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@gmail.com

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 <redacted>

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