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    SubjectRe: [PATCH 06/17] arm: mmu_gather rework
    On Mon, Feb 28, 2011 at 12:44:47PM +0100, Peter Zijlstra wrote:
    > Right, so the normal case is:
    >
    > unmap_region()
    > tlb_gather_mmu()

    The fullmm argument is important here as it specifies the mode.
    tlb_gather_mmu(, 0)

    > unmap_vmas()
    > for (; vma; vma = vma->vm_next)
    > unmao_page_range()
    > tlb_start_vma() -> flush cache range
    > zap_*_range()
    > ptep_get_and_clear_full() -> batch/track external tlbs
    > tlb_remove_tlb_entry() -> batch/track external tlbs
    > tlb_remove_page() -> track range/batch page
    > tlb_end_vma() -> flush tlb range

    tlb_finish_mmu() -> nothing

    >
    > [ for architectures that have hardware page table walkers
    > concurrent faults can still load the page tables ]
    >
    > free_pgtables()

    tlb_gather_mmu(, 1)

    > while (vma)
    > unlink_*_vma()
    > free_*_range()
    > *_free_tlb()
    > tlb_finish_mmu()

    tlb_finish_mmu() -> flush tlb mm

    >
    > free vmas

    So this is all fine. Note that we *don't* use the range stuff here.

    > Now, if we want to track ranges _and_ have hardware page table walkers
    > (ARM seems to be one such), we must flush TLBs at tlb_end_vma() because
    > flush_tlb_range() requires a vma pointer (ARM and TILE actually use more
    > than ->vm_mm), and on tlb_finish_mmu() issue a full mm wide invalidate
    > because the hardware walker could have re-populated the cache after
    > clearing the PTEs but before freeing the page tables.

    No. The hardware walker won't re-populate the TLB after the page table
    entries have been cleared - where would it get this information from if
    not from the page tables?

    > What ARM does is it retains the last vma pointer and tracks
    > pte_free_tlb() range and uses that in tlb_finish_mmu(), which is a tad
    > hacky.

    It may be hacky but then the TLB shootdown interface is hacky too. We
    don't keep the vma around to re-use after tlb_end_vma() - if you think
    that then you misunderstand what's going on. The vma pointer is kept
    around as a cheap way of allowing tlb_finish_mmu() to distinguish
    between the unmap_region() mode and the shift_arg_pages() mode.

    > Mostly because of shift_arg_pages(), where we have:
    >
    > shift_arg_pages()
    > tlb_gather_mmu()

    tlb_gather_mmu(, 0)

    > free_*_range()
    > tlb_finish_mmu()

    tlb_finish_mmu() does nothing without the ARM change.
    tlb_finish_mmu() -> flush_tlb_mm() with the ARM change.

    And this is where the bug was - these page table entries could find
    their way into the TLB and persist after they've been torn down.

    > For which ARM now punts and does a full tlb invalidate (no vma pointer).
    > But also because it drags along that vma pointer, which might not at all
    > match the range its actually going to invalidate (and hence its vm_flags
    > might not accurately reflect things -- at worst more expensive than
    > needed).

    Where do you get that from? Where exactly in the above code would the
    VMA pointer get set? In this case, it will be NULL, so we do a
    flush_tlb_mm() for this case. We have to - we don't have any VMA to
    deal with at this point.

    > The reason I wanted flush_tlb_range() to take an mm_struct and not the
    > current vm_area_struct is because we can avoid doing the
    > flush_tlb_range() from tlb_end_vma() and delay the thing until
    > tlb_finish_mmu() without having to resort to such games as above. We
    > could simply track the full range over all VMAs and free'd page-tables
    > and do one range invalidate.

    No. That's stupid. Consider the case where you have to loop one page
    at a time over the range (we do on ARM.) If we ended up with your
    suggestion above, that means we could potentially have to loop 4K at a
    time over 3GB of address space. That's idiotic when we have an
    instruction which can flush the entire TLB for a particular thread.

    --
    Russell King
    Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
    maintainer of:


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