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* Potential race in TLB flush batching?
@ 2017-07-11  0:52 Nadav Amit
  2017-07-11  6:41 ` Mel Gorman
  0 siblings, 1 reply; 70+ messages in thread
From: Nadav Amit @ 2017-07-11  0:52 UTC (permalink / raw)
  To: Mel Gorman, Andy Lutomirski; +Cc: open list:MEMORY MANAGEMENT

Something bothers me about the TLB flushes batching mechanism that Linux
uses on x86 and I would appreciate your opinion regarding it.

As you know, try_to_unmap_one() can batch TLB invalidations. While doing so,
however, the page-table lock(s) are not held, and I see no indication of the
pending flush saved (and regarded) in the relevant mm-structs.

So, my question: what prevents, at least in theory, the following scenario:

	CPU0 				CPU1
	----				----
					user accesses memory using RW PTE 
					[PTE now cached in TLB]
	try_to_unmap_one()
	==> ptep_get_and_clear()
	==> set_tlb_ubc_flush_pending()
					mprotect(addr, PROT_READ)
					==> change_pte_range()
					==> [ PTE non-present - no flush ]

					user writes using cached RW PTE
	...

	try_to_unmap_flush()


As you see CPU1 write should have failed, but may succeed. 

Now I don’t have a PoC since in practice it seems hard to create such a
scenario: try_to_unmap_one() is likely to find the PTE accessed and the PTE
would not be reclaimed.

Yet, isn’t it a problem? Am I missing something?

Thanks,
Nadav
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^ permalink raw reply	[flat|nested] 70+ messages in thread

end of thread, other threads:[~2017-07-27 17:36 UTC | newest]

Thread overview: 70+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2017-07-11  0:52 Potential race in TLB flush batching? Nadav Amit
2017-07-11  6:41 ` Mel Gorman
2017-07-11  7:30   ` Nadav Amit
2017-07-11  9:29     ` Mel Gorman
2017-07-11 10:40       ` Nadav Amit
2017-07-11 13:20         ` Mel Gorman
2017-07-11 14:58           ` Andy Lutomirski
2017-07-11 15:53             ` Mel Gorman
2017-07-11 17:23               ` Andy Lutomirski
2017-07-11 19:18                 ` Mel Gorman
2017-07-11 20:06                   ` Nadav Amit
2017-07-11 21:09                     ` Mel Gorman
2017-07-11 20:09                   ` Mel Gorman
2017-07-11 21:52                     ` Mel Gorman
2017-07-11 22:27                       ` Nadav Amit
2017-07-11 22:34                         ` Nadav Amit
2017-07-12  8:27                         ` Mel Gorman
2017-07-12 23:27                           ` Nadav Amit
2017-07-12 23:36                             ` Andy Lutomirski
2017-07-12 23:42                               ` Nadav Amit
2017-07-13  5:38                                 ` Andy Lutomirski
2017-07-13 16:05                                   ` Nadav Amit
2017-07-13 16:06                                     ` Andy Lutomirski
2017-07-13  6:07                             ` Mel Gorman
2017-07-13 16:08                               ` Andy Lutomirski
2017-07-13 17:07                                 ` Mel Gorman
2017-07-13 17:15                                   ` Andy Lutomirski
2017-07-13 18:23                                     ` Mel Gorman
2017-07-14 23:16                               ` Nadav Amit
2017-07-15 15:55                                 ` Mel Gorman
2017-07-15 16:41                                   ` Andy Lutomirski
2017-07-17  7:49                                     ` Mel Gorman
2017-07-18 21:28                                   ` Nadav Amit
2017-07-19  7:41                                     ` Mel Gorman
2017-07-19 19:41                                       ` Nadav Amit
2017-07-19 19:58                                         ` Mel Gorman
2017-07-19 20:20                                           ` Nadav Amit
2017-07-19 21:47                                             ` Mel Gorman
2017-07-19 22:19                                               ` Nadav Amit
2017-07-19 22:59                                                 ` Mel Gorman
2017-07-19 23:39                                                   ` Nadav Amit
2017-07-20  7:43                                                     ` Mel Gorman
2017-07-22  1:19                                                       ` Nadav Amit
2017-07-24  9:58                                                         ` Mel Gorman
2017-07-24 19:46                                                           ` Nadav Amit
2017-07-25  7:37                                                           ` Minchan Kim
2017-07-25  8:51                                                             ` Mel Gorman
2017-07-25  9:11                                                               ` Minchan Kim
2017-07-25 10:10                                                                 ` Mel Gorman
2017-07-26  5:43                                                                   ` Minchan Kim
2017-07-26  9:22                                                                     ` Mel Gorman
2017-07-26 19:18                                                                       ` Nadav Amit
2017-07-26 23:40                                                                         ` Minchan Kim
2017-07-27  0:09                                                                           ` Nadav Amit
2017-07-27  0:34                                                                             ` Minchan Kim
2017-07-27  0:48                                                                               ` Nadav Amit
2017-07-27  1:13                                                                                 ` Nadav Amit
2017-07-27  7:04                                                                                   ` Minchan Kim
2017-07-27  7:21                                                                                     ` Mel Gorman
2017-07-27 16:04                                                                                       ` Nadav Amit
2017-07-27 17:36                                                                                         ` Mel Gorman
2017-07-26 23:44                                                                       ` Minchan Kim
2017-07-11 22:07                   ` Andy Lutomirski
2017-07-11 22:33                     ` Mel Gorman
2017-07-14  7:00                     ` Benjamin Herrenschmidt
2017-07-14  8:31                       ` Mel Gorman
2017-07-14  9:02                         ` Benjamin Herrenschmidt
2017-07-14  9:27                           ` Mel Gorman
2017-07-14 22:21                             ` Andy Lutomirski
2017-07-11 16:22           ` Nadav Amit

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