Re: Page fault scalability patch V18: Drop first acquisition of ptl

[Christoph Lameter <clameter@sgi.com>]

On Thu, 3 Mar 2005, Benjamin Herrenschmidt wrote:

> > There is no need to provide pte_cmpxchg. If the arch does not support
> > cmpxchg on ptes (CONFIG_ATOMIC_TABLE_OPS not defined)
> > then it will fall back to using pte_get_and_clear while holding the
> > page_table_lock to insure that the entry is not touched while performing
> > the comparison.
>
> Nah, this is wrong :)
>
> We actually _want_ pte_cmpxchg on ppc64, because we can do the stuff,
> but it requires some careful manipulation of some bits in the PTE that
> are beyond linux common layer understanding :) Like the BUSY bit which
> is a lock bit for arbitrating with the hash fault handler for example.
>
> Also, if it's ever used to cmpxchg from anything but a !present PTE, it
> will need additional massaging (like the COW case where we just
> "replace" a PTE with set_pte). We also need to preserve some bits in
> there that indicate if the PTE was in the hash table and where in the
> hash so we can flush it afterward.

You can define your own pte_cmpxchg without a problem ...