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    SubjectRe: [RFC][PATCH 4/6] arm, mm: Convert arm to generic tlb
    On Thu, May 17, 2012 at 05:00:12PM +0100, Catalin Marinas wrote:
    > On Thu, May 17, 2012 at 12:28:06PM +0100, Peter Zijlstra wrote:
    > > On Thu, 2012-05-17 at 10:51 +0100, Russell King wrote:
    > > > On Thu, May 17, 2012 at 10:30:23AM +0100, Catalin Marinas wrote:
    > > > > Another minor thing is that on newer ARM processors (Cortex-A15) we
    > > > > need the TLB shootdown even on UP systems, so tlb_fast_mode should
    > > > > always return 0. Something like below (untested):
    > > >
    > > > No Catalin, we need this for virtually all ARMv7 CPUs whether they're UP
    > > > or SMP, not just for A15, because of the speculative prefetch which can
    > > > re-load TLB entries from the page tables at _any_ time.
    > >
    > > Hmm,. so this is mostly because of the confusion/coupling between
    > > tlb_remove_page() and tlb_remove_table() I guess. Since I don't see the
    > > freeing of the actual pages being a problem with speculative TLB
    > > reloads, just the page-tables.
    > >
    > > Should we introduce a tlb_remove_table() regardless of
    > > HAVE_RCU_TABLE_FREE which always queues the tables regardless of
    > > tlb_fast_mode()?
    >
    > BTW, looking at your tlb-unify branch, does tlb_remove_table() call
    > tlb_flush/tlb_flush_mmu before freeing the tables? I can only see
    > tlb_remove_page() doing this. On ARM, even UP, we need the TLB flushing
    > after clearing the pmd and before freeing the pte page table (and
    > ideally doing it less often than at every pte_free_tlb() call).

    Catalin,

    The way TLB shootdown stuff works is that _every_ single bit of memory
    which gets freed, whether its a page or a page table, gets added to a
    list of pages to be freed.

    So, the sequence is:
    - remove pte/pmd/pud/pgd pointers
    - add pages, whether they be pages pointed to by pte entries or page tables
    to be freed to a list
    - when list is sufficiently full, invalidate TLBs
    - free list of pages

    That means the pages will not be freed, whether it be a page mapped
    into userspace or a page table until such time that the TLB has been
    invalidated.

    For page tables, this is done via pXX_free_tlb(), which then calls out
    to the arch specific __pXX_free_tlb(), which ultimately then hands the
    page table over to tlb_remove_page() to add to the list of to-be-freed
    pages.

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


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