linux-kernel - Re: [PATCH 0/9] Avoid populating unbounded num of ptes with mmap

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Date:	Fri, 21 Dec 2012 16:36:21 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>, Jorn_Engel <joern@...fs.org>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/9] Avoid populating unbounded num of ptes with mmap_sem held

On Thu, Dec 20, 2012 at 4:49 PM, Michel Lespinasse <walken@...gle.com> wrote:
> We have many vma manipulation functions that are fast in the typical case,
> but can optionally be instructed to populate an unbounded number of ptes
> within the region they work on:
> - mmap with MAP_POPULATE or MAP_LOCKED flags;
> - remap_file_pages() with MAP_NONBLOCK not set or when working on a
>   VM_LOCKED vma;
> - mmap_region() and all its wrappers when mlock(MCL_FUTURE) is in effect;
> - brk() when mlock(MCL_FUTURE) is in effect.
>

Something's buggy here.  My evil test case is stuck with lots of
threads spinning at 100% system time.  Stack traces look like:

[<0000000000000000>] __mlock_vma_pages_range+0x66/0x70
[<0000000000000000>] __mm_populate+0xf9/0x150
[<0000000000000000>] vm_mmap_pgoff+0x9f/0xc0
[<0000000000000000>] sys_mmap_pgoff+0x7e/0x150
[<0000000000000000>] sys_mmap+0x22/0x30
[<0000000000000000>] system_call_fastpath+0x16/0x1b
[<0000000000000000>] 0xffffffffffffffff

perf top says:

 38.45%  [kernel]            [k] __mlock_vma_pages_range
 33.04%  [kernel]            [k] __get_user_pages
 28.18%  [kernel]            [k] __mm_populate

The tasks in question use MCL_FUTURE but not MAP_POPULATE.  These
tasks are immune to SIGKILL.

--Andy
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