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

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Date: Thu, 15 Feb 2024 21:22:07 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Kent Overstreet <kent.overstreet@...ux.dev>,
 Suren Baghdasaryan <surenb@...gle.com>
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Subject: Re: [PATCH v3 31/35] lib: add memory allocations report in show_mem()

On 2/15/24 19:29, 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@...e.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.
> 
> It's completely fine if the allocation fails, there's nothing "unsafe"
> about doing a GFP_ATOMIC allocation here.

Well, I think without __GFP_NOWARN it will cause a warning and thus
recursion into __show_mem(), potentially infinite? Which is of course
trivial to fix, but I'd myself rather sacrifice a bit of memory to get this
potentially very useful output, if I enabled the profiling. The necessary
memory overhead of page_ext and slabobj_ext makes the printing buffer
overhead negligible in comparison?