Re: [PATCH 06/17] arm: mmu_gather rework

From: Peter Zijlstra
Date: Fri Feb 25 2011 - 14:45:49 EST

On Fri, 2011-02-25 at 19:04 +0100, Peter Zijlstra wrote:
> On Thu, 2011-02-24 at 17:34 +0100, Peter Zijlstra wrote:
> > On Thu, 2011-02-17 at 17:23 +0100, Peter Zijlstra wrote:
> > > plain text document attachment
> > > (peter_zijlstra-arm-preemptible_mmu_gather.patch)
> > > Fix up the arm mmu_gather code to conform to the new API.
> >
> > So akpm noted that this one doesn't apply anymore because of:
> >
> > commit 06824ba824b3e9f2fedb38bee79af0643198ed7f
> > Author: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
> > Date: Sun Feb 20 12:16:45 2011 +0000
> >
> > ARM: tlb: delay page freeing for SMP and ARMv7 CPUs
> >
> > We need to delay freeing any mapped page on SMP and ARMv7 systems to
> > ensure that the data is not accessed by other CPUs, or is used for
> > speculative prefetch with ARMv7. This includes not only mapped pages
> > but also pages used for the page tables themselves.
> >
> > This avoids races with the MMU/other CPUs accessing pages after they've
> > been freed but before we've invalidated the TLB.
> >
> > Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
> >
> >
> > Which raises a nice point about shift_arg_pages() which calls
> > free_pgd_range(), the other architectures that look similar to arm in
> > this respect are ia64 and sh, do they suffer the same problem?
> >
> > It doesn't look hard to fold the requirements for this into the generic
> > tlb range support (patch 14 in this series).
> It looks like both ia64 and sh do indeed suffer there.
> I've pulled my generic range tracking to the head of the series so that
> I can convert ARM, IA64 and SH to generic tlb solving it for those.
> Russell, generic tlb doesn't look to need the extra logic you added for
> the fs/exec.c case, but please double check the patches when I post
> them.
> In short, tlb_end_vma() will call flush_tlb_range() on the tracked range
> and clear ->need_flush, so things like zap_page_range() will not then
> also call tlb_flush().
> In case of shift_arg_pages() and unmap_region() however we first call
> free_pgtables() which might end up calling p??_free_tlb() which will
> then set ->need_flush, and tlb_finish_mmu() will then end up calling
> tlb_flush().
> I'm not quite sure why you chose to add range tracking on
> pte_free_tlb(), the only affected code path seems to be unmap_region()
> where you'll use a flush_tlb_range(), but its buggy, the pte_free_tlb()
> range is much larger than 1 page, and if you do it there you also need
> it for all the other p??_free_tlb() functions.
> The tlb flush after freeing page-tables is needed for things like
> gup_fast() which needs to sync against them being freed.
> So the stuff I have now will try its best to track ranges on zap_* while
> clearing the page mapping, will use flush_cache_range() and
> flush_tlb_range(). But when it comes to tearing down the page-tables
> themselves we'll punt and use a full mm flush, which seems a waste of
> all that careful range tracking by zap_*.
> One possibility would be to add tlb_start/end_vma() in
> unmap_page_range(), except we don't need to flush the cache again, also,
> it would be nice to not have to flush on tlb_end_vma() but delay it all
> to tlb_finish_mmu() where possible.
> OK, let me try and hack up proper range tracking for free_*, that way I
> can move the flush_tlb_range() from tlb_end_vma() and into
> tlb_flush_mmu().

Grmbl.. so doing that would require flush_tlb_range() to take an mm, not
a vma, but tile and arm both use the vma->flags & VM_EXEC test to avoid
flushing their i-tlbs.

I'm tempted to make them flush i-tlbs unconditionally as its still
better than hitting an mm wide tlb flush due to the page table free.

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