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Date: Thu, 15 Feb 2024 13:38:03 -0500
From: Kent Overstreet <kent.overstreet@linux.dev>
To: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>, akpm@linux-foundation.org, 
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Subject: Re: [PATCH v3 31/35] lib: add memory allocations report in show_mem()
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On Thu, Feb 15, 2024 at 10:33:53AM -0800, Suren Baghdasaryan wrote:
> On Thu, Feb 15, 2024 at 10:29 AM Kent Overstreet
> <kent.overstreet@linux.dev> wrote:
> >
> > On Thu, Feb 15, 2024 at 08:47:59AM -0800, Suren Baghdasaryan wrote:
> > > On Thu, Feb 15, 2024 at 8:45 AM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > On Thu 15-02-24 06:58:42, Suren Baghdasaryan wrote:
> > > > > On Thu, Feb 15, 2024 at 1:22 AM Michal Hocko <mhocko@suse.com> wrote:
> > > > > >
> > > > > > On Mon 12-02-24 13:39:17, Suren Baghdasaryan wrote:
> > > > > > [...]
> > > > > > > @@ -423,4 +424,18 @@ 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
> > > > > > > +#ifdef CONFIG_MEM_ALLOC_PROFILING
> > > > > > > +     {
> > > > > > > +             struct seq_buf s;
> > > > > > > +             char *buf = kmalloc(4096, GFP_ATOMIC);
> > > > > > > +
> > > > > > > +             if (buf) {
> > > > > > > +                     printk("Memory allocations:\n");
> > > > > > > +                     seq_buf_init(&s, buf, 4096);
> > > > > > > +                     alloc_tags_show_mem_report(&s);
> > > > > > > +                     printk("%s", buf);
> > > > > > > +                     kfree(buf);
> > > > > > > +             }
> > > > > > > +     }
> > > > > > > +#endif
> > > > > >
> > > > > > I am pretty sure I have already objected to this. Memory allocations in
> > > > > > the oom path are simply no go unless there is absolutely no other way
> > > > > > around that. In this case the buffer could be preallocated.
> > > > >
> > > > > Good point. We will change this to a smaller buffer allocated on the
> > > > > stack and will print records one-by-one. Thanks!
> > > >
> > > > __show_mem could be called with a very deep call chains. A single
> > > > pre-allocated buffer should just do ok.
> > >
> > > Ack. Will do.
> >
> > No, we're not going to permanently burn 4k here.
> 
> We don't need 4K here. Just enough to store one line and then print
> these 10 highest allocations one line at a time. This way we can also
> change that 10 to any higher number we like without any side effects.

There's no reason to make the change at all. If Michal thinks there's
something "dangerous" about allocating a buffer here, he needs to able
to explain what it is.