Messages in this thread | | | Subject | Re: [PATCH 3/3] flush_tlb_range() needs ->page_table_lock when ->mmap_sem is not held | From | Benjamin Herrenschmidt <> | Date | Sat, 10 Mar 2012 08:06:06 +1100 |
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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...
Cheers, Ben.
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