linux-kernel - Re: [RFC] mm: Generalize notify_page

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Date:   Fri, 31 May 2019 10:48:54 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Mark Rutland <mark.rutland@....com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC] mm: Generalize notify_page_fault()

On Fri, May 31, 2019 at 02:17:43PM +0530, Anshuman Khandual wrote:
> On 05/30/2019 07:09 PM, Matthew Wilcox wrote:
> > On Thu, May 30, 2019 at 05:31:15PM +0530, Anshuman Khandual wrote:
> >> On 05/30/2019 04:36 PM, Matthew Wilcox wrote:
> >>> The two handle preemption differently.  Why is x86 wrong and this one
> >>> correct?
> >>
> >> Here it expects context to be already non-preemptible where as the proposed
> >> generic function makes it non-preemptible with a preempt_[disable|enable]()
> >> pair for the required code section, irrespective of it's present state. Is
> >> not this better ?
> > 
> > git log -p arch/x86/mm/fault.c
> > 
> > search for 'kprobes'.
> > 
> > tell me what you think.
> 
> Are you referring to these following commits
> 
> a980c0ef9f6d ("x86/kprobes: Refactor kprobes_fault() like kprobe_exceptions_notify()")
> b506a9d08bae ("x86: code clarification patch to Kprobes arch code")
> 
> In particular the later one (b506a9d08bae). It explains how the invoking context
> in itself should be non-preemptible for the kprobes processing context irrespective
> of whether kprobe_running() or perhaps smp_processor_id() is safe or not. Hence it
> does not make much sense to continue when original invoking context is preemptible.
> Instead just bail out earlier. This seems to be making more sense than preempt
> disable-enable pair. If there are no concerns about this change from other platforms,
> I will change the preemption behavior in proposed generic function next time around.

Exactly.

So, any of the arch maintainers know of a reason they behave differently
from x86 in this regard?  Or can Anshuman use the x86 implementation
for all the architectures supporting kprobes?