2009/5/18 Peter Teoh <[email protected]>
> sorry...my bad....u are indeed using GFP_KERNEL for __get_free_pages().
>
> let me make another guess....since u use VMA operation to implement
> the mmapping, did u clear up the (unmap) the VMA before free-ing the
> pages?
The userspace code does the following:
void *p = mmap(0, size / 2, PROT_READ | PROT_WRITE, MAP_SHARED, fd, addr +
size / 2);
if (p != MAP_FAILED) {
p[0] = 0x11223344;
munmap(p, size / 2);
} else {
perror("mmap");
}
So shouldn't take take care of clearing the vma?
Anyway, I did some grepping with kernel sources, and found the following in
drivers/video/vermillion/vermillion.c:
/*
* It seems like __get_free_pages only ups the usage count
* of the first page. This doesn't work with fault mapping, so
* up the usage count once more (XXX: should use split_page or
* compound page).
*/
memset((void *)va->logical, 0x00, va->size);
for (i = va->logical; i < va->logical + va->size; i += PAGE_SIZE) {
get_page(virt_to_page(i));
}
I changed my own code to do the get_page() calls after allocating the memory
and it seems to work with that. (Also need to decrease usage count before
freeing pages.)
this is because kernel will go through the entire chain list of VMA to
>
free it.....and necessarily...it always assum that VMA's memory comes
>
from HIGH mem - which is why u have no problem when vmalloc() is used.
>
> (ie, u have violated one of the rule - never never used lowmem for
>
userspace operation....but it's another big story altogether.....).
>
u are lucky they did not check everywhere in kernel source
>
(performance reason)....
>
I'm not sure I understand. Basically I'm using LDD3 as reference, it has the
scullp example where they implement mmap by nopage method. Isn't fault
method more or less the same operation, only more generalized?
