linux-kernel - Re: [tip:perf/urgent] uprobes: Fix uprobes on MIPS, allow for a cache flush after ixol breakpoint creation

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Date:   Tue, 20 Dec 2016 14:08:20 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     tip-bot for Marcin Nowakowski <tipbot@...or.com>
Cc:     linux-tip-commits@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterz@...radead.org, alexander.shishkin@...ux.intel.com,
        acme@...nel.org, marcin.nowakowski@...tec.com, acme@...hat.com,
        mingo@...nel.org, victor.kamensky@...aro.org, jolsa@...hat.com,
        hpa@...or.com, torvalds@...ux-foundation.org, tglx@...utronix.de
Subject: Re: [tip:perf/urgent] uprobes: Fix uprobes on MIPS, allow for a
 cache flush after ixol breakpoint creation

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,

> Replace a simple copy_to_page() with arch_uprobe_copy_ixol() to allow
> the arch to ensure all caches are updated accordingly.
>
> This change fixes uprobes on MIPS InterAptiv (tested on Creator Ci40).

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?

Oleg.