Re: mmap doesn't "wrap?"

[kanoj@google.engr.sgi.com (Kanoj Sarcar)]

> 
> It looks to me as though mmap will try to grab a region of memory whose
> address is greater than or equal to the suggested start address passed in
> the call.  I suppose it does this because it traverses the free list in
> ascending address order.  If it can't find enough memory above the
> requested address, then the mmap fails, claiming ENOMEM; and it does this
> even when you've got gigs of wide open address space below the suggested
> address.
> 
> One effect of this is that if you specify an address of 0 (saying "I don't
> care"), the mmap seems to start above 0x40000000 by default, and forever
> ignores the gigabyte of address space below that.  This seems undesirable,
> especially when your chunk of memory requested would fit below 0x4000000
> but can't fit above it (due to memory fragmentation, or because you've
> already used up the rest of it!).
> 
> I don't know if the right thing would be to change the behavior of
> get_unmapped_area() to "wrap" from the beginning of the free list when it
> can't find anything above the given address, or just add a failure clause
> into do_mmap() to try again starting from 0 (or something else?).  
> 
> 
> (Yes, I've patched my rlimits so I can access 3GB virtual and yes, I 
> commit the sin of addressing the 32-bit community only on this more 
> general mailing list: apologies to you lucky 64-bit folks.)
> 
> thx,
> grg
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Look at my patch for get_unmapped_area() at

	http://reality.sgi.com/kanoj_engr/garea.html

This has been submitted to Linus previously, unfortunately, it has not
been accepted. The current get_unmapped_area() is not POSIX compliant 
according to my thinking.

Regarding TASK_UNMAPPED_BASE, you can browse

	http://reality.sgi.com/kanoj_engr/tuning.html

which has some pointers on how to use the 3Gb user space judiciously. 
Basically, you can tune TASK_UNMAPPED_BASE to 0x10000000, if your apps
are "well-behaved".

Kanoj
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