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Date: Mon, 5 Mar 2012 20:53:54 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] flush_tlb_range() needs ->page_table_lock when
 ->mmap_sem is not held
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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.

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.