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SubjectRe: Linux-2.5.16

On Mon, 20 May 2002, Paul Mackerras wrote:
> This patch splits up the existing tlb_remove_page into
> tlb_remove_tlb_entry (for pages that are/were mapped into userspace)
> and tlb_remove_page, as you suggested. It also adds the necessary
> stuff for PPC, which has its own include/asm-ppc/tlb.h now. This
> works on at least one PPC machine. :)

Looking into this a bit more, I came to the conclusion that this cannot
actually work on ppc.

The start/end address optimization cannot work the way you intended them
to, because the "tlb_remove_page()" semantics _only_ call it for "real
pages". Thus anything that is unknown to the kernel VM (ie reserved or
outside the kernel mem_map[]) will never cause that interface to be
called, since those pages will not be free'd.

I solved this by splitting up the interface: the tlb_remove_page() thing
still exists and does the same old thing, but I added a _separate_
"tlb_remove_tlb_entry()" which on x86 is a no-op, and that gets called on
each present pte entry.

So the loop basically looks like

tlb = tlb_gather_mmu(mm);
for_each_vma() {
tlb_start_vma(tlb, vma);
for_each_pte() {
tlb_remove_tlb_entry(tlb, pte, address);
if (pte_valid_page()) {
tlb_remove_page(tlb, page);
tlb_end_vma(tlb, vma);
tlb_finish_mmu(tlb, start, end);

where the x86 defines tlb_start_vma/tlb_end_vma/tlb_remove_tlb_entry to be
no-ops, because on the x86 we always just do a full TLB flush when we fill
up the free buffer.

For other architectures, any of the following may be a good idea:

- tlb_end_vma() looks at the type of the VMA, and flushes that type only
(ie if the vma was not executable, you can avoid the ITLB flush)

- tlb_remove_tlb_entry() can try to remove the entry proactively from MMU
hash chains like on the PPC: it gets enough information to do so. In
this case, the tlb_remove_page() thing might decide to only flush the
on-chip TLB, since it knows that the off-chip TLB has already been

(For this reason, tlb_finish_mmu() doesn't actually call the old
"flush_tlb_mm()", but instead calls a "tlb_flush(tlb)", so that the
architecture can know that it doesn't need to flush the whole mm)

- For architectures that hide hints in the PTE ("this pte has been loaded
for a iTLB access"), the tlb_remove_tlb_entry() routine might just or
in that bit into a tlb "status" word, and then tlb_flush(tlb) might
decide to only flush the DTLB if it never saw any TLB entries that had
the iTLB bit set. This should work for ia64 or alpha.

I tried to make the interface fairly generic, so that people could easily
do any (or none, like on x86) of these optimizations with little or no

It's there in 2.5.17..


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