linux-kernel - Re: [PATCH 4/7] mm: Centralize & improve oom reporting in show

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Date:   Tue, 28 Nov 2023 12:54:39 -0500
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Michal Hocko <mhocko@...e.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH 4/7] mm: Centralize & improve oom reporting in show_mem.c

On Tue, Nov 28, 2023 at 11:07:18AM +0100, Michal Hocko wrote:
> On Wed 22-11-23 18:25:09, Kent Overstreet wrote:
> [...]
> > 00177 Shrinkers:
> > 00177 super_cache_scan: objects: 127
> > 00177 super_cache_scan: objects: 106
> > 00177 jbd2_journal_shrink_scan: objects: 32
> > 00177 ext4_es_scan: objects: 32
> > 00177 bch2_btree_cache_scan: objects: 8
> > 00177   nr nodes:          24
> > 00177   nr dirty:          0
> > 00177   cannibalize lock:  0000000000000000
> > 00177
> > 00177 super_cache_scan: objects: 8
> > 00177 super_cache_scan: objects: 1
> 
> It would be really great to provide an example on how these numbers are
> useful for the oom evaluation.

I should've posted an example from the end of the patch series; I'll do
that later today.

> [...]
> > @@ -423,4 +426,21 @@ void __show_mem(unsigned int filter, nodemask_t *nodemask, int max_zone_idx)
> >  #ifdef CONFIG_MEMORY_FAILURE
> >  	printk("%lu pages hwpoisoned\n", atomic_long_read(&num_poisoned_pages));
> >  #endif
> > +
> > +	buf = kmalloc(4096, GFP_ATOMIC);
> 
> I really do not think we want to allow allocations from the OOM context.
> Is there any reason why this cannot be a statically allocated buffer?

You've made this claim before without ever giving any reasoning behind
it.

It's GFP_ATOMIC; it has to work from _interrupt_ context, OOM context is
fine.

And no, we don't want to burn 4k on a static buffer that is almost never
used; people do care about making the kernel run on small systems.