Re: can device drivers return non-ram via vm_ops->nopage?

[Andrea Arcangeli <andrea@suse.de>]

On Sat, Mar 20, 2004 at 12:13:45PM -0800, Andrew Morton wrote:
> fyi, we don't need the check in page_referenced() and try_to_unmap()
> because do_no_page() does not place pages on the LRU.  It is the ->nopage

yes I've noticed.

> I agree that ->nopage implementations should not be doing what that driver
> is doing.  ->nopage is defined to return a page*: it's crazy to be
> returning someting from there which isn't covered by mem_map[].

I may have been wrong about that sorry, it's still not certain though,
but I had a bug in the code that would mistake a sigbus for a page_t
outside the mem_map, so it could have been a sigbus not a non-ram page.

Also in the meantime I noticed with NUMA it's impossible to handle
non-ram correctly in ->nopage, at least if using the current
page_to_pfn.

> I just don't think it's important enough to be able to cope with
> non-mem_map[] "memory" in do_no_page(), so I agree that requiring ->mmap()
> to synchronously instantiate the pte's and retaining the debug check in
> do_no_page() is a good idea.

I agree, I reistantiated the debug check because we cannot handle
non-ram from there if it's numa (actually discontigmem). If alsa uses
non-ram pages it must be fixed, but I've an hope it was a sigbus
trouble. We'll know more in a few more hours.

(btw, Martin definitely triggered the sigbus with numa, the 0x3()
dereference was a the page_t address to read page->flags >> 24)

it's good to have reminded the API cannot handle non-ram.