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Date: Thu, 15 Feb 2024 22:54:53 +0100
From: Michal Hocko <mhocko@suse.com>
To: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Vlastimil Babka <vbabka@suse.cz>,
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Subject: Re: [PATCH v3 31/35] lib: add memory allocations report in show_mem()
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On Thu 15-02-24 15:33:30, Kent Overstreet wrote:
> On Thu, Feb 15, 2024 at 09:22:07PM +0100, Vlastimil Babka wrote:
> > 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@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.
> > > 
> > > 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?
> 
> __GFP_NOWARN is a good point, we should have that.
> 
> But - and correct me if I'm wrong here - doesn't an OOM kick in well
> before GFP_ATOMIC 4k allocations are failing?

Not really, GFP_ATOMIC users can compete with reclaimers and consume
those reserves.

> I'd expect the system to
> be well and truly hosed at that point.

It is OOMed...
 
> If we want this report to be 100% reliable, then yes the preallocated
> buffer makes sense - but I don't think 100% makes sense here; I think we
> can accept ~99% and give back that 4k.

Think about that from the memory reserves consumers. The atomic reserve
is a scarse resource and now you want to use it for debugging purposes
for which you could have preallocated.
-- 
Michal Hocko
SUSE Labs