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    SubjectRe: [PATCH 3/3] flush_tlb_range() needs ->page_table_lock when ->mmap_sem is not held
    On Mon, 2012-03-05 at 20:53 +0000, Al Viro wrote:
    > On Mon, Mar 05, 2012 at 12:30:19PM -0800, Linus Torvalds wrote:
    > > Is this safe? And why does it need it? Please add more explanations.
    > a) safety - as the matter of fact, all other callers either hold either
    > ->mmap_sem (exclusive) or ->page_table_lock. flush_tlb_range() is
    > called under ->page_table_lock in a lot of places, e.g.
    > page_referenced_one() -> pmdp_clear_flush_young_notify() ->
    > -> pmdp_clear_flush_young() -> flush_tlb_range(), with
    > /* go ahead even if the pmd is pmd_trans_splitting() */
    > if (pmdp_clear_flush_young_notify(vma, address, pmd))
    > referenced++;
    > spin_unlock(&mm->page_table_lock);
    > in page_referenced_one().
    > b) there are instances that work with page tables. See e.g.
    > arch/powerpc/mm/tlb_hash32.c, flush_tlb_range() and flush_range() in there.
    > The same goes for uml, with a lot more extensive playing with page tables.

    Yes, we need to make sure they don't go away. Without any of these locks
    page table pages may be freed... however, I don't see the page table
    lock ensuring that anymore. The hugetlb_free_pgd_range implementation in
    powerpc seemed to have old comments about expecting the PTL to be held
    but that doesn't appear to be the case anymore.

    In fact I always worry with the whole walking of page tables vs. freeing
    them. We use sched RCU to delay the freeing so we -should- be ok if we
    keep interrupts off on the walking side but it's fishy.

    > Almost all callers are actually fine - flush_tlb_range() may have no need
    > to bother playing with page tables, but it can do so safely; again, this
    > caller is the sole exception - everything else either has exclusive ->mmap_sem
    > on the mm in question, or mm->page_table_lock is held.

    mmap_sem will protect vs. page tables freeing. page_table_lock on the
    other hand...


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