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    Subjecttrailing flush_tlb_fix_spurious_fault in handle_pte_fault (was Re: [PATCH 1/3] x86/mm: only do a local TLB flush in ptep_set_access_flags())
    On Monday 29 October 2012 08:53 PM, Rik van Riel wrote:
    > On 10/26/2012 03:18 PM, Linus Torvalds wrote:
    >> On Fri, Oct 26, 2012 at 12:16 PM, Rik van Riel <riel@redhat.com> wrote:
    >>>
    >>> I can change the text of the changelog, however it looks
    >>> like do_wp_page does actually use ptep_set_access_flags
    >>> to set the write bit in the pte...
    >>>
    >>> I guess both need to be reflected in the changelog text
    >>> somehow?
    >>
    >> Yeah, and by now, after all this discussion, I suspect it should be
    >> committed with a comment too. Commit messages are good and all, but
    >> unless chasing a particular bug they introduced, we shouldn't expect
    >> people to read them for background information.
    >
    > Now that we have the TLB things taken care of, and
    > comments to patches 10/31 and 26/31 have been addressed,
    > is there anything else that needs to be done before
    > these NUMA patches can be merged?
    >
    > Anyone, this is a good time to speak up. We have some
    > time to address whatever concern you may have.
    >

    Hi,

    I know I'm very late in speaking up - but still I'll hazard a try. This is not
    exactly the same topic but closely related.

    There is a a different call to flush_tlb_fix_spurious( ), towards the end of
    handle_pte_fault( ) which commit 61c77326d "x86, mm: Avoid unnecessary TLB flush"
    made no-op for X86. However is this really needed for any arch at all - even if we
    don't know all the arch specific quirks.

    Given the code flow below

    handle_pte_fault( )
    ....
    ....
    if ptep_set_access_flags()-> if PTE chg remote TLB shot (pgtable-generic.c ver)
    update_mmu_cache -> if PTE chg local TLB possibly shot too
    else
    flush_tlb_fix_spurious_fault -> PTE didn't change - still remote TLB shotdown

    So for PTE unchanged case, we default to doing remote TLB IPIs (barring X86) -
    unless arch makes this macro NULL.

    Thing is, in case of SMP races - due to PTE being different - any fixups to
    local/remote will be handled within ptep_set_access_flags( ) - arch-specific or
    generic versions. What I fail to understand is need to do anything - specially a
    remote shootdown, for PTE not changed case.

    I could shut up and just make it NO-OP for ARC, but ....

    Please note that for the record, the addition of this special case was done via
    following change. It might help answer what I feel to comprehend.

    2005-10-29 1a44e14 [PATCH] .text page fault SMP scalability optimization

    I might be totally off track so please feel free to bash me - but atleast I would
    end up knowing more !

    Thx,
    -Vineet


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