linux-kernel - Re: [PATCH RFC] mm: mmap: Change DEFAULT_MAX_MAP_COUNT to INT

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Message-ID: <53f4ixl6heo5yvhp7j2c7j4q7ftai6nztmpc22jtaj2u3og5go@4kmzjef43hky>
Date: Mon, 2 Sep 2024 12:05:45 +0100
From: Pedro Falcato <pedro.falcato@...il.com>
To: Petr Špaček <pspacek@....org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>, 
	Liam Howlett <liam.howlett@...cle.com>
Subject: Re: [PATCH RFC] mm: mmap: Change DEFAULT_MAX_MAP_COUNT to INT_MAX

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