how has set_pgdir been replaced in 2.4.x

how has set_pgdir been replaced in 2.4.x

[Martin Maletinsky]

Hello,

In the 2.2.x Linux kernel there used to be a function set_pgdir(), in charge of keeping all page global directories consistent. I.e. when modifying the kernel page tables
(e.g. in vmalloc, or ioremap), set_pgdir() was called, to update the corresponding entry in any processes global page directory, as well as in the cached global page
directories.

I noticed that in the 2.4.x Linux kernel the function set_pgdir() has gone (at least for most platforms). When looking at code that modifies kernel page tables (e.g.
vmalloc_area_pages) I could not figure out, how the page global directories are kept consistent. It looks to me as if
global page directory entries were modified in one global page directory (the swapper_pg_dir) only. If this is the case, I wonder how the modifications are 'propagated'
into all the other global page directories (I think they *must* at one moment be copied into all other global page directories, in order to make the new memory mapping
visible in all process contexts).

I thought that maybe one global page directory was used for all processes, but than I wonder how the user space mappings in the beginning of the virtual address space
(which are process specific) can be handled.

Please put me on cc: in your reply, since I am not subscribed to the list.

thank you in advance, regards
Martin

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Re: how has set_pgdir been replaced in 2.4.x

[Stephen C. Tweedie]

Hi,

On Mon, Dec 10, 2001 at 09:53:25AM +0100, Martin Maletinsky wrote:
 
> I noticed that in the 2.4.x Linux kernel the function set_pgdir() has gone (at least for most platforms). When looking at code that modifies kernel page tables (e.g.
> vmalloc_area_pages) I could not figure out, how the page global directories are kept consistent. It looks to me as if
> global page directory entries were modified in one global page directory (the swapper_pg_dir) only. If this is the case, I wonder how the modifications are 'propagated'
> into all the other global page directories

They are now faulted on demand for vmalloc.  The cost of manually
updating all the pgds for every vmalloc is just too expensive if
you've got tens of thousands of threads in the system.

Cheers,
 Stephen
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RE: how has set_pgdir been replaced in 2.4.x

[Martin Frey]

Hi,

>> I noticed that in the 2.4.x Linux kernel the function 
>set_pgdir() has gone (at least for most platforms). When 
>looking at code that modifies kernel page tables (e.g.
>> vmalloc_area_pages) I could not figure out, how the page 
>global directories are kept consistent. It looks to me as if
>> global page directory entries were modified in one global 
>page directory (the swapper_pg_dir) only. If this is the case, 
>I wonder how the modifications are 'propagated'
>> into all the other global page directories
>
>They are now faulted on demand for vmalloc.  The cost of manually
>updating all the pgds for every vmalloc is just too expensive if
>you've got tens of thousands of threads in the system.
>

Is there an implication on drivers, e.g. not accessing vmalloc'd
memory from within a page fault handler? A page fault from a
page fault handler is quite ugly...

Best regards, Martin
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Re: how has set_pgdir been replaced in 2.4.x

[Stephen C. Tweedie]

Hi,

On Mon, Dec 10, 2001 at 01:35:19PM +0100, Martin Frey wrote:
 
> >They are now faulted on demand for vmalloc.  The cost of manually
> >updating all the pgds for every vmalloc is just too expensive if
> >you've got tens of thousands of threads in the system.
> 
> Is there an implication on drivers, e.g. not accessing vmalloc'd
> memory from within a page fault handler? A page fault from a
> page fault handler is quite ugly...

arch/i386/mm/fault.c is where the magic happens.  The vmalloc fault
path is really, really careful not to access any potentially volatile
data, so it is supposed to be safe for use even inside traps and
interrupts.

There _is_ one place where it can break down: if you have a driver
which walks page tables manually after a vmalloc, expecting to
translate virtual addresses into physical ones before the VA has been
faulted in.  Other than that, it should be safe.

Cheers,
 Stephen
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