vmalloc size

[Subramaniam Appadodharana]

On Tue, Jun 26, 2012 at 12:41 AM, Dave Hylands <dhylands at gmail.com> wrote:

> Hi Subbu,
>
> On Mon, Jun 25, 2012 at 7:45 PM, Subramaniam Appadodharana
> <c.a.subramaniam at gmail.com> wrote:
> ...snip...
> >> However, if you call vmalloc and lets suppose that vmalloc just
> >> happens to return 0xE0000000. The physical address of the first page
> >> might be 0xD2345000.
> >>
> >> What's important is that the physical pages which back up the vmalloc
> >> area all come from the kernel direct mapped area. They won't ever be
> >> backed by pages from high-memory. So the physical addresses will all
> >> be in the range 0x40000000 thru 0x5FFFFFFF.
> >>
> > This is the one that I had a completely wrong understanding of!!!!!!!
> > I understand from the above statement that  the vmalloc'ed virtual
> address
> > will _ALWAYS_  correspond to a  physical address from the lowmem region!
> I
> > was under the impression that the carved out region for the vmalloc,
> > is the one that would back any vmalloc'ed virtual address, which is
> > absolutely wrong by what you are saying.
>
> Actually, I was the one that was wrong. vmalloc pages can come from
> low or highmem.
>
Thanks for clarifying! Not a problem.

>
>
> Now this also means that increasing vmalloc inadvertently reduces lowmem.
> > Why is this designed such a way?
>
> It may or may not depending on the amount of physical memory and the
> size of the vmalloc space.
>
> vmalloc space will normally increase vmalloc_end, which won't reduce
> lowmem.
> If the end can't advance any further, then I believe that the start
> can be reduced. This will reduce lowmem, if the lowmem overlaps with
> vmalloc memory.
>
> Okay! got that!

> > Essentially, the idea that we increase vmalloc is because we expect more
> > memory to be consumed via vmalloc
> > calls, and hence we might need more physical address backing. But
> increasing
> > vmalloc decreases low mem, which would also mean that we have less
> backing.
> > Am I missing something here too :)?
>
> No - that was my mistake.
>
> No problem again. Thanks for correcting it!

> --
> Dave Hylands
> Shuswap, BC, Canada
> http://www.davehylands.com
>
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