linux-kernel - Re: [PATCH] mm: memcg: dump memcg protection info on oom or alloc failures

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Message-ID: <tawekrqql6efopwac3zrqgrszueampnadqp7s3g7wfvohsiqbt@22ai2cryardu>
Date: Fri, 7 Nov 2025 21:45:04 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: SeongJae Park <sj@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	David Rientjes <rientjes@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org, 
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH] mm: memcg: dump memcg protection info on oom or alloc
 failures

On Fri, Nov 07, 2025 at 06:26:38PM -0800, SeongJae Park wrote:
> On Fri,  7 Nov 2025 15:40:41 -0800 Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> 
> > Currently kernel dumps memory state on oom and allocation failures. One
> > of the question usually raised on those dumps is why the kernel has not
> > reclaimed the reclaimable memory instead of triggering oom. One
> > potential reason is the usage of memory protection provided by memcg.
> > So, let's also dump the memory protected by the memcg in such reports to
> > ease the debugging.
> > 
> > Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> > ---
> [...]
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index c34029e92bab..623446821b00 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5636,3 +5636,16 @@ bool mem_cgroup_node_allowed(struct mem_cgroup *memcg, int nid)
> >  {
> >  	return memcg ? cpuset_node_allowed(memcg->css.cgroup, nid) : true;
> >  }
> > +
> > +void mem_cgroup_show_protected_memory(struct mem_cgroup *memcg)
> > +{
> > +	if (mem_cgroup_disabled() || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
> > +		return;
> > +
> > +	if (!memcg)
> > +		memcg = root_mem_cgroup;
> > +
> > +	pr_warn("Memory cgroup min protection %lukB -- low protection %lukB",
> > +		K(atomic_long_read(&memcg->memory.children_min_usage)*PAGE_SIZE),
> > +		K(atomic_long_read(&memcg->memory.children_low_usage)*PAGE_SIZE));
> > +}
> 
> I didn't expect this function is showing the information by calling pr_warn().
> To me, "show" feels like something for file operations, like memory_min_show().
> 
> What about s/show/dump/ on the name?  It makes it more consistent with the
> subject of this patch, and other similar functions like dump_page() ?
> 
> No strong opinion.  The current name is also ok for me, but I'm just curious your thought.
> 

I just took the inspiration from show_mem(). Initially I was trying to
put these pr_warn in show_mem() but noticed that it was called from more
places than I intend to print this info, so decided to have a separate
function.

Thanks for taking a look.