Re: [PATCH bpf-next v5 4/7] memcg: Use trylock to access memcg stock_lock.

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Wed, 15 Jan 2025 18:22:28 -0800 Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> On Wed, Jan 15, 2025 at 4:12 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > On Tue, Jan 14, 2025 at 06:17:43PM -0800, Alexei Starovoitov wrote:
> > > From: Alexei Starovoitov <ast@kernel.org>
> > >
> > > Teach memcg to operate under trylock conditions when spinning locks
> > > cannot be used.
> > >
> > > local_trylock might fail and this would lead to charge cache bypass if
> > > the calling context doesn't allow spinning (gfpflags_allow_spinning).
> > > In those cases charge the memcg counter directly and fail early if
> > > that is not possible. This might cause a pre-mature charge failing
> > > but it will allow an opportunistic charging that is safe from
> > > try_alloc_pages path.
> > >
> > > Acked-by: Michal Hocko <mhocko@suse.com>
> > > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> >
> > Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> >
> > > @@ -1851,7 +1856,14 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> > >  {
> > >       unsigned long flags;
> > >
> > > -     local_lock_irqsave(&memcg_stock.stock_lock, flags);
> > > +     if (!local_trylock_irqsave(&memcg_stock.stock_lock, flags)) {
> > > +             /*
> > > +              * In case of unlikely failure to lock percpu stock_lock
> > > +              * uncharge memcg directly.
> > > +              */
> > > +             mem_cgroup_cancel_charge(memcg, nr_pages);
> >
> > mem_cgroup_cancel_charge() has been removed by a patch in mm-tree. Maybe
> > we can either revive mem_cgroup_cancel_charge() or simply inline it
> > here.
> 
> Ouch.
> 
> this one?
> https://lore.kernel.org/all/20241211203951.764733-4-joshua.hahnjy@gmail.com/
> 
> Joshua,
> 
> could you hold on to that clean up?
> Or leave mem_cgroup_cancel_charge() in place ?

Hi Alexei,

Yes, that makes sense to me. The goal of the patch was to remove the
last users and remove it, but if there are users of the function, I
don't think the patch makes any sense : -)

Have a great day!
Joshua



Hi Andrew,

I think that the patch was moved into mm-stable earlier this week.
I was wondering if it would be possible to revert the patch and
replace it with this one below. The only difference is that I leave
mem_cgroup_cancel_charge untouched in this version.

I'm also not sure if this is the best way to send the revised patch.
Please let me know if there is another way I should do this to make
it easiest for you!

Thank you for your time!
Joshua

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index d1ee98dc3a38..c8d0554e5490 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -620,8 +620,6 @@ static inline bool mem_cgroup_below_min(struct mem_cgroup *target,
 		page_counter_read(&memcg->memory);
 }

-void mem_cgroup_commit_charge(struct folio *folio, struct mem_cgroup *memcg);
-
 int __mem_cgroup_charge(struct folio *folio, struct mm_struct *mm, gfp_t gfp);

 /**
@@ -646,9 +644,6 @@ static inline int mem_cgroup_charge(struct folio *folio, struct mm_struct *mm,
 	return __mem_cgroup_charge(folio, mm, gfp);
 }

-int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg, gfp_t gfp,
-		long nr_pages);
-
 int mem_cgroup_charge_hugetlb(struct folio* folio, gfp_t gfp);

 int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm,
@@ -1137,23 +1132,12 @@ static inline bool mem_cgroup_below_min(struct mem_cgroup *target,
 	return false;
 }

-static inline void mem_cgroup_commit_charge(struct folio *folio,
-		struct mem_cgroup *memcg)
-{
-}
-
 static inline int mem_cgroup_charge(struct folio *folio,
 		struct mm_struct *mm, gfp_t gfp)
 {
 	return 0;
 }

-static inline int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg,
-		gfp_t gfp, long nr_pages)
-{
-	return 0;
-}
-
 static inline int mem_cgroup_swapin_charge_folio(struct folio *folio,
 			struct mm_struct *mm, gfp_t gfp, swp_entry_t entry)
 {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index dd171bdf1bcc..aeff2af8d722 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2402,18 +2402,6 @@ static void commit_charge(struct folio *folio, struct mem_cgroup *memcg)
 	folio->memcg_data = (unsigned long)memcg;
 }

-/**
- * mem_cgroup_commit_charge - commit a previously successful try_charge().
- * @folio: folio to commit the charge to.
- * @memcg: memcg previously charged.
- */
-void mem_cgroup_commit_charge(struct folio *folio, struct mem_cgroup *memcg)
-{
-	css_get(&memcg->css);
-	commit_charge(folio, memcg);
-	memcg1_commit_charge(folio, memcg);
-}
-
 static inline void __mod_objcg_mlstate(struct obj_cgroup *objcg,
 				       struct pglist_data *pgdat,
 				       enum node_stat_item idx, int nr)
@@ -4501,7 +4489,9 @@ static int charge_memcg(struct folio *folio, struct mem_cgroup *memcg,
 	if (ret)
 		goto out;

-	mem_cgroup_commit_charge(folio, memcg);
+	css_get(&memcg->css);
+	commit_charge(folio, memcg);
+	memcg1_commit_charge(folio, memcg);
 out:
 	return ret;
 }
@@ -4527,40 +4517,6 @@ bool memcg_accounts_hugetlb(void)
 #endif
 }

-/**
- * mem_cgroup_hugetlb_try_charge - try to charge the memcg for a hugetlb folio
- * @memcg: memcg to charge.
- * @gfp: reclaim mode.
- * @nr_pages: number of pages to charge.
- *
- * This function is called when allocating a huge page folio to determine if
- * the memcg has the capacity for it. It does not commit the charge yet,
- * as the hugetlb folio itself has not been obtained from the hugetlb pool.
- *
- * Once we have obtained the hugetlb folio, we can call
- * mem_cgroup_commit_charge() to commit the charge. If we fail to obtain the
- * folio, we should instead call mem_cgroup_cancel_charge() to undo the effect
- * of try_charge().
- *
- * Returns 0 on success. Otherwise, an error code is returned.
- */
-int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg, gfp_t gfp,
-			long nr_pages)
-{
-	/*
-	 * If hugetlb memcg charging is not enabled, do not fail hugetlb allocation,
-	 * but do not attempt to commit charge later (or cancel on error) either.
-	 */
-	if (mem_cgroup_disabled() || !memcg ||
-		!cgroup_subsys_on_dfl(memory_cgrp_subsys) || !memcg_accounts_hugetlb())
-		return -EOPNOTSUPP;
-
-	if (try_charge(memcg, gfp, nr_pages))
-		return -ENOMEM;
-
-	return 0;
-}
-
 int mem_cgroup_charge_hugetlb(struct folio *folio, gfp_t gfp)
 {
 	struct mem_cgroup *memcg = get_mem_cgroup_from_current();