Re: [PATCH v2 4/4] mm: fix KSM data corruption

[Minchan Kim]

Hi Andrea,

On Tue, Aug 01, 2017 at 09:33:41PM +0200, Andrea Arcangeli wrote:
> Hello,
> 
> On Tue, Aug 01, 2017 at 02:56:17PM +0900, Minchan Kim wrote:
> > CPU0		CPU1		CPU2		CPU3
> > ----		----		----		----
> > Write the same
> > value on page
> > 
> > [cache PTE as
> >  dirty in TLB]
> > 
> > 		MADV_FREE
> > 		pte_mkclean()
> > 
> > 				4 > clear_refs
> > 				pte_wrprotect()
> > 
> > 						write_protect_page()
> > 						[ success, no flush ]
> > 
> > 						pages_indentical()
> > 						[ ok ]
> > 
> > Write to page
> > different value
> > 
> > [Ok, using stale
> >  PTE]
> > 
> > 						replace_page()
> > 
> > Later, CPU1, CPU2 and CPU3 would flush the TLB, but that is too late. CPU0
> > already wrote on the page, but KSM ignored this write, and it got lost.
> > "
> > 
> > In above scenario, MADV_FREE is fixed by changing TLB batching API
> > including [set|clear]_tlb_flush_pending. Remained thing is soft-dirty part.
> > 
> > This patch changes soft-dirty uses TLB batching API instead of flush_tlb_mm
> > and KSM checks pending TLB flush by using mm_tlb_flush_pending so that
> > it will flush TLB to avoid data lost if there are other parallel threads
> > pending TLB flush.
> > 
> > [1] http://lkml.kernel.org/r/BD3A0EBE-ECF4-41D4-87FA-C755EA9AB6BD@xxxxxxxxx
> > 
> > Note:
> > I failed to reproduce this problem through Nadav's test program which
> > need to tune timing in my system speed so didn't confirm it work.
> > Nadav, Could you test this patch on your test machine?
> 
> Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>

Thanks for the review!