Can i allocate 4GB virtual addresses (more than a certain limit) using vmalloc?

[subin gangadharan]

Hi Dave,

>Also keep in mind that when you vmalloc stuff you're allocating in
>units of pages, and there is a guard page between each allocation. So
>if you had 512 Mb of virtual space (which is 131,072 pages), you'll be
>able to vmalloc a maximum of 65536 objects or 1 page each. Fewer
>objects if they're bigger than a page.

A quick question, Suppose if I alloc 1MB of memory using vmalloc,Is there a
guard page between each page or is it after the 1MB.

With Regards,
Subin K G




On Tue, May 31, 2011 at 10:19 AM, Dave Hylands <dhylands at gmail.com> wrote:

> Hi Sandeep,
>
> On Mon, May 30, 2011 at 10:54 PM, sandeep kumar
> <coolsandyforyou at gmail.com> wrote:
> > Hi all,
> > The following link gives the memory map for the arm architecture.
> > http://www.arm.linux.org.uk/developer/memory.txt
> >
> > I have the following doubts..
> > 1) Any chipset(based on arm) manufacturer(qualcom,samsung..) should
> follow
> > the same memory map.
> > Is it hardly constrained or can be changed?
> > Where are this constraints are implemented in the kernel source tree?
>
> You can have 2 different configurations for the same chip which
> present totally different memory maps. Where things are physically in
> memory varies from chip to chip, regardless of the fact that they
> happen to use the same processor.
>
> > 2) while i was student, i read in OS concepts that, "Virtual memory
> gives an
> > illusion to a process,
> > that it has always a larger continuous address space (even more than RAM)
> > available to it."
> > So i thought i could allocate howmuch ever memory i want.
> > But seeing the above link,i observed there is some limitation in the
> address
> > space created by the vmalloc().
> > So i m now thinking that vmalloc has some limit.
>
> Absolutely. You can only vmalloc as much memory as there is virtual
> space reserved for vmalloc. And your vmalloc memory space can become
> fragmented, so you could have lots of space, but not have enough
> contiguous virtual space left to allocate a large object.
>
> Also keep in mind that when you vmalloc stuff you're allocating in
> units of pages, and there is a guard page between each allocation. So
> if you had 512 Mb of virtual space (which is 131,072 pages), you'll be
> able to vmalloc a maximum of 65536 objects or 1 page each. Fewer
> objects if they're bigger than a page.
>
> The actual allocation of vmalloc memory is determined by the macros
> VMALLOC_START and VMALLOC_END. VMALLOC_START is defined in
> arch/arm/include/asm/pgtable.h:
> http://lxr.linux.no/linux+v2.6.39/arch/arm/include/asm/pgtable.h#L41
> although it can be overriden by a particular architecture.
>
> VMALLOC_END is normally defined in the machine's memory.h file, and it
> sits somewhere between VMALLOC_START and the beginning of I/O space.
>
> --
> Dave Hylands
> Shuswap, BC, Canada
> http://www.davehylands.com
>
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>



-- 
With Regards
Subin Gangadharan

Everything should be made as simple as possible,but not simpler.
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