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    SubjectRe: page table lock patch V15 [0/7]: overview
    On Thu, 13 Jan 2005, Andi Kleen wrote:

    > The rule in i386/x86-64 is that you cannot set the PTE in a non atomic way
    > when its present bit is set (because the hardware could asynchronously
    > change bits in the PTE that would get lost). Atomic way means clearing
    > first and then replacing in an atomic operation.

    Hmm. I replaced that portion in the swapper with an xchg operation
    and inspect the result later. Clearing a pte and then setting it to
    something would open a window for the page fault handler to set up a new
    pte there since it does not take the page_table_lock. That xchg must be
    atomic for PAE mode to work then.

    > This helps you because you shouldn't be looking at the pte anyways
    > when pte_present is false. When it is not false it is always updated
    > atomically.

    so pmd_present, pud_none and pgd_none could be considered atomic even if
    the pm/u/gd_t is a multi-word entity? In that case the current approach
    would work for higher level entities and in particular S/390 would be in
    the clear.

    But then the issues of replacing multi-word ptes on i386 PAE remain. If no
    write lock is held on mmap_sem then all writes to pte's must be atomic in
    order for the get_pte_atomic operation to work reliably.
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