PG_reserved in PCI DMA-able mem

Davide Rossetti (davide.rossetti@roma1.infn.it)
Fri, 25 Jun 1999 00:35:25 +0200

In porting a mixed user/kernel driver for a PCI BusMastering device,
after some struggle and a look to the sound driver, I realized that
PG_reserved is necessary in kernel memory which has to be mapped from
within user space as in:

kvaddr = kmalloc(size, ...) // where size is n*PAGE_SIZE
...
{
void* adr= kvaddr;
unsigned long page;
unsigned int sz = size;
while (sz > 0)
{
mem_map_reserve(MAP_NR(adr));
adr += PAGE_SIZE;
sz -= PAGE_SIZE;
}
}

what is the sense of the comment in include/linux/mm.h ?

/*
* Various page->flags bits:
*
* PG_reserved is set for a page which must never be accessed (which
* may not even be present).
*
...

"which must never be accessed" refers to the I/O subsystem, doesn't it ?
or to generically to code instructions ?

I'm curious, what is that bit for ?
I experimented the effect: user space doesn't see any modification made
from kernel code or PCI DMA access.

ciao

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