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From: Oleg Nesterov <>
To: Marcin Nowakowski <>
Cc: tip-bot for Marcin Nowakowski <>,,,,,,,,,,,,
Subject: Re: [tip:perf/urgent] uprobes: Fix uprobes on MIPS, allow for a cache flush after ixol breakpoint creation
Date: Tue, 20 Dec 2016 18:50:05 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 12/20, Marcin Nowakowski wrote:
> Hi Oleg,
> On 20.12.2016 14:08, Oleg Nesterov wrote:
>> On 12/19, tip-bot for Marcin Nowakowski wrote:
>>> uprobes: Fix uprobes on MIPS, allow for a cache flush after ixol breakpoint creation
>>> Commit:
>>>   72e6ae285a1d ('ARM: 8043/1: uprobes need icache flush after xol write'
>>> ... has introduced an arch-specific method to ensure all caches are
>>> flushed appropriately after an instruction is written to an XOL page.
>> when this page is already mmaped,
>>> However, when the XOL area is created and the out-of-line breakpoint
>>> instruction is copied, caches are not flushed at all and stale data may
>>> be found in icache.
>> but in this case the page is not mmaped yet, the probed application will
>> take a page fault if it tries to execute this insn,
> In case of MIPS (and AFAICT ARM as well, and these are the only  
> architectures that implement arch_uprobe_copy_ixol), the cache flushing  
> is done through the kernel addresses of that page, so the fact that it  
> is not mapped yet is not an issue.

OK, thanks,

> Do I understand correctly that your statement implies that after the  
> page fault and mmapping the xol page, the page is guaranteed to be  
> updated in the cache? As definitely that is not something that is  
> happening at the moment.

Well, I do not know. Let me repeat I don't understand this flush_.*cache

But. do_read_fault() does

	__do_fault(..., &fault_page, ...);


and alloc_set_pte() does flush_icache_page(vma, page)... Hmm, which is nop
on MIPS.

>> OK, I know nothing about MIPS, but could you help me understand this change?
>> See above. If we really need flush_icache_range() here then perhaps we should
>> modify install_special_mapping() and/or __do_fault/special_mapping_fault paths
>> instead?
> Are you suggesting that those should be updated to force a cache update?

Again, I do not know. But perhaps it makes more sense to actually implement
flush_icache_page() ? Otherwise another user of install_special_mapping()
can hit the same problem?


  reply	other threads:[~2016-12-20 17:51 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-12-13 10:40 [PATCH] uprobes: " Marcin Nowakowski
2016-12-20  7:58 ` [tip:perf/urgent] uprobes: Fix uprobes on MIPS, " tip-bot for Marcin Nowakowski
2016-12-20 13:08   ` Oleg Nesterov
2016-12-20 15:21     ` Marcin Nowakowski
2016-12-20 17:50       ` Oleg Nesterov [this message]
2016-12-21 11:24         ` Ralf Baechle

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