Re: [PATCH RFC] mm: mmap: Change DEFAULT_MAX_MAP_COUNT to INT_MAX

[Pedro Falcato <pedro.falcato@gmail.com>]

On Mon, Sep 02, 2024 at 12:37:48PM GMT, Petr Špaček wrote:
> On 30. 08. 24 19:00, Petr Špaček wrote:
> > On 30. 08. 24 17:04, Pedro Falcato wrote:
> > > On Fri, Aug 30, 2024 at 04:28:33PM GMT, Petr Špaček wrote:
> > > 
> > > Can you get us a dump of the /proc/<pid>/maps? It'd be interesting
> > > to see how
> > > exactly you're hitting this.
> 
> https://users.isc.org/~pspacek/sf1717/bind-9.18.28-jemalloc-maps.xz
> 
> RSS was about 8.9 GB when the snapshot was taken.
> 
> I'm curious about your conclusions from this data. Thank you for your time!

I'm not a jemalloc expert (maybe they could chime in) but a quick look suggests
jemalloc is poking _a lot_ of holes into your memory map (with munmap).
There were theories regarding jemalloc guard pages, but these don't even seem
to be it. E.g:

7fa95d392000-7fa95d4ab000 rw-p 00000000 00:00 0
7fa95d4ac000-7fa95d4b7000 rw-p 00000000 00:00 0
7fa95d4b8000-7fa95d4dd000 rw-p 00000000 00:00 0
7fa95d4de000-7fa95d4f2000 rw-p 00000000 00:00 0
7fa95d4f3000-7fa95d4f9000 rw-p 00000000 00:00 0
7fa95d4fa000-7fa95d512000 rw-p 00000000 00:00 0
7fa95d513000-7fa95d53d000 rw-p 00000000 00:00 0
7fa95d53e000-7fa95d555000 rw-p 00000000 00:00 0
7fa95d556000-7fa95d5ab000 rw-p 00000000 00:00 0
7fa95d5ac000-7fa95d5b4000 rw-p 00000000 00:00 0

Where we have about a one page gap between every vma. Either jemalloc is a big fan
of munmap on free(), or this is some novel guard page technique I've never seen before :)
MADV_DONTNEED should work just fine on systems with overcommit on.

-- 
Pedro