linux-kernel - Re: /proc/pid/maps containg anonymous maps that have PROT

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Date:	Wed, 05 Nov 2008 20:47:44 +0200
From:	Török Edwin <edwintorok@...il.com>
To:	Hugh Dickins <hugh@...itas.com>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: /proc/pid/maps containg anonymous maps that have PROT_NONE

On 2008-11-05 19:56, Török Edwin wrote:
> On 2008-11-05 18:12, Hugh Dickins wrote:
>   
>>>    
>>>       
>> mmap PROT_NONE to reserve an arena, munmap to trim off top and bottom,
>> mprotect to make areas read+writable, madvise 0x4 to say MADV_DONTNEED
>> on some parts.  gcc?  Or the application itself (clamd) and its libs?
>>
>>   
>>     
>   
>> Why does it mmap too much then trim it down?  Perhaps it's trying to
>> minimize pagetable usage, perhaps it's internally convenient to base
>> on rounded addresses, I don't know.
>>
>> But the mmap is there: just easily overlooked because of the way it
>> munmaps too (with strace showing hex addresses but decimal sizes).
>>     
>
> I will get some stacktraces and figure out, know that I know which mmap
> to look for (the one with MAP_NORESERVE).
>   

I found it, glibc: arena.c:669 does it, there's a comment explaining why:

/* If consecutive mmap (0, HEAP_MAX_SIZE << 1, ...) calls return decreasing
   addresses as opposed to increasing, new_heap would badly fragment the
   address space.  In that case remember the second HEAP_MAX_SIZE part
   aligned to HEAP_MAX_SIZE from last mmap (0, HEAP_MAX_SIZE << 1, ...)
   call (if it is already aligned) and try to reuse it next time.  We need
   no locking for it, as kernel ensures the atomicity for us - worst case
   we'll call mmap (addr, HEAP_MAX_SIZE, ...) for some value of addr in
   multiple threads, but only one will succeed.  */

Anyway it is MAP_NORESERVE, and PROT_NONE so it doesn't waste physical
or swap memory.

Best regards,
--Edwin
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