linux-kernel - Re: [PATCH v3 31/35] lib: add memory allocations report in show

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Message-ID: <CAJuCfpH2EF8DZhBp_7324ka7mnMkUdWyqTs+ZiMhwjm_nmcwZQ@mail.gmail.com>
Date: Thu, 15 Feb 2024 10:49:59 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>, akpm@...ux-foundation.org, vbabka@...e.cz, 
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Subject: Re: [PATCH v3 31/35] lib: add memory allocations report in show_mem()

On Thu, Feb 15, 2024 at 10:41 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Thu 15-02-24 13:29:40, Kent Overstreet 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@...e.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@...ecom> 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.
> >
> > It's completely fine if the allocation fails, there's nothing "unsafe"
> > about doing a GFP_ATOMIC allocation here.
>
> Nobody is talking about safety. This is just a wrong thing to do when
> you are likely under OOM situation. This is a situation when you
> GFP_ATOMIC allocation is _likely_ to fail. Yes, yes you will get some
> additional memory reservers head room, but you shouldn't rely on that
> because that will make the output unreliable. Not something you want in
> situation when you really want to know that information.
>
> More over you do not need to preallocate a full page.

Folks, please stop arguing about it. We have more important things to
do. I'll fix it to use a small preallocated buffer.

>
> --
> Michal Hocko
> SUSE Labs