folio);
> > -static inline int mem_cgroup_try_charge_swap(struct folio *folio)
> > +int __mem_cgroup_try_charge_swap(struct folio *folio, long *swap_margin);
> > +static inline int mem_cgroup_try_charge_swap(struct folio *folio,
> > +                                          long *swap_margin)
>
> Am I the only one that feel this returning argument is a bit ugly? See below..
>
> > +/**
> > + * mem_cgroup_get_folio_swap_margin - get a folio's memcg swap margin
> > + * @folio: folio whose memcg margin is queried
> > + *
> > + * Return: Remaining chargeable pages in the folio's memcg hierarchy.
> > + */
> > +long mem_cgroup_get_folio_swap_margin(struct folio *folio)
> > +{
> > +     long swap_margin = PAGE_COUNTER_MAX;
> > +     struct mem_cgroup *memcg;
> > +     struct obj_cgroup *objcg;
> > +
> > +     if (mem_cgroup_disabled() || do_memsw_account())
> > +             return swap_margin;
> > +
> > +     objcg = folio_objcg(folio);
> > +     if (!objcg)
> > +             return swap_margin;
> > +
> > +     rcu_read_lock();
> > +     memcg = obj_cgroup_memcg(objcg);
> > +     swap_margin = page_counter_margin(&memcg->swap);
> > +     rcu_read_unlock();
> > +
> > +     return swap_margin;
> > +}
> > +
>
> Will is be good if we just always check the margin use this helper
> on alloc failure? Alloc failure should be a rather cold path I think?
>

The only disadvantage is that we don't get the margin together with
the charging failure. If this happens infrequently, it may not matter.