> On Mar 11, 2022, at 11:07 AM, Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>
> From: Nadav Amit <namit@xxxxxxxxxx>
>
> Currently, using mprotect() to unprotect a memory region or uffd to
> unprotect a memory region causes a TLB flush. At least on x86, as
> protection is promoted, no TLB flush is needed.
>
> Add an arch-specific pte_may_need_flush() which tells whether a TLB
> flush is needed based on the old PTE and the new one. Implement an x86
> pte_may_need_flush().
>
> For x86, besides the simple logic that PTE protection promotion or
> changes of software bits does require a flush, also add logic that
> considers the dirty-bit. If the dirty-bit is clear and write-protect is
> set, no TLB flush is needed, as x86 updates the dirty-bit atomically
> on write, and if the bit is clear, the PTE is reread.
>
>
[snip]
> + */
> +static inline bool pte_needs_flush(pte_t oldpte, pte_t newpte)
> +{
> + /* !PRESENT -> * ; no need for flush */
> + if (!pte_present(oldpte))
> + return false;
> +
> + /* PRESENT -> !PRESENT ; needs flush */
> + if (!pte_present(newpte))
> + return true;
> +
> + /* PFN changed ; needs flush */
> + if (pte_pfn(oldpte) != pte_pfn(newpte))
> + return true;
> +
> + return pte_flags_need_flush(pte_flags(oldpte), pte_flags(newpte),
> + _PAGE_ACCESSED);
> +}
Looking again at this patch, I think the PRESENT -> !PRESENT is
not needed:
1. It might trigger unnecessary flushes for PROT_NONE (or NUMA)
2. Real PRESENT -> !PRESENT is already checked by
pte_flags_need_flush().
Let me know if I am missing something. Otherwise I will change it
for v4.
