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Subject: Re: [PATCH 2/6] uprobes: introduce is_swbp_at_addr_fast()
From: Peter Zijlstra <peterz@infradead.org>
Date: Mon, 16 Apr 2012 16:55:09 +0200
In-Reply-To: <20120416144457.GA7018@redhat.com>
References: <20120405222024.GA19154@redhat.com>
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To: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>, Srikar Dronamraju <srikar@linux.vnet.ibm.com>, Andrew Morton <akpm@linux-foundation.org>, Linus Torvalds <torvalds@linux-foundation.org>, Ananth N Mavinakayanahalli <ananth@in.ibm.com>, Jim Keniston <jkenisto@linux.vnet.ibm.com>, LKML <linux-kernel@vger.kernel.org>, Linux-mm <linux-mm@kvack.org>, Andi Kleen <andi@firstfloor.org>, Christoph Hellwig <hch@infradead.org>, Steven Rostedt <rostedt@goodmis.org>, Arnaldo Carvalho de Melo <acme@infradead.org>, Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>, Thomas Gleixner <tglx@linutronix.de>, Anton Arapov <anton@redhat.com>

On Mon, 2012-04-16 at 16:44 +0200, Oleg Nesterov wrote:
> On 04/16, Peter Zijlstra wrote:
> >
> > On Fri, 2012-04-06 at 00:21 +0200, Oleg Nesterov wrote:
> > > +int __weak is_swbp_at_addr_fast(unsigned long vaddr)
> > > +{
> > > +       uprobe_opcode_t opcode;
> > > +       int fault;
> > > +
> > > +       pagefault_disable();
> > > +       fault =3D __copy_from_user_inatomic(&opcode, (void __user*)va=
ddr,
> > > +                                                       sizeof(opcode=
));
> > > +       pagefault_enable();
> > > +
> > > +       if (unlikely(fault)) {
> > > +               /*
> > > +                * XXX: read_opcode() lacks FOLL_FORCE, it can fail i=
f
> > > +                * we race with another thread which does mprotect(NO=
NE)
> > > +                * after we hit bp.
> > > +                */
> > > +               if (read_opcode(current->mm, vaddr, &opcode))
> > > +                       return -EFAULT;
> > > +       }
> > > +
> > > +       return is_swbp_insn(&opcode);
> > > +}
> >
> > Why bother with the pagefault_disable() and unlikely fault case and not
> > simply do copy_from_user() and have it deal with the fault if its neede=
d
> > anyway?
>=20
> But we can't do this under down_read(mmap_sem) ?
>=20
> If another thread waits for down_write() then do_page_fault() can't take
> this lock, right?

Ah, indeed, I thought read_opcode() would do the fault, but that's
get_user_pages() which requires the caller to hold mmap_sem instead.

Can't we 'optimize' read_opcode() by doing the pagefault_disable() +
__copy_from_user_inatomic() optimistically before going down the whole
gup()+lock+kmap path?

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