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SubjectRe: page fault scalability patch final : i386 tested, x86_64 support added
On Wed, 1 Sep 2004, Benjamin Herrenschmidt wrote:

> The removal of the page table lock has other more subtle side effects
> on ppc64 (and ppc32 too) that aren't trivial to solve. Typically, due
> to the way we use the hash table as a TLB cache.
>
> For example, out ptep_test_and_clear will first clear the PTE and then
> flush the hash table entry. If in the meantime another CPU gets in,
> takes a fault, re-populates the PTE and fills the hash table via
> update_mmu_cache, we may end up with 2 hash PTEs for the same linux
> PTE at least for a short while. This is a potential cause of checkstop
> on ppc CPUs.
>
> There may be other subtle races of that sort I haven't encovered yet.
>
> We need to spend more time on our (ppc/ppc64) side to figure out what
> is the extent of the problem. We may have a cheap way to fix most of the
> issues using the PAGE_BUSY bit we have in the PTEs as a lock, but we
> don't have that facility on ppc32.
>
> I think there wouldn't be a problem if we could guarantee exclusion
> between page fault and clearing of a PTE (that is basically having the
> swapper take the mm write sem) but I don't think that's realistic, oh
> well, not that I understand anything about the swap code anyways...

We may be able to accomplish that by generic routines for
ptep_cmpxchg and so on that would use the page table lock for platforms
that do not support atomic pte operations.

Something along the lines of:

pte_t ptep_xchg(struct mm_struct *mm, pte_t *ptep, pte_t new) {
pte_t old;
spin_lock(&mm->page_table_lock);
old = *ptep
set_pte(ptep, new_pte);
/* Do rehashing */
spin_unlock(&mm->page_table_lock);
return old;
}
This would limit the time that the page_table_lock is held to a minimum
and may still offer some of the performance improvements.

Would that be acceptable?
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