Re: [PATCH 2/4] mm: Send a single IPI to TLB flush multiple pages when unmapping

[Mel Gorman]

On Thu, Apr 16, 2015 at 03:38:26PM +0900, Minchan Kim wrote:
> Hello Mel,
> 
> On Wed, Apr 15, 2015 at 10:28:55PM +0100, Mel Gorman wrote:
> > On Wed, Apr 15, 2015 at 02:16:49PM -0700, Hugh Dickins wrote:
> > > On Wed, 15 Apr 2015, Rik van Riel wrote:
> > > > On 04/15/2015 06:42 AM, Mel Gorman wrote:
> > > > > An IPI is sent to flush remote TLBs when a page is unmapped that was
> > > > > recently accessed by other CPUs. There are many circumstances where this
> > > > > happens but the obvious one is kswapd reclaiming pages belonging to a
> > > > > running process as kswapd and the task are likely running on separate CPUs.
> > > > > 
> > > > > On small machines, this is not a significant problem but as machine
> > > > > gets larger with more cores and more memory, the cost of these IPIs can
> > > > > be high. This patch uses a structure similar in principle to a pagevec
> > > > > to collect a list of PFNs and CPUs that require flushing. It then sends
> > > > > one IPI to flush the list of PFNs. A new TLB flush helper is required for
> > > > > this and one is added for x86. Other architectures will need to decide if
> > > > > batching like this is both safe and worth the memory overhead. Specifically
> > > > > the requirement is;
> > > > > 
> > > > > 	If a clean page is unmapped and not immediately flushed, the
> > > > > 	architecture must guarantee that a write to that page from a CPU
> > > > > 	with a cached TLB entry will trap a page fault.
> > > > > 
> > > > > This is essentially what the kernel already depends on but the window is
> > > > > much larger with this patch applied and is worth highlighting.
> > > > 
> > > > This means we already have a (hard to hit?) data corruption
> > > > issue in the kernel.  We can lose data if we unmap a writable
> > > > but not dirty pte from a file page, and the task writes before
> > > > we flush the TLB.
> > > 
> > > I don't think so.  IIRC, when the CPU needs to set the dirty bit,
> > > it doesn't just do that in its TLB entry, but has to fetch and update
> > > the actual pte entry - and at that point discovers it's no longer
> > > valid so traps, as Mel says.
> > > 
> > 
> > This is what I'm expecting i.e. clean->dirty transition is write-through
> > to the PTE which is now unmapped and it traps. I'm assuming there is an
> > architectural guarantee that it happens but could not find an explicit
> > statement in the docs. I'm hoping Dave or Andi can check with the relevant
> > people on my behalf.
> 
> A dumb question. It's not related to your patch but MADV_FREE.
> 
> clean->dirty transition is *atomic* as well as write-through?

This is the TLB cache clean->dirty transition so it's not 100% clear what you
are asking. It both needs to be write-through and the TLB updates must happen
before the actual data write to cache or memory and it must be ordered.

> I'm really confusing.
> It seems most arches use xchg for ptep_get_and_clear so it's
> atomic but some of arches without defining __HAVE_ARCH_PTEP_GET_AND_CLEAR
> will use non-atomic version in include/asm-generic/pgtable.h.
> 
>         #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
>         static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>                                                unsigned long address,
>                                                pte_t *ptep)
>         {
>                 pte_t pte = *ptep;
>                 pte_clear(mm, address, ptep);
>                 return pte;
>         }
>         #endif
> 

And if they are using this, they need to be ok that it's not atomic but
it's not clear what you are asking.

> I hope they have own lock or something to protect a race between software
> and hardware(ie, CPU set dirty bit by itself).
> 

Or they're UP.

-- 
Mel Gorman
SUSE Labs
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