Re: [2.6 patch] i386: always use 4k stacks

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Fri, 16 Dec 2005, David Lang wrote:

> On Fri, 16 Dec 2005, linux-os (Dick Johnson) wrote:
>
>> On Fri, 16 Dec 2005, Horst von Brand wrote:
>>
>>> linux-os \(Dick Johnson\) <linux-os@analogic.com> wrote:
>>>
>>> [...]
>>>
>>>> Throughout the past two years of 4k stack-wars, I never heard why
>>>> such a small stack was needed (not wanted, needed). It seems that
>>>> everybody "knows" that smaller is better and most everybody thinks
>>>> that one page in ix86 land is "optimum". However I don't think
>>>> anybody ever even tried to analyze what was better from a technical
>>>> perspective. Instead it's been analyzed as religious dogma, i.e.,
>>>> keep the stack small, it will prevent idiots from doing bad things.
>>>
>>> OK, so here goes again...
>>>
>>> The kernel stack has to be contiguous in /physical/ memory. Keep the
>> stack
>>> /one/ page, that way you can always get a new stack when needed (==
>> each
>>> fork(2) or clone(2)). If the stack is 2 (or more) pages, you'll have
>> to
>>> find (or create) a multi-page free area, and (fragmentation being what
>> it
>>> is, and Linux routinely running for months at a time) you are in a
>> whole
>>> new world of pain.
>>
>> The interrupt stack needs to be non-paged. Are you sure the user-stacks
>> need to be 'physical', non-paged too? If so, that's probably the
>> problem. All addresses accessed by the CPUs in the kernel are virtual
>> which means one needs some mapping anyway.
>
> actually, the kernel always uses real addresses, userspace uses virtual
> addresses.
>

No. Hint: What is PAGE_OFFSET?
Everything the CPU executes/reads/writes is translated from physical (bus)
addresses to virtual addresses.

What you may have heard or read is that the address-space that the
kernel uses for its code and resident data has fixed translation tables
which means one doesn't have to scan a bunch of tables to locate the
bus address, given the virtual address.

physical_address = virtual_address  - PAGE_OFFSET;
virtual_address  = physical_address + PAGE_OFFSET;

This is totally an artifact of how the page-tables are set up.

> This came up recently with the page tables, Linus said that he was
> absolutly opposed to adding the complication and overhead of changine the
> kernel to user virtual addresses instead of real addresses for it's data
> structures. it would add an extra level of redirection to just about every
> memory access (which also means an additional load on the cache to store
> the mapping info to resolve this redirection). The performance hit for
> this would be considerable.
>
> David Lang
>

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.56 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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